Porsche training p90 electrical systems

Current When there is a voltage electrical pressure, and electrons are given a path to flow a conductor, current flows.. • Resistance a load or consumer, used to do work and control curr

AfterSales Training

Electrical Systems

P90

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Important Notice: Some of the contents of this AfterSales Training brochure was originally written by Porsche AG for its

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Electrical Troubleshooting Logic

1 -Do you understand how the electrical consumer is expected to operate?

2 -Do you have the correct wiring diagram?

3 -If the circuit contains a fuse, is the fuse okay & of the correct amperage?

4 -Is there power provided to the circuit? Is the power source the correct voltage?

5 -Is the ground(s) for the circuit connected? Is the connection tight & free of resistance?

6 -Is the circuit being correctly activated by a switch, relay, sensor, microswitch, etc.?

7 -Are all electrical plugs connected securely with no tension, corrosion, or loose wires?

Description Page

Section 1 – Basic Electricity

Introduction 2

Voltage 5

Current 5

Resistance 5

What is a Circuit? 6

Ohm’s Law 7

Units of Measurement 8

Types of Circuits 9

Series Circuits and Ohm’s Law 10

Parallel Circuits 11

Series/Parrallel Circuits 12

Digital Volt-Ohm Meter 13

Voltage Testing 13

Amperage Testing 14

Inductive Clamp 15

Resistance Testing 15

Section 2 – Electrical Components Introduction 2

Switches 2

Relays 3

Resistors 4

Lights 5

Capacitors 5

Inductive Sensors 6

Temperature Sensors 7

Photo Diode 7

Potentiometers 8

Hall-Effect Sensors 8

Active Wheel Speed Sensors 9

Solenoids 10

Electric Motors 10

Semiconductor 11

Diodes 12

Zener Diodes 13

Transistors 14

Section 3 – Batteries, Starters and Generators General Information 2

Battery 2

Starter 4

Starter Operation 6

Generator 7

Gateway Control Unit 7

Section 4 – Control Units

Control Units 2

Function Flow Diagram 3

Section 5 – BUS Communications CAN Bus Networking 2

Network Architechure 3

Bus Logic 4

MOST Bus Networking 5

Boxster (981) Network Topology 8

Cayman (981) Network Topology 9

911 (991) Network Topology 10

Cayenne (92A) Network Topology 11

Macan (95B) 12

Panamera (970) Network Topology 13

Section 6 – Fuses & Relays Sports Cars (9x1) 2

Cayenne (92A) 6

Macan (95B) 9

Panamera (970) 10

Panamera S E-Hybrid (970) 12

Section 7 – PIWIS Tester Wiring Diagram Viewer Selecting Language 2

Selecting Wiring Diagram 2

Wiring Diagram Viewer/Functions 3

Wiring Diagram Viewer Navigation 6

Zooming the Wiring Diagram 7

Lines (Wires) 8

Selecting & Deselecting a Line/Filter Function 9

Following the Line Routing into Another Wiring Diagram 11

Internal Circuitry 13

Connector View 14

Printing 15

Help 16

Operating Manual 17

Function Flow 18

Section 8 – Worksheets Worksheet 1 - Series Circuit Breadboarding Exercise 2

Worksheet 2 - Parallel Circuit Breadboarding Exercise 4

Worksheet 3 - Relay Circuit Breadboarding Exercise 6

Worksheet 4 - Transistor Circuit Breadboarding Exercise 9

Section 9 – Appendix

Subject Page

Introduction 2

Voltage 5

Current 5

Resistance 5

What is a Circuit? 6

Ohm’s Law 7

Units of Measurement 8

Types of Circuits 9

Series Circuits and Ohm’s Law 10

Parallel Circuits 11

Series/Parrallel Circuits 12

Digital Volt-Ohm Meter 13

Voltage Testing 13

Amperage Testing 14

Inductive Clamp 14

Resistance Testing 15

Electrical power is essential for the operation of today’s

vehicles Electrical energy must be available to provide

enough power to operate the control units, sensors,

motors, and accessories on the vehicle When the vehicle

is not operating, there must be sufficient power to monitor

certain systems, to provide power for accessories, and to

allow the vehicle to be restarted

Electricity is a form of energy similar to light, heat,

mechanical and chemical energy Electricity has

several advantages compared to these other forms

of energy:

• Large amounts of energy can be transported over long

distances

• It is easy to transform to other forms of energy, such as

light, heat, mechanical and chemical

• It can be used to operate electrical circuits and motors

• It can be easily generated and stored (chemically in a

battery)

While mechanical systems are generally easily explained,

electrical and electronic systems remain invisible while

performing work This class will simplify electricity and

electronics and assist you in testing, diagnosing and

repairing Porsche vehicles

What is Electricity?

Electricity is the movement of electrons from one atom to

another In order to understand electricity, we need to

look at atoms

An atom is the smallest particle of matter Matter is

anything that has mass (weight) and occupies space

Matter that is made up of only one kind of atom is called

an element Copper, aluminum and oxygen are examples

of elements Matter that is made up of more than one

kind of atom is called a compound Water (which contains

the elements hydrogen and oxygen) is a compound

Parts of an atom:

• Electrons - Negatively charged particles orbiting around

the nucleus

• Protons - Positively charged particles in the nucleus

• Neutrons - Uncharged particles in the nucleus

Protons and electrons have equal but opposite magneticcharges Opposite (or unlike) charges attract, so the nega-tively charged electrons are held in their orbits around thenucleus by their attraction to the positively chargedprotons in the nucleus Since neutrons have no electricalcharge, they do not affect this relationship

Different elements have different numbers of protons,neutrons and electrons Hydrogen is the simplest atomwith one proton, one electron and no neutrons Helium hastwo protons, two electrons and two neutrons, while copperhas 29 protons, 29 electrons, and a varying number ofneutrons

An atom is balanced (and has a neutral charge) when thenumber of protons and electrons are equal For example,

a balanced hydrogen atom has one proton and oneelectron A balanced copper atom has 29 protons, and 29electrons

ElectronNucleus

Copper

Nu cleu s (29 Protons)

