6.2l Cec Testing GMC Magnavan G3500

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DIAGNOSTIC CIRCUIT CHECK

The ECM provides the diagnostic logic to detect faults in the Diesel Electronic Control system that the ECM monitors or controls. The ECM, when it recognizes a fault, has the ability of turning the "SERVICE ENGINE SOON" light on and storing a code. If the condition corrects itself, the "SERVICE ENGINE SOON" light will be immediately turned off.

The ECM recognizes errors in engine speed, vacuum errors in the EGR vacuum loop, and electrical faults involving the 5-volt reference circuit. The ECM controls the following

1. Exhaust Gas Recirculation (EGR)
2. Exhaust Pressure Regulation (EPR)
3. Torque Converter Clutch (TCC)
4. System Diagnosis
5. Cold Advance & Glow Plugs

To allow proper engine control, the ECM monitors the following inputs

1. Engine RPM
2. Manifold Absolute Pressure (MAP)
3. Throttle Position Sensor (TPS)
4. Vehicle Speed Sensor (VSS)
5. Coolant Temperature Sensor (CTS)

All diagnosis should start with the "DIESEL DIAGNOSTIC CIRCUIT CHECK" chart. After any repair to the Diesel Electronic Control system, the diesel diagnostic circuit check procedure must be repeated.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks for proper operation of "SERVICE ENGINE SOON" light. With ignition on and engine not running, the light should be on.
2. Grounding the diagnostic terminal will allow ECM to flash Code 12 and any stored codes. The light must go on and off for a proper code. If light goes from bright to dim, this is not considered a code.

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"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.

1. This test checks for an open ECM fuse or an open in the "SERVICE ENGINE SOON" light circuit, including instrument panel connector, printed circuit and "SERVICE ENGINE SOON" light. Normal response is light on.
2. This test checks 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.
3. This test checks 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, a bad ground, or a faulty remote lamp driver. A normal reading is 9-11 volts, because of the voltage drop through the upper resistor in the remote lamp driver. A reading of over 11 volts indicates there is no voltage drop in the remote lamp driver. This indicates a bad ground or faulty remote lamp driver.
4. This test checks for an open wire to terminal "B" of remote lamp driver. Normal reading should be close to battery voltage.
5. This test checks for a grounded circuit No. 487 from terminal "C" of remote lamp driver to ECM terminal "A10". Normal response is light on.

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ECM CHECK "SERVICE ENGINE SOON" LIGHT ON AT ALL TIMES OR WON'T FLASH CODE 12

The ECM check is made to determine 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.

1. This test checks for a short to battery voltage in wire to terminal "C" of remote lamp driver or for a faulty remote lamp driver. Normal reading is 9-11 volts.
2. This test checks if problem is related to the ECM or the remote lamp driver. Normally, grounding terminal "C" of remote lamp driver should turn light off. If it does, problem is related to the ECM and its wiring. If not, it is related to the remote lamp driver and its wiring.
3. Grounding terminal "A10" at ECM and finding light on, indicates an open in circuit No. 487 to terminal "C" of remote lamp driver. Normally, grounding ECM terminal "A10" should turn light off.
4. This test checks for open circuit No. 451 from ECM to diagnostic terminal in ALDL connector. The light should flash Code 12 when ECM terminal "A6" is grounded.
5. This test checks for proper voltage supply to ECM. Both readings should be over 9 volts. Voltage to ECM terminal "C14" comes from ignition switch, and terminal "C16" has constant battery voltage for long term memory.
6. This test checks for a bad ground to ECM. Terminal "C2" is connected in the ECM.
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.

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EXHAUST PRESSURE REGULATOR (EPR) SOLENOID ELECTRICAL CIRCUIT CHECK

The Exhaust Pressure Regulation (EPR) solenoid controls vacuum to the EPR valve. The EPR solenoid, when energized, allows vacuum to close the EPR valve, which increases exhaust backpressure for proper EGR operation. The solenoid is supplied with 12 volts by the ignition switch. The ECM completes the 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.

