CH26 detailed diagnostic and sensors STUDENT VERSION rev

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Chapter 26

Detailed Diagnosis and Sensors

Symptom-Based Diagnosis

• Before diagnosing symptoms, ensure:

– ECM and MIL are operating correctly

– No stored DTCs

– Scan data within normal ranges

– Scan tool to OBD system communication – No TSBs for current symptom

– All ECM/PCM grounds are correct

– Vehicle modifications identified

Common Symptoms

• Hard start/Long crank hard start

– Cranks normally but does not start quickly

• No crank

– Starter does not turn engine over

• No start (engine cranks)

– Engine turns over but does not start even after prolonged cranking

Common Symptoms (cont’d)

• Lack of or loss of power

• Cuts out, misses

– Voltage or frequency generators

– Some inputs are simply connections from other modules or battery voltage and ground

Diagnosis of Computer Voltage Supply and Ground Wires

• Verify the power and ground circuits to the ECM/PCM

• Check power and ground circuit voltage

drop

• Good ground circuits is critical for all

reference voltage sensors

• Either normally open or normally closed

• Send digital signal to the PCM

• Some are provided with a reference voltage

• Can be on ground or power circuit

Temperature Sensors

• Most are NTC thermistors

– High resistance cold

– Low resistance hot

• PCM supplies 5 volts reference

• Voltage changed by sensor

• Signal returns on the ground wire

• Scan tool values

• Highest possible value = short in circuit

• Lowest possible value = open in circuit

• Place in water

• Never apply an open flame to a sensor

Pressure Sensors

• Most are piezometric sensors

• The amount of flex dictates the voltage

signal

• One side exposed to reference pressure and other to a calibrated pressure or vacuum

MAP Sensors

• Sense air pressure in the intake manifold

• Not significantly affected by altitude or barometric pressure

• Intake pressure varies based on engine speed and load

• Used to calculate fuel delivery

Testing MAP Sensors

• Can cause rich, lean, or surging conditions

• Check vacuum supply

• PCM supplies 5 volt reference and ground

• MAP can be monitored with scan tool

• Frequency output can be scoped

Barometric Pressure (BARO)

Sensor

• Used on some engines to alter fuel and

ignition timing based on altitude

• Higher elevations have less atmospheric pressure

• Often located in PCM and not separately replaceable

Vapor Pressure Sensor (VPS)

• Measures vapor pressure in EVAP system

• Responds to slight changes

• May be mounted on fuel pump or remotely

• Remote sensors have hose connections

• Check hoses and connections

Mass Airflow (MAF) Sensors

• Measures air entering the engine

• Used to calculate engine load

• Sensors are placed between air cleaner and throttle plate

• Common MAF sensors include - hot wire, vane-type, and Karmen vortex type

Titanium Dioxide (TiO2) Sensors

• Do not generate voltage

• Alters a 5 volt reference signal

• Low oxygen content equals low voltage,

below 2.5 volts

• High oxygen content equals voltage above 2.5 volts

Air/Fuel Ratio (A/F) Sensors

• Changes current in the circuit

• Can be used in very lean conditions

• At stoichiometric, no current is generated and the voltage detection circuit is 3.3 volts

• When rich, there is negative current flow and voltage is below 3.3 volts

Checking O2 Sensors and Circuits

• Things that can cause an O2 sensor to appear faulty:

– Incorrect fuel pressure

– Malfunctioning AIR system

Heated Oxygen Sensors (HO2S)

• To generate an accurate signal, the sensor must operate at 750ºF (400ºC) or higher

• Current HO2S have a built-in heating

element

• The PCM controls the heater ground

Testing with a Lab Scope

Position Sensors

• TP Sensors

– Provides rate of opening and position

– Three wires

• TP Sensors for electronic throttle control

– Uses two resistors in one housing

Testing Position Sensors

• Check voltage reference

and ground

• Check resistance

• Connect a lab scope and

watch pattern as throttle

is opened and closed

Accelerator Pedal Position Sensor

• Similar to TP sensor operation, uses two

potentiometers

• PCM determines power or torque demand

• The PCM then commands the throttle plate motors to open or close as needed

Speed Sensors

• Vehicle speed sensor

• Can be either a magnetic pulse generator or hall-effect sensor

• Provides a waveform proportional to

vehicle speed

• Used for cruise control, EGR, EVAP,

transmission, and torque converter

Position/Speed Sensors

• Crankshaft position (CKP) sensor

– Used for misfire detection

– Can be checked with scan tool or lab scope

• Camshaft position (CMP) sensor

– Used for fuel injection control

Computer Outputs and Actuators

• Air management

solenoids

• Evaporative emissions

system purge valve

• EGR flow solenoid or

Testing Actuators with a Lab Scope

• Most computer-controlled circuits are ground controlled

• The PCM energizes the actuator by

providing the ground

• Understand what the actuator is before trying to test

• Some are pulse width controlled