Positive and Negative Ions

Atoms can sometimes lose an electron and become

posi-tively charged because of the greater number of protons

These atoms are called Positive Ions Atoms can also pick

up or gain extra electrons and become negatively

charged These atoms are called Negative Ions Negative

ions will attempt to repel extra electrons and positive ions

will attempt to attract them

Valence Ring

Electrons orbit the atom in distinct rings Each ring can

hold a specific maximum number of electrons and each

ring is located progressively further from the nucleus The

outermost ring is called the valence ring The valence ring

can hold a maximum of eight electrons

Electrons in the valence ring determine the atom's

electrical properties If the ring has fewer than four

electrons, then these electrons can be easily forced from

their orbits and the element is therefore a conductor of

electricity Progressively fewer electrons in the valance

ring make the material a better conductor (e.g., copper,

gold) If the valence ring has more than four electrons, its

electrons are hard to force from their orbits and the

element is an electrical insulator Progressively more

electrons in the valence ring make the material a better

insulator (e.g., glass, plastic)

Atoms with four electrons in the valence ring are a special

case and are considered to be semiconductors (e.g.,

silicon) They can be made into either conductors or

insu-lators by adding impurities These materials are used in

the manufacture of diodes, transistors, and integrated

circuit chips

Electron

Positive Ion

Electrons

3 Protons

3 Neutrons

Negative Ion

Electrons

Additional Electron

Ne utrons

3 Protons

3 Neutrons

Balanced Atom

Electricity Defined

A flow of electricity is the movement of electrons from the

valence ring of one atom to the valence ring of another

atom When a large number of electrons move together in

the same direction, an electric current flows This does

not mean that a single electron will travel the entire length

of a wire Rather, electrons flow from atom to atom,

displacing other electrons in the valence rings as a large

number of electrons all move in basically the same

direction

Voltage is the force, or electrical pressure that makes

valence electrons move from atom to atom When a

voltage is applied, this happens to billions of atoms

simul-taneously

Atoms that lose valence electrons become positive ions,and atoms that receive extra valence electrons becomenegative ions The positive ion will draw one of the freeelectrons from from another atom and become balanced.The process repeats as electrons move from atom toatom

Voltage, or electromotive force (EMF) is the electrical

pressure or potential created by the difference between

positive and negative charges The greater the difference

between the positive and negative charge, the higher the

attraction, and the higher the voltage In a vehicle, the

voltage is typically created by the battery (a chemical

source of electricity) or the generator (an electromagnetic

source of electricity)

To illustrate this principle, compare electricity to a water

system Electrical voltage is similar to water pressure The

water in the pipe is under pressure when the valve is

turned off, but water is not flowing Similarly, in an

electrical circuit, there can be electrical pressure (voltage),

but no flow of electrical current

Current

When there is a voltage (electrical pressure), and electrons

are given a path to flow (a conductor), current flows

Current is the actual flow of electrons as they move from

negative ions to positive ions Since current is the

movement of a negatively charged particle towards a

posi-tively charged particle, it moves from negative to positive

This is called electron flow However, for many years,

conventional electrical theory has described current flow

from positive to negative This is the established standard

in the automotive industry, and this is how we will refer to

current flow in this class

In the water system, water flows when the valve isopened The flow of water is similar to the flow of electriccurrent

Resistance

A good conductor should not slow down or resist the flow

of electricity In other words, a good conductor shouldhave low resistance As we have said, good conductorshave valence electrons that are easy to move from atom

to atom Anything that opposes the movement ofelectrons through a conductor has resistance Resistancereduces the number of electrons that flow

Using our analogy, the water pipe is a certain size and willallow a certain volume of water to flow If we add a restric-tion, we are effectively reducing the diameter of the pipe

at that point and less water can flow This restriction isequivalent to electrical resistance In a circuit, voltagedrops across each resistance when current flows, just likewater pressure drops because of a restriction

Voltage drops across each resistance only when currentflows This will be discussed in greater detail when wediscuss circuits

Various factors affect electrical resistance in a

circuit:

• Circuit length - Increasing the conductor length

increases the resistance

• Diameter (gauge) of conductor - Decreasing the

conductor diameter (gauge) increases the resistance

• Temperature - For most materials, increasing the

temperature will increase the material’s resistance

• Physical damage - Any damage will increase

resistance

• Material - Materials have a wide range of resistances.

Summary

• The unit of measure for electrical pressure or potential

is measured in Volts (V or E)

• The unit of measure for current flow is the Ampere, or

Amp (A or I)

• The resistance of a circuit is measured in Ohms (W or

R)

What is a Circuit?

A circuit is a complete path for electron (electrical

current) flow A complete electrical circuit must

have three things:

• Voltage (electrical pressure, the “push” to move

electrons in the circuit)

• Resistance (a load or consumer, used to do work and

control current flow)

• Conductors (wiring that forms a completed pathway

between the power supply and the load)

When all three items are present, current (electrons) will

flow in the circuit

In automotive circuits, the power supply or source ofvoltage is usually the vehicle battery or generator Thecircuit resistance could be many things: light bulbs,electric motors, resistors (these are loads, or devices that

do work in a circuit) The path or circuit itself may beinsulated wires or it may be the the vehicle body orchassis

The applied voltage in the circuit drops (or is “used up”)across each load or resistance in the circuit This is calledVoltage Drop In a perfect circuit, all voltage drops willoccur across the loads and not in wiring and connectors.Most of the circuits that you will see have additionalcomponents such as switches, relays, connectors, etc.,and will usually have a circuit protection device (a fuse orcircuit breaker) to protect against short circuits and highcurrent flow Switches and relays allow the current to beswitched on or off by opening and closing the circuit

Notes:

Switch

Conductor(wiring)

Resistance(Load)

12V

Ohm’s Law

Ohm’s Law is a very useful and simple formula that allows

us to predict the behavior of electrical circuits

mathemati-cally Ohm's Law defines the relationship between voltage,

amperage, and resistance in an electrical circuit In a

complete circuit, the voltage “E” causes a current “I” to

flow through a resistance “R”

Ohm's Law states that a potential of one Volt will cause

one Amp of current to flow through a resistance of one

Ohm Stated another way, the current flow is directly

proportional to the applied voltage and inversely

proportional to the resistance

Ohm’s Law can be very useful in helping you to figure out

what is happening in a circuit To help you to remember,

look at the Ohm's Law wheel below When you know two

of the values, you can easily find the third with simple

math To use it, go to the Ohm's Law wheel, cover the

unknown, and the formula shows you what to do If the

two elements in the formula are side by side, multiply

them together If one is over the other, divide

Look at the graphics below to see the Ohm's Law formula

expressed three different ways Each different expression

of Ohm’s Law allows you to find a different unknown value

We will apply Ohm’s Law to actual circuits later, but here is

an example to show how easy this calculation is

Example: A 12 Volt circuit has a 3 Ohm resistance.

