LV33 electrical and electronic systems (2)

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Student Workbook

LV33 Electrical and Electronic

Systems (2)

kap all phase 2 & 3 6/11/03 11:38 am Page 43

Student Workbook for Technical Certificates in

Light Vehicle Maintenance and Repair

MODULE LV33 ELECTRICAL AND ELECTRONIC

Voltage: 3 Accessing the terminals 19

After the resistance but before the Removing the starter motor from the

Fault Finding: 9 Progress check 3 26

Checking the Wiring Loom 14 Good Practice: 31

Checking the Alternator: 14

Introduction

Diagnosis is a significant part of a vehicle technician’s responsibility With the proliferation of electrical systems on a modern vehicle, an ability to diagnose electrical faults quickly and accurately has become most valued

General Fault Finding Procedures

To be able to fault diagnose electrical circuits accurately and quickly, an

understanding of voltage is critical An understanding of voltage coupled with the correct use of a voltmeter will enable a technician to find virtually any fault

Voltage

A few voltage rules must be understood and remembered:

• voltage is electrical pressure – it ‘pushes’ current around a circuit

• voltage will drop across a resistor

• volts drop will only occur if current is flowing

The amount the voltage drops by across a resistor in a circuit is dictated by the comparative value of that resistor i.e its value when compared to that of any other resistors in the circuit The bigger the value of a resistor, the

greater the amount of the available voltage will be used by it, when compared

to the other resistors in the circuit

The voltage after the last resistor in a circuit will be 0 volts so long as current can flow

If there is only one resistor in a circuit it is also the last resistor and therefore the voltage after it will be zero so long as current is flowing

A voltmeter displays the difference in voltage between the positions of the red and black probes

Exercise 1

A simple earth switched lamp circuit Consider this circuit as serviceable and

in the condition pictured (switch closed or on) Mark on the voltages at all the key points

Now consider an open circuit fault in connector B (poor terminal contact)

Mark on the voltages at all the key points

Now consider an open circuit fault in connector F (poor terminal contact)

Mark on the voltages at all the key points

Now consider a high resistance fault in connector D (poor terminal contact)

Mark on the voltages at all the key points

Now consider a high resistance fault in connector F (poor terminal contact)

Mark on the voltages at all the key points

Short circuits

Short faults can occur in three areas on a circuit and each area will have a

different effect on that circuit

Before the resistance

This will result in a blown fuse, popped circuit breaker or a serious fire!

Because the short to ground has reduced the circuit resistance to virtually

nothing, there is no limiting factor to the amount of current that the battery will produce (except the battery’s amp hour rating) This will result in a serious

over-current situation in the affected circuit

After the resistance but before the switch (earth switched circuit)

This means that the driver cannot switch off the affected circuit The

resistance has its usual permanent supply but it now has a permanent earth courtesy of the fault If the circuit is one where the consumer (lamp for

example) is not normally visible to the driver – such as a luggage

compartment light – the driver will complain that the battery keeps going flat This type of short circuit is known as a ‘parasitic drain’ fault as the affected

circuit is draining the battery (like a parasite)

After the resistance and after the switch (earth switched circuit)

This is probably the best kind of short circuit fault as it isn’t actually a fault! In fact, it can be considered something of a bonus because if the proper earth for the circuit should develop a problem (such as a resistance build up at the

earthing bolt) we have our ‘fault’ to fall back on!

Exercise 2

1 Draw these three differing short circuit faults on the circuits below:

Progress check 1

Answer the following questions:

1 With an earth switched circuit, if the switch is open what should the voltage

be just after the resistance?

2 With an earth switched circuit, if the switch is closed what should the

voltage be after the last resistance?

Fault Finding

Battery - charging systems

When faced with a flat battery the key to successful diagnosis is to ascertain

why the battery has gone flat Seems like an obvious thing to say, but ‘why’ is

an often forgotten word A general rule that is well worth applying with

diagnosis of any fault (electrical or otherwise) is this:

Do not only cure the fault, cure the cause of the fault

If this approach is adopted, you will have very few vehicles being returned to you with the same fault that you claimed you had cured!

