SERVICE ENGINE SOON LIGHT INOPERATIVE
When engine is started, the ECM grounds terminal A10 to turn off the SERVICE ENGINE SOON light. When ALDL terminal "B" is grounded it alternately grounds and opens ECM terminal A10 to flash a code.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) This tests for an open ECM fuse or an open in the SERVICE ENGINE SOON light circuit, including instrument panel connector and printed circuit board. Normal response is light on.
Test Number 2) This tests for a shorted ECM. Grounded ECM terminal A10 will turn the SERVICE ENGINE SOON light off. If disconnecting ECM turns light on, ECM is shorted. Normal response is light on.
Test Number 3) This tests for a grounded circuit No. 487 from terminal "C" of remote lamp driver to ECM terminal A10. It also checks for an open circuit No. 439 to terminal "B" of remote lamp driver module, a bad ground, or a faulty remote lamp driver. A normal reading is 9-11 volts because of the voltage drop through the remote lamp driver upper resistor. A reading of over 11 volts indicates there is no voltage drop in the remote lamp driver module caused by bad ground or faulty module.
Test Number 4) This tests for an open wire to remote lamp driver terminal "B". Normal reading should be close to battery voltage.
Test Number 5) This tests for a grounded circuit No. 487 from terminal "C" of remote lamp driver to ECM terminal A10. Normal response is light on.
Flow Chart Service Engine Soon Light Inop. Scheme 38
ECM CHECK, SERVICE ENGINE SOON LIGHT ON AT ALL TIMES OR WON'T FLASH CODE 12
The ECM check determines why the SERVICE ENGINE SOON light remains on or does not flash Code 12. Normally, the ECM will not recognize a fault for at least 10 seconds after start up. If the SERVICE ENGINE SOON light remains on, the ECM has lost battery power, ground or the signal that turns the SERVICE ENGINE SOON light off. When engine is started, the ECM grounds terminal A10 to turn off the SERVICE ENGINE SOON light. It alternately grounds and opens terminal ECM A10 to flash a code.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) This tests for a short to battery voltage in wire to remote lamp driver terminal "C" or a faulty remote lamp driver. Normal reading is 9-11 volts.
Test Number 2) This tests if problem is related to the ECM or the remote lamp driver. Normally, grounding remote lamp driver terminal "C" should turn light off. If it does, problem is related to the ECM and its wiring. If not, problem is related to the remote lamp driver and its wiring.
Test Number 3) Grounding terminal A10 at ECM and finding light on, indicates an open in circuit No. 487 to remote lamp driver terminal "C". Normally, grounding ECM terminal A10 should turn light off.
Test Number 4) This tests for open ECM circuit No. 451 to diagnostic terminal in ALDL connector. The light should flash Code 12 when ECM terminal A6 is grounded.
Test Number 5) This tests for proper voltage supply to ECM. Both readings should be more than 9 volts. Voltage to ECM terminal C14 comes from ignition switch. Terminal C16 has constant battery voltage for long term memory.
Test Number 6) This tests for a bad ground to ECM. Terminal C2 is connected in the ECM.
Test Number 7) This test distinguishes between a faulty ECM and PROM. Normal response is for Code 51 to flash, even though the PROM is not installed in the ECM. If it does not, it means that the ECM is faulty.
Flow Chart ECM Check, Svc Eng Soon Lt On all Times or Won't Flash Code 12. Scheme 39
EPR SOLENOID ELECTRICAL CHECK
The Exhaust Pressure Regulation (EPR) solenoid controls vacuum to the EPR valve. When energized, EPR solenoid allows vacuum to close the EPR valve, increasing exhaust backpressure for proper EGR operation. The solenoid is supplied with 12 volts by ignition switch. The ECM completes ground circuit to energize the solenoid and turn EPR on when needed (EGR operation command).
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) This tests for short to ground or a faulty ECM signal to EPR solenoid. Test light should normally be off.