How much current flows?

I = E/R

I = 12/3 = 4 Amps

And one more example A circuit with 6 Ohms

resis-tance has 2 Amps flowing What is the voltage?

E = I x R

E = 2 x 6 = 12 Volts

Ohm's Law can also help you to understand how changingone variable in a circuit can affect another

Remember what Ohm’s Law says:

Current flow is directly proportional to the applied voltageand inversely proportional to the resistance When thevoltage stays the same, such as in a vehicle with aproperly operating charging system, then current goes up

as resistance goes down, and current goes down as tance goes up A short circuit reduces resistance, causinghigh current Loose or corroded connections increaseresistance, causing low current

resis-To illustrate this, refer to the balance graphics below

If you increase circuit resistance, the balance beam showsthat current flow will decrease (it is inversely proportional)

If you decrease the resistance, the balance beam showsthat current flow will increase

Units of Measurement

Values for electrical measurements use a series of

prefixes as shorthand to simplify reading of very large and

very small numbers The most common prefixes in

automotive work are milli, kilo, and mega

m – milli – 0.001

The milli prefix is used when measuring units smaller than

1 One milli is one thousandth of a unit For example, 0.5

Volts = 500 millivolts, or 500 mV Another example, 0.05

Amps = 50 milliamps, or 50mA

k – kilo – 1,000

The Kilo prefix is used when measuring units larger than

1,000 For example, 1,000 Ohms can also be stated as

1K Ohm, or 1KΩ

M – Mega – 1,000,000

The most common use for this is when measuring units

over 1,000,000 For example, 1,000,000 Ohms can also

be stated as 1M Ohm, or 1MΩ

Volts Basic Unit Units For Units For

Small Amounts Large Amounts

Symbol V mV kV

Pronounced Volt Milli-Volt Kilo-Volt

Multiplier 1 0.001 1,000

Current Basic Unit Units For Units For

Small Amounts Large Amounts

Symbol A mA kA

Pronounced Amp Milli-ampere Kilo-ampere

Multiplier 1 0.001 1,000

Resistance Basic Unit Units For Units For

Small Amounts Large Amounts

Symbol Ω – kΩ

Pronounced Ohm – Kilo-ohm

Multiplier 1 – 1,000

Notes:

Types of Circuits

Series Circuit

The simplest circuit is called a series circuit In this type of

circuit, current has only one path to flow That path is from

the source of the current, through a load or loads, and

back to the source In a series circuit, the current

(measured in Amps) is the same at any point in the circuit

In other words, the Amps measured in any two places in

the circuit will be equal

Characteristics of series circuits:

• Current is the same everywhere in the circuit Since

there is only one path for current flow, the same amount

of current must be flowing in all parts of the circuit

• The sum of the voltage drops in the circuit equal the

source voltage Voltage drops may vary from load to

load if the individual resistances vary

• The total circuit resistance is the sum of all the

individual resistances in the series circuit

In the second example, if the switch is open (or if a wire isbroken), no current can flow, and the light will not be illumi-nated

In the third example, there are multiple loads (two light).Any lack of continuity or open (i.e one bad light) in theseries circuit will cause both lights to not illuminate Abreak anywhere in the circuit causes an open circuit and

no current will flow

When a circuit is complete and current is flowing, voltage

drops occur across the loads Ideally, we want all voltage

drops to occur only at the loads, and we do not want any

voltage drops in the circuit and wiring itself In the real

world, there is always some loss or voltage drop in the

wiring, and we try to minimize these losses in order to

provide full voltage to the loads To illustrate this, look at

the first example

The circuit is complete and current is flowing Ideally, all of

the applied 12 Volts will drop across the light load (from

Point 1 to Point 2)

In the second example, the circuit is open, no current is

flowing and now there is no voltage drop across the load

(the light)

In the third example, there are two loads (two light), and

the circuit is complete If we assume that the resistance of

both lights is the same, then exactly half of the applied

voltage (6 Volts) drops across each light

Series Circuits and Ohm’s Law

Let’s apply Ohm’s Law to series circuits

In the example above, we have replaced the light with a 3Ohm resistor, and the battery voltage (source) is 12 Volts

Ohm’s Law can be used determine the current flow

in Amps in this circuit by dividing the source voltage

by the resistance:

I = E/R

I = 12/3 = 4 AmpsBecause there is only one load, we know the voltage dropacross the resistor is the full 12 Volts (source)

In the second example, we have added a second load tothe circuit Let’s determine the voltage drop across bothloads We will use a 2 Ohm resistor and a 4 Ohm resistor.Remember, the sum of the the voltage drops must equalthe source voltage, or 12 Volts

First, let’s find the circuit current.

Total resistance = R = 2 + 4 = 6 OhmsTotal current = I = E/R = 12/6 = 2 Amps

Now we can calculate the voltage drops over each resistor For the 2 Ohm resistor we use Ohm’s Law to calculate:

E = I x R = 2 x 2 = 4 Volts

And for the 4 Ohm resistor we use Ohm’s Law to calculate:

E = I x R = 2 x 4 = 8 VoltsThe total voltage drop is 4V + 8V which equals the sourcevoltage of 12 volts

Parallel Circuits

A Parallel circuit provides more than one path for current

to flow in a circuit In a Parallel circuit, all of the

component’s positive terminals are connected to one point

and all of the component’s negative terminals are

connected to a different common point

Characteristics of parallel circuits:

• The voltage applied to, or measured across, each

branch of the circuit is the same

• The total current in a parallel circuit is the sum of the

currents in each branch

• The total resistance of a parallel circuit is always less

than the value of the lowest individual resistance When

you add resistance in parallel, you are actually adding

more conductors (or paths) in which current can flow,

which reduces the total circuit resistance

Notes:

Calculating Total Resistance in a Parallel Circuit

There are two ways to calculate the total resistance in aparallel circuit Method 1 (sometimes called “the productover the sum”) can be used when the circuit has only twobranches Method 2 is used when there are more than twobranches in the circuit

Method 1 Rtotal = (R1 X R2)/(R1 + R2) Method 2 Rtotal = 1/(1/R1 + 1/R2 + 1/R3 + ) Refer to the parallel circuit shown on this page First, let’s find the total circuit resistance using Method 1.