1. This test checks for short to ground or a faulty ECM signal to EPR solenoid. Test light should normally be off.
2. This test checks for signal to energize EPR solenoid with engine at idle. If test light is on, electrical circuits to the solenoid are okay.
3. This test checks for voltage or open circuit from terminal "B" of EPR solenoid to ECM terminal "C12".

Scheme 197

EXHAUST PRESSURE REGULATOR (EPR) VACUUM CIRCUIT CHECK

The Exhaust Pressure Regulation (EPR) solenoid controls vacuum to the EPR valve. The EPR solenoid, when energized, allows vacuum to close the EPR valve, which increases exhaust backpressure for proper EGR operation. When vacuum is applied to the actuator, the restrictor plate closes to increase exhaust system backpressure to allow the EGR valve to function more efficiently.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks for normal EPR vacuum at idle. Since electrical circuit was verified as being okay on prior chart, 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.
2. The EPR solenoid is de-energized, so no vacuum should be present.
3. This test checks for normal operation of EPR valve. When vacuum is applied to vacuum, valve actuator should move and hold.

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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.

1. This test checks for a good 5-volt reference. Normally, the ECM should be at about 5 volts for fully charged batteries.
2. This test checks 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.
3. This test checks 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.

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CODE 14 - COOLANT TEMPERATURE 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.

1. Code 14 will set if signal voltage indicates a coolant temperature above 275°F (135°C) for 3 minutes.
2. This test will determine if circuit No. 410 is shorted to ground, which will cause the conditions for Code 14.

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Diagnostic Aids

Check circuit No. 410 routing for a potential short to circuit No. 452 or ground.

"SCAN" tester displays engine temperature in degrees Centigrade. After engine is started, the temperature should rise steadily to about 90°C then stabilize when thermostat opens.

The "COOLANT SENSOR TEMPERATURE TO RESISTANCE VALUES" chart may be used to test coolant sensor at various temperature levels to evaluate the possibility of a mis-scaled sensor. A mis-scaled sensor could result in poor driveability complaints.

CODE 15 - COOLANT TEMPERATURE SENSOR (SIGNAL VOLTAGE HIGH)

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.

1. Code 15 will set if engine has run for longer than 5 minutes and coolant temperature is less than -22°F (-30°C) for 5 minutes.
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.
3. This test determines if circuit No. 410 is open. There should be 5 volts present at sensor connector, if measured with a DVOM.

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"SCAN" tester displays engine temperature in degrees Centigrade. After engine is started, the temperature should rise steadily to about 90°C then stabilize when thermostat opens.

If Code 12 or 21 is also set, check circuit No. 452 for faulty wiring or connections. Check terminals at sensor for good contact.

The "COOLANT SENSOR TEMPERATURE TO RESISTANCE VALUES" chart may be used to test coolant sensor at various temperature levels to evaluate the possibility of a mis-scaled sensor. A mis-scaled sensor could result in poor driveability complaints.

CODE 21 - THROTTLE POSITION SENSOR (SIGNAL VOLTAGE HIGH)

The Throttle Position Sensor (TPS) is a variable resistor that informs the ECM the degree of throttle opening. The sensor is connected to a 5-volt reference signal and 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 that the 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.

1. This test confirms Code 21 and that fault is present.
2. This test checks 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.
3. This step checks if the low reference voltage is due to an open wire or the ECM.

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CODE 22 - THROTTLE POSITION SENSOR (SIGNAL VOLTAGE LOW)

The Throttle Position Sensor (TPS) is a variable resistor that informs the ECM the degree of throttle opening. The sensor is connected to a 5-volt reference signal and 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 that the 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.

1. This test confirms Code 22 and that fault is present.
2. This test simulates Code 21 (high voltage). If the ECM recognizes the high signal voltage, the ECM and wiring are okay. If signal voltage is still low, Code 23 will set, because test was performed below 1250 RPM.