Test Number 2) This tests for signal to energize EPR solenoid with engine at idle. If test light is on, electrical circuits to the solenoid are okay.
Test Number 3) This tests for voltage or open circuit from terminal "B" of EPR solenoid to ECM terminal C12.
Flow Chart EPR Solenoid Electrical Check. Scheme 40
EPR VACUUM CIRCUIT CHECK
The Exhaust Pressure Regulation (EPR) solenoid controls vacuum to the EPR valve. When energized, EPR solenoid allows vacuum to close EPR valve, increasing exhaust backpressure for proper EGR operation. When vacuum is applied to the actuator, the restrictor plate closes to increase exhaust system backpressure, allowing EGR valve to function more efficiently.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) This tests for normal EPR vacuum at idle. Since electrical circuit was proven okay in prior flowchart, if no vacuum is present, it is due to no source vacuum (vacuum pump), a restriction or leak in the vacuum hose to valve, including a leak in solenoid.
Test Number 2) The EPR solenoid is de-energized, so no vacuum should be present.
Test Number 3) This tests for normal operation of EPR valve. When vacuum is applied to vacuum, valve actuator should move and hold.
Flow Chart EPR Vacuum Circuit Check. Scheme 41
CODE 12, NO REFERENCE PULSE
Code 12 indicates that the ECM is on and sees no reference pulse from the engine speed sensor. Code display is normal with ignition on and engine not running. Code 12 is not stored and will only flash when fault is present. With engine running, Code 12 could mean an open or ground in engine speed sensor reference circuit.
The engine speed sensor is a camshaft driven pick-up, mounted at center rear of engine. Sensor is sourced by a 5-volt reference voltage and allows the ECM to measure engine RPM by the number of times voltage is pulsed. The engine speed sensor pulses 4 times per revolution.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) This tests for a good 5-volt reference. Normally, the ECM should be at about 5 volts for fully charged batteries.
Test Number 2) This tests for proper ECM voltage to the engine speed sensor. If the circuit to the ECM is complete, normal voltage will be about 5 volts with harness disconnected from sensor.
Test Number 3) This tests for good sensor ground circuit No. 452, from sensor to ECM. Since test 2) indicated an open, the results of this test indicates whether the open is in the wire or at the ECM.
Code 12: Circuit Diagram & Flow Chart No Reference Pulse. Scheme 42
CODE 14, COOLANT TEMP SENSOR SIGNAL VOLTAGE LOW
The coolant temperature sensor uses a thermistor to control signal voltage to the ECM. The ECM applies voltage over circuit No. 410 to the sensor. When engine is cold, sensor resistance is high. The ECM will see a high signal voltage. As the engine warms-up, sensor resistance becomes less and the voltage drops. At normal engine operating temperature, voltage will measure about 1.5-2.0 volts.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) Code 14 will set if signal voltage indicates a coolant temperature above 275°F (135°C) for 3 minutes.
Test Number 2) This test determines if circuit No. 410 is shorted to ground, causing conditions for Code 14.
CODE 15, COOLANT TEMP SENSOR SIGNAL VOLTAGE HIGH
The coolant temperature sensor is a thermistor which control signal voltage to the ECM. The ECM applies voltage over circuit No. 410 to the sensor. When engine is cold, sensor resistance is high. The ECM will see a high signal voltage. As the engine warms, sensor resistance reduces and voltage drops. At normal engine operating temperature, voltage should be 1.5-2.0 volts.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) Code 15 will set if engine has run for more than 5 minutes and coolant temperature is less than -22°F (-30°C).
Test Number 2) This test simulates Code 14. If the ECM recognizes the low signal voltage (high temperature) and the SCAN tester reads 130°C or above, the ECM and wiring are okay.
Test Number 3) This tests if circuit No. 410 is open. Five volts present at sensor connector, if measured with a Digital Volt-Ohmmeter (DVOM).