Rtotal = (R1 X R2)/(R1 + R2)Rtotal = (2 x 3)/(2 + 3) = 6/5 = 1.2 OhmsNotice that the total resistance is less than the smallestresistor in the parallel circuit

Knowing that E = 12 Volts and Rt = 1.2 Ohms, we can use Ohm’s Law to calculate total current in the circuit.

I = E/R = 12/1.2 = 10 Amps

The total current flow in the circuit is 10 Amps Now let’s calculate how many Amps flow through each resistor:

Resistor 1: I = E/R1 = 12/2 = 6 Amps Resistor 2: I = E/R2 = 12/3 = 4 Amps

As you can see, the sum of the currents in the two branches equals the total current flow in the circuit Method 2 gives us the same answer for the total circuit resistance:

Rtotal = 1/(1/R1 + 1/R2 + 1/R3 + )Rtotal = 1/(1/2 + 1/3) =

1/(3/6 + 2/6) =1/(5/6) =6/5 = 1.2 Ohms

Series/Parallel Circuits

A Series-Parallel circuit contains a parallel circuit that is

also in series with another component or resistance

A headlight circuit would typically be a Series/Parallel

circuit The headlight switch is in series with the

headlights, and the headlights are in parallel with each

other Both lights are controlled by the switch, but one

lamp will still illuminate if the other is burned out

Series/Parallel Circuits and Ohm’s Law

Refer to the graphic below In this example we have added

a resistor in series with the parallel circuit discussed

previ-ously

To calculate total circuit current, we will first

determine the total circuit resistance Reduce the

parallel branches of this circuit to the equivalent

series resistance:

Rparallel = (R2 X R3)/(R2 + R3)

Rparallel = (6 x 3/6 + 3) = 2 Ohms

Then this equivalent resistance value of the parallel

resistors will be added to the value of the resistor in

E = I x R1 = 3 x 2 = 6 Volts

This results in a 6 Volt drop over the series resistor.

We also know that there are only 6 more Volts to drop across the parallel portion of the circuit Calcu- lating current for each parallel resistor individually:

I = E/R2 = 6/6 = 1 Amp

I = E/R3 = 6/3 = 2 Amps

The sum of the values for current in the parallel branchesequals the total current found at any point in the seriescircuit

Digital Volt-Ohm Meter (DVOM)

The ability to measure voltage, current, and resistance is

important when diagnosing electrical problems The

instru-ment most commonly used to make these electrical

measurements is called the Digital Volt-Ohm Meter

(DVOM) DVOMs have very high internal resistance, so they

are safe to use when measuring computer circuits

Old-style analog meters (meters with needles) do not have

high internal resistance and should not be used to

measure computer circuits

Basic DVOMs are capable of measuring the

We will deal specifically with the proper methods of

measuring Voltage, Resistance and Current

Voltage Testing

The voltmeter function of the DVOM is used to measurethe voltage potential at different points in a circuit You canalso think of this as using the DVOM to measuring thedifference in voltage between two points The DVOM isconnected in parallel (across the points being measured)when measuring voltage

The selector knob on the DVOM must be set to read either

AC volts or DC volts When testing automotive systems,you will almost always use the DC volts setting The blacklead is connected to the “COMMON” connection on theDVOM, and the red lead is connected to the “VOLT-OHM-DIODE” connection

The red probe is placed on the more positive test point inthe circuit, with the black lead on the more negative testpoint If the meter leads are reversed, which reverses thepolarity, a negative value will be displayed on the meter.The DVOM can be used to measure applied voltage orvoltage drop across a load, a connector or a portion ofthe circuit

On some DVOMs, the voltage scale will auto-range Onothers, you select the voltage scale or range Always usethe lowest possible scale for the greatest accuracy

!

!

A mA A COM V

1000V MAX 400mA MAX FUSED 10A MAX FUSED CAT II

Hz ms-PULSE ±TRIGGER SMOOTH

X

mA A mV V V OFF

88 AUTOMOTIVE METER

! 1000V MAXCA

MIN MAX ZERO RANGE HOLD H

RPM % DUTY ALERT

Hz ms-PULSE ±TRIGGER SMOOTH

X

mA A mV V V OFF

Positive Lead for current testing

(Amps, Milliamps, Microamps)

Amperage Testing

The ammeter function of the DVOM is used to measure the

flow of current in Amps When placed in series in a circuit,

all current in the circuit also passes through the meter

itself An ammeter is NEVER connected in parallel.

Always take initial measurements with the highest scale to

avoid blown meter fuses or damage to the meter After

determining the amount of current in the circuit, switch to

a lower scale for the most accurate readings

Always be sure that the meter leads are connected to the

proper terminals on the DVOM The black lead is

connected to the “COMMON” connection on the DVOM,

and the red lead is connected to the “AMPS” or

“MILLIAMPS” connection If the meter leads are reversed,

which reverses the polarity, a negative value will be

displayed on the meter Typically, ammeters are rated for

no more than 10 Amps Current flow above 10 Amps will

blow the internal fuse of the meter, which must then be

replaced

Note:

Always replace blown fuses with the correct “fast blow”

type Using an incorrect type of fuse can be dangerous

Inductive Clamp

Another method of measuring current is with an inductivecurrent clamp Unlike the previous "in series" measurementtechnique, this device is clamped around the wire andmeasures current flow by sensing the magnetic fieldaround the wire with Hall-effect technology The circuitdoes not have to be broken or disturbed in any way

An inductive AC/DC current clamp connects to the Voltsinputs of the DVOM An internal battery powers the clamp.Select the Vdc or mVdc scale on the DVOM to measure

DC current, or select Vac to measure AC current

A 1 mV reading on the meter is equal to an actual 1 Ampcurrent measurement

!

!

A mA A COM V

1000V MAX 400mA MAX FUSED 10A MAX FUSED CAT II

88 AUTOMOTIVE METER

!

T II

- Always measure current in series

- Be sure meter leads are in the proper

- Set multimeter selector switch to the

with a circuit.

location on the multimeter.

Amps (A) position.

12 v

MIN MAX ZERO RANGE HOLD H RPM % DUTY ALERT

Hz ms-PULSE ±TRIGGER SMOOTH

X

mA A mV V V OFF

A

X

Resistance Testing

The ohmmeter function of the DVOM is used to measure

component or circuit resistance or continuity An

ohm-meter is always used in an open or unpowered circuit

Never use an ohmmeter in a live or powered circuit.