So why has the battery gone flat? Is it because the vehicle has been parked

up for four months while the owner has been abroad avoiding the cold

weather? If so, the cure would be to charge the battery Simple

Has the battery’s performance degenerated over time? If so, the chances are the battery has succumbed to general wear and tear and is no longer able to receive a charge successfully (plate sulphation or an overly dilute electrolyte

is normally the reason)

Has the battery gone flat overnight having performed brilliantly the day

before? If so, the driver may have left something switched on all night, such

as an interior light If not, you could be looking for a parasitic drain fault (see page 11)

1 litre of pure water weighs 1 Kg

Therefore, if 1 litre of a given fluid weighs 2 Kg then it is twice as dense and therefore has a specific gravity of 2 (no units)

Battery electrolyte is dilute sulphuric acid It is therefore a mix of sulphuric

acid and distilled water Concentrated sulphuric acid is considerably more

dense than water and therefore its presence increases the density (and S.G.)

of the electrolyte A fully charged battery in good order will have a S.G of

1.27 to 1.28 As a battery discharges more water is produced Therefore, the S.G will reduce as the batteries charge state reduces As the hydrometer is capable of drawing electrolyte up from each individual cell and allowing us to read the S.G., it is most suited to the inspection of a battery It also enables

us to view the electrolyte If it appears brown (muddy) then the plates are

sulphated and the battery should be replaced

Parasitic drain

To confirm that parasitic drain is the reason for the flat battery an ammeter

should be used Fit the ammeter in series with the battery negative post and the battery negative cable

Parasitic battery drain

The ammeter will now tell you exactly how much current the vehicle is drawing from the battery With most modern vehicles this should be no more than

0.5A It is vital that all systems that should be switched off are switched off

during this test Do not forget that if you are working with the driver’s door

open, the interior light could well be on! Ensure that the keys are removed

from the ignition – if you do not, the immobiliser will be drawing considerable current

Occasionally the act of disconnecting the battery will temporarily cure the

parasitic drain fault – especially if it is an ECU that is pulling current This will make the fault nearly impossible to find To this end, ensure that the battery connection is never lost But how? Connect a piece of wire to the negative cable and then wrap it around the base of the battery negative post As you lift the negative cable clear this wire will maintain the link Now connect the ammeter and then remove the wire At no point has the connection been

broken (remember, the ammeter is a conductor)

If the current draw exceeds 0.5A (check with manufacturers’ specifications if available) you could well have a parasitic circuit on the vehicle

Finding the problem circuit

Start pulling fuses whilst observing the ammeter The fuse that you pull that drops current flow to an acceptable level is protecting your faulty circuit But

be careful! Some of the fuses that you pull will be protecting circuits that pull current legitimately, such as the engine ECU (memory function) and the dash mounted clock Refer to the wiring diagram

Charging system faults

Use a voltmeter to confirm that the battery is going flat because the alternator

is not producing a charge Once you have confirmed this, you now have to

discover why it is not charging

11.5v

A typical IC regulated charging circuit (see Phase 1 Electrical and Electronic Systems LV19 for details on its operation)

With such a circuit, the easiest thing to do is consider it in two parts – the IC regulator and the rest With a Japanese manufactured alternator the easiest way to ascertain whether the fault lies in the regulator or elsewhere is to short the ‘F terminal’ to ground Looking at the diagram above, it can be seen that the F terminal is connected directly to the earth path of the rotor coil

Normally the IC regulator controls this earth path, but when you short the F

terminal to ground the IC regulator is effectively bypassed If the alternator

Checking the Wiring Loom

The IG terminal of the alternator should show battery voltage with the ignition switch turned on, the S terminal should show battery voltage continually, the L terminal should show 0 volts with the ignition turned on and the charge

warning light illuminated and the B terminal (the heavy duty cable) should

have no resistance (0 ohms) This cable has to carry a large amount of

current so even a small amount of resistance will cause significant volts drop (see Ohm’s Law)

Checking the Alternator

An oscilloscope can give some indication of problem areas within an

alternator The diagram above illustrates the typical charge patterns and the indicated problem

The diagram shows tests to be carried out in order to confirm the nature of the problem