CODE 21, THROTTLE POSITION SENSOR SIGNAL VOLTAGE HIGH
The Throttle Position Sensor (TPS), a variable resistor, informs the ECM of throttle opening. The TPS, connected to a 5-volt reference signal, has a high resistance value at closed throttle. At wide open throttle, TPS resistance is at its lowest and output to the ECM will be close to 5 volts. When Code 21 is set, EPR is turned off. Code 21 means ECM has seen the following: high voltage at ECM terminal A2 for at least 2 minutes with engine speed less than 1120 RPM.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) This test confirms Code 21 and fault are present.
Test Number 2) This tests for 5-volt reference signal at TPS harness connector and separates an electrical circuit problem from a faulty TPS. If circuit is okay, normal voltage reading will be 5 volts.
Test Number 3) This checks if low reference voltage is due to an open wire or ECM.
Code 21: Circuit Diagram & Flow Chart TPS Signal Voltage High. Scheme 43
CODE 22, THROTTLE POSITION SENSOR SIGNAL VOLTAGE LOW
The Throttle Position Sensor (TPS), a variable resistor, informs the ECM of throttle opening. The TPS, connected to a 5-volt reference signal, has a high resistance value at closed throttle. At wide open throttle, TPS resistance is at its lowest and output to the ECM will be close to 5 volts. When Code 21 is set, EPR is turned off. Code 22 means ECM has seen the following: low voltage at ECM terminal A2 for at least 2 minutes with engine speed more than 1250 RPM.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) This test confirms Code 22 and fault are present.
Test Number 2) This test simulates Code 21 (high voltage). If the ECM recognizes the high signal voltage, ECM and wiring are okay. If signal voltage is still low, Code 23 will set because test was performed at less than 1250 RPM.
Code 22: Circuit Diagram TPS Signal Voltage High. Scheme 44
Code 22: Flow Chart TPS Signal Voltage High. Scheme 45
CODE 23, THROTTLE POSITION SENSOR MISADJUSTED
The Throttle Position Sensor (TPS), a variable resistor, informs the ECM of throttle opening. The TPS, connected to a 5-volt reference signal, has a high resistance value at closed throttle. At wide open throttle, TPS resistance is at its lowest and output to the ECM will be close to 5 volts. Code 23 means the ECM has seen the following: voltage is not .25-1.35 volts at ECM terminal A2 for at least 30 seconds at 550-650 RPM.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) This confirms Code 23 and fault are present.
Test Number 2) This test determines if sensor signal line is shorted to ground.
Test Number 3) See THROTTLE POSITION SENSOR in N - REMOVE/INSTALL/OHAUL.
CODE 24, VEHICLE SPEED SENSOR CIRCUIT
The ECM applies and monitors 12 volts on circuit No. 437 to the Vehicle Speed Sensor (VSS). The circuit is opened and closed at approximately 2000 times per mile when the drive wheels are turning. The ECM calculates vehicle speed based on the time between pulses. SCAN tester reading should closely match speedometer reading with drive wheels turning. Code 24 will set if: circuit No. 437 is constant, engine speed is more than 200 RPM, vehicle speed signal at terminal ECM A9 is less than 10 MPH (16 KM/H), all conditions are met for at least 10 seconds.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) This test monitors the ECM voltage on circuit No. 437. With wheels turning, the pulsating action causes varying voltage. The variation will be greater at low wheel speeds to an average of 4-6 volts at about 20 MPH (32 KM/H).
Test Number 2) Less than one volt at ECM connector indicates circuit No. 437 wire is shorted to ground. Disconnect VSS connector. If voltage now reads more than 10 volts, the VSS is faulty. If voltage remains less than 10 volts, then wire is grounded. If circuit No. 437 is not grounded, check for a faulty ECM connector or ECM.
Test Number 3) A steady 8-12 volt reading at the ECM connector indicates the circuit No. 437 is open or VSS is faulty.