To measure resistance, the ohmmeter uses a battery topass a small amount of current through the circuit beingtested The meter leads are placed across, or in parallel,

to the component or circuit to be measured Disconnect atleast one side of the circuit to avoid false readings fromcircuit power or other circuit components

The black lead is connected to the “COMMON” connection

on the DVOM, and the red lead is connected to the OHM-DIODE” connection When testing resistance, polarity

“VOLT-is usually not important

Place the DVOM selector in the Ohms position Use thelowest scale for greatest accuracy On some DVOMs, theOhms scale will auto-range On others, you select theOhms scale or range

!

!

A mA A COM V

1000V MAX 400mA MAX FUSED 10A MAX FUSED CAT II

88 AUTOMOTIVE METER

!

- Measure resistance with no

- Be sure multimeter selector switch is in

power applied to the circuit (switch open).

the Ohms ( ) position.

12 v

10 resistor

MIN MAX ZERO RANGE HOLD H RPM % DUTY ALERT

Hz ms-PULSE ±TRIGGER SMOOTH

X

mA A mV V V OFF

A

X

Notes:

Notes:

Subject Page

Introduction 2

Switches 2

Relays 3

Resistors 4

Lights 5

Capacitors 5

Inductive Sensors 6

Temperature Sensors 7

Photo Diode 7

Potentiometers 8

Hall-Effect Sensors 8

Active Wheel Speed Sensors 9

Solenoids 10

Electric Motors 10

Semiconductor 11

Diodes 12

Zener Diodes 13

Transistors 14

Sensors are devices that output a voltage signal that

changes with changing physical conditions Sensors can

be designed to respond to changes in temperature,

pressure, light, motion, etc Usually, sensors send their

signals to a control unit, which uses this input information

to make control decisions The coolant temperature

sensor and camshaft position sensor are examples of

sensors

Actuators are devices that receive output signals from

control units in order to do work or modify a condition

Fuel injectors and e-throttle bodies are examples of

Switches are mechanical devices used to start, stop or

redirect current flow A switch may be used to directly

control a load, or to control a relay that in turn operates a

higher current device A switch may be installed on the

power side (positive, feed or insulated side), or on the

ground side (negative side) of the circuit

Switches can be normally open (NO) or normally closed

(NC) There are many different types of switches including

momentary, multiple-position, pressure activated,

tempera-ture activated, and mechanically activated

Typical switches used include the clutch pedal positionswitch, door close switch, brake pedal switch, and batterycutoff switch

The 3-way refrigerant pressure switch has two functions,one switch controls the compressor by shutting off thecompressor when the refrigerant pressure drops below, orrises above safe levels for compressor operation

A - Medium pressure switch

B - High/low pressure switch

a - To DME control unit to control the condensor fan

b - To compressor relay

Mechanically Operated Switch

Normally open

switch Normally closedswitch

A relay is an electromagnetic switch that uses a small

current to control or switch a large current A small control

current through a coil (the electromagnet) moves an

armature against spring tension to open or close

load-carrying contact points When the control current is

inter-rupted, the relay returns to its rest state (unless it is a

latching relay)

Relay configurations may vary (number of pins, normally

open or closed state, rated current), and relays should not

be substituted except with an exact replacement

Common relay pin configurations include SPST (Single

Pole Single Throw), and SPDT (Single Pole Double Throw)

Typical relay pin identification:

• Pin 30 (switch or load side battery power)

• Pin 85 (solenoid or control side output)

• Pin 86 (solenoid or control side input)

• Pin 87 (switch or load side output, normally open)

• Pin 87a (switch or load side output, normally closed)

Relay Failures

If the control side of a relay fails, the relay will not operate

On the load-side, contacts can weld together, or the loadcircuit can open

Dual Five Pin Relays For Controlling Motor Rotation

When we need to control the direction of rotation of anelectric motor, we usually employ two five-pin relays in theconfiguration below This is used to control cabriolet tops,electric windows (when we have digital control), door lockmotors, or whenever we need to control an electricmotors direction of rotation When we energize one relaythe motor spins clockwise, and when we energize theother, the motor spins counterclockwise

Notes:

Resistors limit current flow in a circuit Resistors are

available in a fixed resistance value, or variable resistance

Fixed value resistors are color coded to indicate the

resis-tance value

Stepped Resistors

A stepped or tapped resistor has two or more fixed taps

that provide different resistance values These taps allow

current to flow through all or part of the resistor, which

changes the amount of current flowing through the circuit

Stepped resistors can also be encased in ceramic and are

nothing more than a series of fixed resistors placed end to

end

An example of a stepped resistor in operation is the

blower motor circuit shown here The blower resistor is in

series with the blower motor Adding resistance in series

with the motor will lower the current flow Higher blower

circuit resistance will result in lower blower speeds

In low speed 1, current flows

through the entire resistor from

(2) though (4) before powering

the motor The motor will run at

low speed because less current

is flowing Moving the blower

switch to medium speed 2

removes one of the resistors

(shown as 17 ohm) from the

circuit The motor will now run

faster compared to low speed

because more current is flowing

through the circuit

Fixed Resistors

Fixed resistors are often used in voltage divider circuits.One example is the computer sensor circuit shown below.Note that two resistors, R1 and R2, are placed in series Afixed pull-up resistor R1 is used to create a voltage drop

As the resistance of coolant temperature sensor R2changes, the voltage drop across R2 also changes Amonitor circuit inside the computer measures this voltagedrop between the two resistors

Resistor

Typically, blowers are controlled with stepped resistors on vehicles with manual A/C.

Variable Resistors

Variable resistors change their resistance value with

changes in some physical condition

There are three main types of variable resistors:

• Thermistors - vary resistance with changes in

tempera-ture

• Photo resistors - vary resistance with changes in light

• Potentiometers - vary resistance with changes in

position

Variable resistors are usually used as sensors which

provide information to control units Variable resistors will

be discussed in detail later in this section

Lights

Lights provide illumination or signal functions Typical

lights are incandescent, halogen, Xenon or Light Emitting

Diodes (LEDs) LEDs will be discussed in detail in the

Semiconductors and Control Units section.