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CODE 23 - THROTTLE POSITION SENSOR MISADJUSTED

The Throttle Position Sensor (TPS) is a variable resistor that informs the ECM the degree of throttle opening. The sensor is connected to a 5-volt reference signal and 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 that the ECM has seen the following: voltage not between .25-1.35 volts at ECM terminal "A2" for at least 30 seconds, with engine speed between 550-650 RPM.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms Code 23 and that fault is present.
2. This test will determine if sensor signal line is shorted to ground.
3. See THROTTLE POSITION SENSOR installation and adjustment under REMOVAL & INSTALLATION in this article.

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Disregard Code 23 if "SERVICE ENGINE SOON" light goes out as soon as throttle is returned to idle.

CODE 24 - VEHICLE SPEED SENSOR (VSS) CIRCUIT

The ECM applies and monitors 12 volts on circuit No. 437. This circuit connects the Vehicle Speed Sensor (VSS), which alternately grounds circuit No. 437 when drive wheels are turning. This pulsating action takes place about 2000 times per mile and the ECM will calculate 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 k/mh), all conditions are met for at least 10 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test monitors the ECM voltage on circuit No. 437. With wheels turning, the pulsating action will result in a 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).
2. A voltage of less than one volt at ECM connector indicates that circuit No. 437 wire is shorted to ground. Disconnect wire at VSS. If voltage now reads above 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.
3. A steady 8-12 volt reading at the ECM connector indicates the circuit No. 437 is open or VSS is faulty.
4. Normal voltage indicates a possible intermittent condition.

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With drive wheels turning, "SCAN" tester reading should closely match with speedometer reading.

CODE 31 - MANIFOLD ABSOLUTE PRESSURE SENSOR (SIGNAL VOLTAGE LOW)

The Manifold Absolute Pressure (MAP) sensor is used to monitor the amount of 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.

1. This test confirms Code 31 and that fault is present.
2. If the ECM recognizes and sets Code 33 (high MAP signal), the ECM, MAP sensor and wiring are okay.
3. If the ECM recognizes and sets Code 33 (high MAP signal), the ECM and wiring are okay.
4. This test checks for 5-volt reference signal to MAP sensor. Normally, about 5 volts should be present, with key on at MAP sensor terminal "C".
5. This test checks for an open in the EGR solenoid circuit.

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CODE 32 - EGR CIRCUIT LOOP ERROR

During normal operation, the ECM compares its EGR duty cycle signal with the manifold absolute pressure signal and makes corrections 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.

1. This test determines if Code 32 can reset.
2. This test checks EGR solenoid electrical control circuit. Test light should flicker faintly if the ECM harness and connections are okay. Flicker faintly is defined as a slightly pulsating glow, as opposed to a bright steady glow from a continuous ground path.

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Diagnostic Aids

A vacuum leak may cause a Code 32. Check all vacuum hoses and components connected to vacuum hoses for leaks. This check should include cruise control and A/C systems, if installed.

CODE 33 - MANIFOLD ABSOLUTE PRESSURE SENSOR (SIGNAL VOLTAGE HIGH)

The Manifold Absolute Pressure (MAP) sensor is used to monitor the amount of 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.

1. This step confirms Code 33 and that fault is present.
2. If the ECM recognizes and sets Code 33 (low MAP signal), the ECM and wiring are okay.
3. This test determines if solenoids are stuck closed.
4. This test determines if there is a short circuit to ground in either solenoid circuit, or if there is a fault in the ECM.

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CODE 51 - PROM PROBLEM

Check that all pins are fully inserted in socket. If okay, replace PROM and recheck. If problem is not corrected, replace ECM.

CODE 52 - ECM FAULT

Check that ECM connectors are fully inserted. Clear ECM memory. Start engine and check for "SERVICE ENGINE SOON" light. If light and Code 52 reappears, 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.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test confirms that Code 53 is still present.
2. This test confirms if there is a 5-volt reference signal from the ECM.
3. This test determines if there is a short circuit to ground in circuit No. 416, or a short to ground in the ECM.

