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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 Hesitation or stumble Surges Detonation/spark knock Poor fuel economy Testing Sensors • All sensors detect a condition and change it into an electrical signal • Sensors may be: – – – – Switches Variable resistors 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 Switches • • • • 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 Testing Switches • With an ohmmeter • With a voltmeter • Some switches are adjustable Temperature Sensors • Most are NTC thermistors – High resistance cold – Low resistance hot • PCM supplies volts reference • Voltage changed by sensor • Signal returns on the ground wire 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 Knock Sensor (KS) • Converts knock vibration into a voltage • Mounted in block, intake, or cylinder head Computer Outputs and Actuators • Air management solenoids • Evaporative emissions system purge valve • EGR flow solenoid or EGR valve • Fuel injectors • • • • • • Fuel pump Idle speed controls Ignition module Warning lights Other solenoids Electronic throttle control 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 ... 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 •... 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... 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