Headlights: halogen, Litronic or Xenon

Lights used for Boxster taillight assembly.

is measured in units called the Farad (F)

At high frequencies a capacitor acts like a short circuit,making it ideal for use as a voltage spike protector Anexample of this is the generator noise suppressor, whichlimits radio interference

Notes:

Analog Sensors

Inductive Sensors

Inductive sensors have a coil winding around a permanent

magnet core The permanent magnet creates a magnetic

field, and as impulse wheel teeth move past the sensor,

the strength of the magnetic field increases and

decreases Magnetic lines of force to cut through the

turns of wire in the coil winding, inducing a voltage The

voltage signal produced is an alternating current (AC) sine

wave

How fast the impulse wheel teeth move past the sensor

core affects the signal voltage The amplitude (height) of

the signal increases with increasing speed The signal

frequency (how many cycles or waves per unit of time)

also increases with increasing speed In most analog

circuits, a reference signal (usually 5-Volts) is provided,

and the control unit then compares the sensor signal to

the reference voltage

Typical applications for inductive sensors include:

• Crankshaft Position Sensor

• Camshaft Position Sensor

• Transmission Input/Output Speed Sensor

• Wheel Speed Sensor

Porsche uses various pulse generators, such as this wheel sensor.

Inductive crankshaft sensor.

Typical analog signal from inductive sensor.

Notes:

Temperature Sensors

Temperature sensors (thermistors) change resistance with

changes in temperature The sensors are analog, and

sensor resistance varies continuously as temperature

changes The resistance of Negative Temperature

Coeffi-cient (NTC) sensors decreases as the temperature

increases This decrease in resistance causes the voltage

drop across the sensor to decrease and the input signal

voltage at the control unit also decreases

Typical applications for NTC Type sensors:

• Engine Coolant Temperature Sensor

• Transmission Temperature Sensor

• Intake Air Temperature Sensor

The signals from Positive Temperature Coefficient (PTC) sensors

are also analog, but increase as the temperature increases.

Photo Sensitive Diode

Porsche uses photo diodes to sense the amount ofsunlight entering the passenger compartment Photodiodes generate voltage when exposed to light Thestronger the light the stronger the voltage

Notes:

Temperature Dependent Resistor

Potentiometers are three-wire variable resistors that

output an analog voltage signal with changes in the sensor

wiper position One terminal is the supply voltage

(reference voltage) This is typically a 5 Volt regulated

supply The second terminal is ground, and the third is the

input signal to the control unit The signal voltage will vary

continuously as the wiper moves across the fixed resistor

Potentiometers can be used to measure mechanical

movement, such as in the throttle position sensor or the

accelerator pedal position sensor

Typical applications for potentiometer sensors:

• Throttle Position Sensor

• Accelerator Pedal Position Sensor

Digital Sensors

Hall-Effect Sensors

Like an inductive sensor, a Hall-effect sensor outputs asignal in proportion to movement of shutter wheel teethpast the sensor A Hall-effect sensor receives an inputvoltage to a semiconductor chip, and has a permanentmagnet and a metal trigger wheel with teeth, notches orholes

The advantage to a Hall-effect sensor is that it produces awell-defined digital square wave “ON-OFF” signal Also, thesignal strength does not depend on trigger wheel speed AHall-effect sensor will provide a reliable signal even at verylow rotational speeds

Typical Hall-Effect sensor and signal

Å S Å

V

G R

5v 5v

1v

2v 3v 4v

+

Hall-Effect Sensors (cont’d)

The Hall effect semiconductor chip receives the input

voltage and outputs a digital signal as the magnetic field

from the permanent magnet alternately passes through,

and is blocked from the semiconductor chip

Typical applications for Hall-effect sensors:

• Crankshaft Position Sensors

• Camshaft Position Sensors

• Wheel Speed Sensors

Active Wheel Speed Sensors

The wheel speed sensors on the 911 Carrera (997) and

Cayenne are described as “active speed sensors.” Active

speed sensors require an external power supply A

two-wire cable provides the connection to the control unit The

active speed sensor is supplied with power and ground by

the PSM control unit

The change in resistance is converted into a digital speedsignal by an electronic circuit in the sensor element and

transmitted to the PSM control unit Following the

instal-lation/replacement of the PSM multiple sensor, the integral sensors for linear and lateral acceleration must be calibrated.

Caution!

• Do not measure the resistance of the active speedsensor This will destroy the sensor

• Observe installation orientation of the wheel bearing

• Do not expose wheel bearing to a strong magnetic field

Notes:

Solenoids

Arrows indicate direction of solenoid plunger movement

A solenoid uses an electromagnet to produce mechanical

movement Solenoids can be used to physically move a

component Solenoids can also combine electrical

switching functions with the mechanical movement

function

A solenoid consist of a coil winding around a metal plunger

with a return spring When current flows through the

solenoid winding, a magnetic field attracts the plunger,

moving it against spring tension toward the center of the

coil When current flow stops, the magnetic field collapses

and the spring returns the plunger to the rest position

Solenoids are commonly used on starter motors, fuel

injectors, oil control solenoids and purge valves

Electric Motors

Electric motors change electrical energy to motion As wehave said, a basic electric motor consists of a current-carrying conductor loop within a stationary magnetic field.When the windings in the motor armature carry current, itsmagnetic field interacts with the stationary magnetic fieldand the motor armature turns

A rear wiper motor on the Cayenne is shown.

The commutator and brushes keep current flowing in theright direction in the armature loop The stationarymagnetic field may be created with permanent magnets orthe fields may be electromagnets with windings Windowmotors and blower motors are examples of electricmotors

Motor symbol in a wiring diagram.

Motor M

As we discussed earlier in this course, the number of

electrons in the valence (outer) ring of atoms determines

whether a material is a conductor or an insulator Materials

with one to three electrons in the outer ring readily accept

free electrons, making it easy for electrons to move from

atom to atom The electrons in the outer ring are loosely

held, and even a small potential difference (voltage) can

make free electrons flow These materials are conductors

Most metals are good conductors (copper, silver, gold,

aluminum)

Atoms with five to eight electrons in the outer ring have

valence electrons bound tightly to the atom These

materials are insulators These atoms do not easily accept

free electrons Insulators tend to stop the flow of free

electrons Rubber, glass, and many plastics are good

insu-lators

Atoms with exactly four electrons in the outer ring are not

conductor, nor are they insulators These materials can be

classified as semiconductors Silicon and germanium are

examples of semiconductor materials with four valence

electrons The four valence electrons in these materials

give them special electrical properties which can be very

useful in making electrical circuits and components

Semiconductor Doping

Pure silicon atoms with four electrons in the outer ring

tend to form crystalline structures The four electrons in

the outer ring are shared with neighboring atoms This

makes the crystal form of these materials an excellent

insulator because there are no free electrons to move

Crystalline silicon is an excellent insulator.