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TORQUE CONVERTER CLUTCH (TCC) ELECTRICAL DIAGNOSIS

The purpose of the Torque Converter Clutch (TCC) is to eliminate the power loss of the torque converter stage when the vehicle is in a cruise condition. This allows the convenience of an automatic transmission and the fuel economy of a manual transmission.

Fused battery voltage is supplied to the TCC solenoid through the TCC brake switch. The ECM will engage TCC by grounding circuit No. 422 to energize the TCC apply solenoid inside the transmission. The ECM completes circuit whenever TPS exceeds a calibrated value for throttle opening.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks for a complete circuit, from ignition switch, through solenoid, and up to test point. Test light should normally be on, since the ECM has not completed circuit.
2. This test checks for continuity through TCC brake switch and TCC apply solenoid.
3. This test checks for ECM to complete circuit to ground, to energize TCC apply solenoid and engage TCC. Test light should normally go out when ECM completes circuit.
4. This test checks for TPS signal. If signal to ECM is correct, fault is in ECM connection or ECM. If TPS voltage signal to ECM is incorrect, proper operation will not occur.
5. This test checks for ground in circuit to ECM terminal "C5". Test light should normally be off.
6. This test checks for voltage to terminal "A" of TCC connector. Test light should normally be on.
7. This test checks for a complete circuit from voltage to ground, via TCC test terminal in ALDL connector. Normally, test light should come on, if harness is good.

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Solenoid coil resistance must measure more than 20 ohms. A lower resistance value will cause early failure of the ECM driver. Check the solenoid coil resistance of all ECM controlled solenoids and relays, before installing replacement ECM. Replace any solenoid or relay that measures less than 20 ohms resistance.

GLOW PLUG CIRCUIT ELECTRICAL CHECK

With ignition on and coolant temperature below 150°F (65°C), the ECM turns on the cold advance relay to supply 12 volts to the glow plug controller and the cold advance solenoid. The controller, in turn, will cycle the glow plugs on and off for a pre-determined amount of time. After engine is cranked, the glow plugs will again cycle on and off.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Disconnecting the coolant temperature sensor sends a low temperature signal to the ECM. The ECM turns on the cold advance relay which sends 12 volts to terminal "D" of the glow plug controller. If test light comes on during this test, the cold advance circuit and ECM are okay.
2. If relay contacts are shorted together, there will be 12 volts supplied to the glow plugs at all times. Since glow plugs are 6 volt components, they will be damaged by the constant high voltage.
3. This test checks for high resistance in the feedback circuit to the glow plug controller. High resistance in this circuit will signal the glow plug controller to turn off.
4. This test determines if the glow plug controller is cycling its internal relay. Test light should cycle on and off if the glow plug controller is operating.
5. This test checks for a complete circuit between the glow plug controller and each glow plug. If test light fails to come on, that glow plug feed circuit is open. It may be necessary to turn ignition off for 10 seconds and on again to keep the glow plug controller cycling.

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COLD ADVANCE SYSTEM ELECTRICAL CHECK

With ignition on and coolant temperature below 80°F (27°C), the ECM grounds circuit No. 901 to the cold advance relay. Grounding circuit No. 901 turns on cold advance relay, to supply 12 volts to the cold advance solenoid (in the injection pump), and the glow plug controller. The cold advance solenoid is now energized, which causes the injection pump timing to be advanced about 4 degrees.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test will determine if problem is electrical or a malfunctioning injection pump.
2. This test will determine if the ECM is able to turn on the cold advance relay.

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If there is no electrical problem found, check the operation of the cold advance solenoid in injection pump. Check the solenoid coil resistance of all ECM controlled solenoids and relays, before installing replacement ECM. Replace any solenoid or relay that measures less than 20 ohms resistance.

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6.2L Diesel Electronic Control System Wiring Diagram ("C/K" Series). Scheme 220

6.2L Diesel Electronic Control System Wiring Diagram ("G" Series). Scheme 221

6.2L Diesel Electronic Control System Wiring Diagram ("R/V" Series). Scheme 222