Short circuits in the rectifier pack can be confirmed by connecting an

ohmmeter in the way shown in the previous two diagrams Connect the

positive lead of the ohmmeter to the N (Neutral) terminal and the negative

lead to the E (Earth) terminal These two terminals and the circuitry to which they are connected should be entirely separate due to the action of the

blocking diodes Therefore the ohmmeter should read infinite resistance

(open circuit)

Note: Some ohmmeters display ‘O/L’; this means ‘Off Limits’ (more resistance than the meter is capable of displaying) Any other reading confirms a faulty rectifier diode(s)

Measure the resistance of the coil, slip rings and brushes by connecting the ohmmeter across terminals ‘F’ (Field) and ‘E’ (Earth) Typically this value

should be less than 10 ohms Ensure that this figure does not fluctuate too wildly when you rotate the rotor If it does, the slip rings and brushes are

probably faulty (you can expect some fluctuation – it is a dynamic contact and far from perfect)

If you strip the alternator, you can also carry out the tests shown which are

Progress check 2

Answer the following questions:

1 What should the S.G of a fully charged battery be?

2 When measuring parasitic drain, where should the ammeter be fitted?

3 Between which two terminals on an alternator should an ohmmeter be

fitted to test the serviceability of the rotor, slip rings and brushes?

Starting Systems – The Circuit

Exercise 3

Minimum acceptable cranking voltages

A typical relay operated starter circuit

Write onto the circuit the minimum acceptable cranking voltages

The reason that such a significant voltage loss can be expected is due to the large amount of current that is flowing in the heavy-duty side of the circuit

The cables are of course conductors but all conductors have some resistance and with such a large amount of current flowing even a small amount of

resistance will cause a significant voltage drop (see Ohm’s Law)

Exercise 4

Engine does not crank – automatic gearbox

0v

Term 50 0v Term 50

3v 12v

12v

3v

Study the voltmeter readings on the circuit

Mark onto the circuit the area where the problem lies

Circuit testing

The tests being performed on the circuit suggest that access must be gained

to the relay terminals On a vehicle this is very difficult to do because the

terminals are buried in the relay block to which the relay is fitted Pulling the relay out in order to access these terminals is not the answer as you are

physically breaking the circuit when you do this Breaking the circuit prevents current from flowing, and volts drop will not occur Therefore the reading at the relay coil terminal would have been battery voltage (no volts drop shown across the high resistance fault in the neutral start switch) This could easily lead you to believe that there is nothing wrong with the circuit up to this point The golden rule:

NEVER DISCONNECT A CIRCUIT WHEN MEASURING VOLTAGES

Accessing the terminals

Pull the relay out and remake the circuit using a test light The test light has been fitted across the terminals of the relay block that the coil of the relay

would normally occupy The test light does not get in the way like the relay does so you can access the terminals with your voltmeter Also, the

brightness of the test light when you turn the starter key to the crank position

is a good indication of the condition of that side of the circuit Never be

tempted to bridge these two terminals with a wire – if there is nothing wrong with that side of the circuit (i.e no high resistance or breaks) you have just

created a dead short and you will blow a fuse or perhaps cause a fire

Remember, your test light has resistance

Testing the starter motor

Once you have confirmed that the fault does not lie in the starter circuit, you can now be certain that the fault must lie in the starter motor itself A good

number of workshops at this point will simply replace the starter motor as it is not economically viable to repair If your workshop does have a repair policy then you need to take your diagnosis to the next level

You can very quickly ascertain whether the fault lies in the starter motor itself

or the starter circuit by bridging together terminals 50 and 30 on the starter

motor If the engine cranks, then the starter motor is serviceable When

doing this ensure that you make your last connection at terminal 30 - a

sizeable spark is often created when carrying out this test and this can

damage a small terminal such as terminal 50 to the point where you cannot refit the plug! Make sure that you use a heavy-duty bridge that has plenty of insulation – an electrical screwdriver is ideal Mainstream tool manufacturers will happily sell you a heavy-duty switch complete with crocodile clips to help you to carry out this test

Pull-in test

Hold-in coil supply

Pull-in coil supply

Terminal 30 disconnected