When making semiconductor circuits, impurities are added

to the silicon atoms This is called doping Doping eitheradds free electrons or creates holes (missing electrons),depending on the impurity that is added These impuritiesallow the semiconductor to carry current

N-Type Material

If the semiconductor is doped with substances such asphosphorous or antimony, N-Type material with excessvalence electrons is formed N-Type semiconductors haveextra electrons that move easily in the crystal structure N-Type materials attract positive charges

N-Material has an extra, or free electron.

P-Type Material

If the semiconductor is doped with substances such asboron or indium, P-Type material with missing valenceelectrons, called holes, is formed P-Type semiconductormaterials attract negative charges (free electrons)

Useful semiconductor components such as diodes andtransistors can be made when layers of P-Type and N-Typematerial are combined

P-Material has a hole in some of it’s valence rings.

A diode allows current to flow in only one direction and

stops current flow in the opposite direction Diodes are

used to rectify or change AC (alternating current) to DC

(direct current) Diodes are also used to block current flow

in one direction in a circuit

A diode is made by combining a layer of N-Type

semicon-ductor material with a layer of P-Type semiconsemicon-ductor

material The line along which P-Type and N-Type material

meet is called the junction

Simplest semiconductor device is a diode It is formed by joining

chips of P and N materials.

A diode is forward-biased and allows current flow when the

diode’s N-Type layer is connected to the negative side of

the circuit, and the P-Type layer is connected to the

positive side of the circuit When connected like this, extra

electrons from the circuit are provided to the N-Type layer

These extra electrons are attracted to the more positive

P-Type layer in the diode and current flows

Current can flow in one direction.

Diode in forward direction.

A diode is reverse-biased when the N-Type layer isconnected to the positive side of the circuit, and the P-Type layer is connected to the negative side of the circuit.When connected like this, free electrons from the N-Typelayer are attracted to the positive side of the circuit and donot flow through the diode No current flows and the diodeblocks current flow

Diodes are rated for specific voltage and current

Exceeding the ratings can damage the diode

Diode in reverse direction.

Zener Diodes

A Zener diode is a special type of diode that works like a

pressure relief valve Below a preset breakdown voltage,

the zener diode conducts and blocks current like any

diode But when the reverse-bias voltage exceeds a

threshold or breakdown voltage value, the zener diode will

conduct in the reverse direction

Zener diodes are commonly used in charging system

voltage regulator circuits They are also used in inductive

circuits to reduce voltage spikes

Light Emitting Diodes (LED)

LEDs emit visible light when forward biased As current

flows through the diode, electrical energy is converted to

light that is radiated through the thin positive material layer

in the diode

Photodiode

Diodes designed specifically to detect light are called

photodiodes They include a glass or plastic window

through which light enters the diode

Notes:

NPN Transistor

A transistor is a three-element semiconductor with three

layers of semiconductor material Transistors are

sometimes described as two diodes connected

The emitter and collector are the outer layers, with the

base layer in the middle Transistors are either NPN or

PNP-Types, with alternating layers of N-Type and P-Type

material If the emitter and collector are N-Type material,

then the base is P-Type material, and vice versa

Transistors are used to control current flow and act as a

relay, or act as an amplifier to vary the current output

depending on base voltage variations A transistor can

also switch or control a large current with a small signal

current

Transistor Operation

The transistor load circuit is through the emitter-collector.The emitter-collector circuit is controlled by a smallemitter-base circuit current, and it works just like a relay.When the emitter-base circuit is forward-biased with asmall current, this allows a larger current to flow throughthe emitter-collector circuit

When the emitter-base circuit is reverse-biased or notpowered, then the emitter-collector circuit is open and nocurrent flows

Small input current yields high output current.

Current is blocked.

General Information

The battery, starter, generator system is the heart of the

vehicle electrical system The Starter turns over the

engine allowing it to start The battery stores electrical

energy to operate the starter and provide power to vehicle

systems when the engine is not running and the generator

is not producing power And the generator provides power

to vehicle systems when the engine is running and charges

the battery

These components are a balanced system the generator

must produce enough energy to charge the battery and

operate vehicle systems at the same time The battery

must be able to supply enough energy to operate the

starter and allow vehicle systems to function when the

engine is not running A battery with plenty of storage

capacity is desirable so we have more capacity than

required to start the engine in case of unforeseen

circum-stances But a batteries weight, cost and size grow with

capacity

The starter must be able to provide enough torque to spin

the engine fast enough to start but not be too heavy or

expensive The components of the battery, starter,

generator system must have the lowest weight possible

and with a reasonable cost The battery, starter, generator

system must be designed to operate together the battery

must produce sufficient amperage for the starter and the

correct internal resistance and capacitance for the

generator to operate correctly

Understanding the relationship between the system

components and the restrictions imposed by design

requirements is useful for diagnostics We need to be able

to test the components of the charging starting system,

understanding the system dynamic will help us correctly

diagnose the battery, starter, generator system

For example the order of testing is essential the firstcomponent we need to test is the battery if the battery isdefective none of our other measurements will be valid.Next we can test the starter and finally the generator.Then we perform voltage drop test on the cables andconnections When we have completed this test sequence

we can make a definitive diagnosis of any defectivecomponents in the battery, starter, generator system

Battery

Battery Testing Inspection:

The first battery test is a through visual inspection of thebattery for damage to the case or terminals And avoltage drop test across the terminal to determine ifcorrosion is causing a resistive connection from thebattery post to clamp

Open circuit voltage test:

If when the prerequisite conditions are met the open circuitvoltage is 12.5 Volts or higher the battery is unlikely tohave a problem

Battery load test:

This is a generic test utilizing a tester (for example SUNVAT 40) If this test and the open circuit Voltage are passed

a battery is in good condition Load battery to 3 times theAmpere Hour rating for 15 seconds if battery can maintainthe amps draw and voltage remains above 10.5 volts itpasses and starter testing can be performed

Valve-Regulated Lead–Acid Battery (VRLA)