Test Number 4) Normal voltage indicates a possible intermittent condition.
CODE 31, MANIFOLD ABSOLUTE PRESSURE SENSOR SIGNAL VOLTAGE LOW
The Manifold Absolute Pressure (MAP) sensor monitors vacuum in the EGR circuit. It senses the actual vacuum in the EGR vacuum line and sends a signal back to the ECM. The signal is compared to the EGR duty cycle calculated by the ECM. If there is a difference in the ECM command and what vacuum is at the EGR valve sensed by the MAP sensor, the ECM makes minor adjustments. When a major difference is sensed, the ECM recognizes a fault and sends a full EGR signal.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) This confirms Code 31 and fault are present.
Test Number 2) If the ECM recognizes and sets Code 33 (high MAP signal), the ECM, MAP sensor and wiring are okay.
Test Number 3) If the ECM recognizes and sets Code 33 (high MAP signal), the ECM and wiring are okay.
Test Number 4) This tests for 5-volt reference signal to MAP sensor. Normally, about 5 volts should be present with key on at MAP sensor terminal "C".
Test Number 5) This tests for an open in the EGR solenoid circuit.
Code 31: Circuit Diagram MAP Sensor Signal Voltage Low. Scheme 46
Code 31: Flow Chart MAP Sensor Signal Voltage Low. Scheme 47
CODE 32, EGR CIRCUIT LOOP ERROR
During normal operation, the ECM compares its EGR duty cycle signal with the Manifold Absolute Pressure (MAP) signal and makes adjustments in the duty cycle. If the actual EGR control pressure (line vacuum) varies from what the ECM has previously determined the pressure should be and this variance continues for 10 seconds or more, a Code 32 will be set and the ECM will shut down EGR.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) This determines if Code 32 can reset.
Test Number 2) This tests EGR solenoid electrical control circuit. Test light should flicker faintly if the ECM harness and connections are okay. Flicker faintly is a slightly pulsating glow, as opposed to a bright steady glow from a continuous ground path.
CODE 33, MAP SENSOR SIGNAL VOLTAGE HIGH
The Manifold Absolute Pressure (MAP) sensor monitors vacuum in the EGR circuit. It senses the actual vacuum in the EGR vacuum line and sends a signal back to the ECM. The signal is compared to the EGR duty cycle calculated by the ECM. If there is a difference in the ECM command and what vacuum is at the EGR valve sensed by the MAP sensor, the ECM makes minor adjustments. When a major difference is sensed, the ECM recognizes a fault and sends a full EGR signal.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) This step confirms Code 33 and fault are present.
Test Number 2) If the ECM recognizes and sets Code 33 (low MAP signal), the ECM and wiring are okay.
Test Number 3) This determines if solenoids are stuck closed.
Test Number 4) This determines if a short circuit to ground exists in either solenoid circuit or ECM is faulty.
Code 33: Flow Chart MAP Sensor Signal Voltage High. Scheme 48
CODE 51, PROM PROBLEM
Ensure all pins are fully inserted in socket. If okay, replace PROM and recheck. If problem is not corrected, replace ECM.
CODE 52, ECM FAULT
Ensure ECM connectors are fully inserted. Clear ECM memory. Start engine and check for SERVICE ENGINE SOON light. If light and Code 52 reappear, replace ECM. After repairs, clear ECM memory to confirm no SERVICE ENGINE SOON light.
CODE 53, VOLTAGE REFERENCE OVERLOAD
The 5-volt reference (Vref) circuit is overloaded (grounded circuit). It takes 10 seconds before Code 53 will set. DVM is Digital Voltmeter.
Note. Test numbers refer to test numbers on diagnostic chart.
Test Number 1) This confirms Code 53 is still present.
Test Number 2) This confirms 5-volt reference signal from the ECM.
Test Number 3) This tests if a short circuit to ground exists in circuit No. 416 or ECM.