Porsche vehicles after 2010 with Start/Stop have VRLA

(valve regulated lead acid), AMG (absorbed glass matt)

batteries These batteries require battery chargers

designed for use with AGM batteries They must also have

the battery data entered into the gateway when replaced

General Information

A VRLA battery (valve-regulated lead–acid battery) more

commonly known as a sealed battery is a lead–acid

rechargeable battery Because of their construction, VRLA

batteries do not require regular addition of water to the

cells, and vent less gas than flooded lead-acid batteries

The reduced venting is an advantage since they can be

used in confined or badly ventilated spaces But sealing

cells and preventing access to the electrolyte also has

several considerable disadvantages as discussed below

An absorbed glass mat battery has the electrolyte

absorbed in a fiber-glass mat separator While these

batteries are often colloquially called sealed lead–acid

batteries, they always include a safety pressure relief

valve

As opposed to vented (also called flooded) batteries, aVRLA cannot spill its electrolyte if it is turned upside down.Because AGM VRLA batteries use much less electrolyte(battery acid) than traditional lead–acid batteries, they arealso occasionally referred to as an "acid-starved" design

The name "valve regulated" does not wholly describe thetechnology; these are really "recombinant" batteries, whichmeans that the oxygen evolved at the positive plates willlargely recombine with the hydrogen ready to evolve onthe negative plates, creating water and so preventingwater loss The valve is a safety feature in case the rate ofhydrogen evolution becomes dangerously high In floodedcells, the gases escape before they have a chance torecombine, so water must be periodically added

Construction

These batteries have a pressure relief valve which willactivate when the battery is recharged at high voltage,typically greater than 2.30 volts per cell Valve activationallows some of the gas or electrolyte to escape, thusdecreasing the overall capacity of the battery Rectangularcells may have valves set to operate as low as 1 or 2 psi

At high overcharge currents, electrolysis of water occurs,expelling hydrogen and oxygen gas through the battery'svalves Care must be taken to prevent short circuits andrapid charging Constant-voltage charging is the usual,most efficient and fastest charging method for VRLAbatteries, VRLA batteries may be continually "float"

charged at around 2.35 volts per cell at 77° F (25° C.).Sustained charging at 2.7 V per cell will damage the cells.Constant-current overcharging at high rates (rates fasterthan restoring the rated capacity in three hours) willexceed the capacity of the cell to recombine hydrogen andoxygen

Notes:

Starter Testing

A defective starter will usually not operate at all or

intermittently not operate or have reduced speed when

cranking For the first case a problem in the control circuit

is indicated In the second case a starter draw test is

needed

Starter control circuit testing:

Connect a voltmeter negative to the 50 circuit terminal of

the starter Connect the voltmeter positive to the battery

positive terminal Activate the starter and read voltage the

value should be no more than 800 millivolts if higher there

is a defect in the starter activation circuit If there is no

voltage drop an open circuit or control system problem is

indicated

Starter draw test:

Connect an inductive ammeter with a range of at least

250 amps on the positive or negative battery cable,

prevent the engine from starting (best is to remove the

fuel pump fuse) and operate the starter Read the amps

draw Allowable draw is approximately 15% to 20% lower

than the amps load applied to the battery (three times the

ampere hour rating) and cranking speed should be normal

If the draw is low and cranking speed is low, voltage drop

testing of the cables, solenoid and connections for

excessive resistance are indicated

If there is no problem with the cables or connections, a

resistive solenoid switch is indicated To get specifications

for normal voltage drops, test a known good vehicle If the

draw is high and cranking speed is low, excessive

mechan-ical resistance in the starter or engine is indicated For

example the starter bushings could be worn or the engine

could have carbon build up in the combustion chambers

raising the compression of the engine

Voltage drop test of the starter cables:

The large cables from the battery to the starter on thepositive side and battery to body and body to engine onthe negative side supply the high amperage for starteroperation They are tested by measuring their voltagedrop Correct connection of the volt meter is shown in thefollowing diagrams

When we measure the voltage drop of a cable we see itsdynamic resistance (its resistance when amperage isactually flowing) this could not be done by using anohmmeter

Voltage drop testing is a versatile and powerful diagnostictool when we measure a voltage drop we see the

resistance of the circuit or component we measure thevoltage drop of When the electrical engineer designs thecircuit he must select a conductor cross section largeenough to carry the amperage that flows in the circuit.This keeps the voltage drop on any circuit leg under 200

mV only high amperage circuits (for example alternator orstarter) have higher voltage drops To measure a voltagedrop the voltmeter must be connected in parallel to the

component being measured and the circuit must have

current flowing in it That is it must be a “live” circuit in

other words we test the circuit when it is operating Some

problems are only seen when the circuit is operational

resistive connections and conductors have a PTC

tempera-ture behavior

That is their resistance increases when their temperature

rises and current flow through the resistive components

causes them to heat up We could use an Ohmmeter to

measure the resistance of the circuit or connection but we

would not see the PTC effect We would also need to have

a different specification for every circuit depending on the

cross section and length of the conductor connections of

course have no allowable voltage drop

A voltage drop check is quick and easy the specificationsfor voltage drops are for the most part the same 200 mV(.2 Volts) and the outcome of the test is conclusive that is

if you see a voltage drop there definitely is resistance

The three requirements for a voltage drop test are:

1 You must use a voltmeter

2 It must be connected in parallel to the component beingtested

3 The circuit must be live (current must be flowing

Wiring from light switch K1.30 to lights < 15W

Wiring from light switch K1.30 to lights > 15W

Wiring from light switch K1.30 to main beam

Charging wire from 3-phase generator K1.B+ to battery

Main starter cable

Starter control wire from start switch to starter K1.50

latching relay with pull-on and hold winding

Other control wiring from switch to relay, horn, etc.

0.1 V 0.5 V 0.3 V 0.4 V 0.5 V 1.4 V 1.5 V 0.5 V

0.6V 0.9V 0.6V

1.7V 1.9V 1.5V

Maximum Voltage Drop in Pos Circuit

Notes:

Starter Operation

When the starter 50 terminal is energized and

current flows in the solenoid windings a

magnetic field builds up This magnetic field

pulls the solenoid plunger against the return

spring (3) moving the pinion engaging lever and

pulling the starter pinion into engagement with

the engine ring gear The solenoid plunger

bot-toms against the high current switch (1) closing

the circuit from the battery to the starter motor

The starter motor generates torque turning the

engine initiating engine start

When current flows through the motor a voltage

drop is generated that changes the potential at

the bottom of the pull in solenoid winding (a)

from ground to system voltage and current flow

ceases in the pull in winding Current continues

to flow in the hold in winding (b) and the

mag-netic field generated by the hold in winding is

sufficient to hold the solenoid plunger in

pos-ition until the 50 circuit is shut down at end of

starter operation