GMC Rally G2500 1990 - 1990 ENGINE PERFORMANCE Self-Diagnostics - Diesel

GMC Rally G2500 1990 - INTRODUCTION

The Diesel Electronic Control (DEC) system, on the 6.2L with light duty emissions, electronically controls EGR system operation, Torque Converter Clutch (TCC) engagement, cold advance and glow plug operation.

Most engine control problems are NOT computer related, but result from mechanical breakdowns, poor electrical connections or damaged vacuum hoses. Before condemning the computer system, carefully perform visual and mechanical inspections. Failure to perform these inspections can result in lost diagnostic time.

GMC Rally G2500 1990 - SELF-DIAGNOSTIC SYSTEM

The 6.2L light duty emissions system includes a self-diagnostic system which can determine input signal circuit malfunctions. Input signal circuits determine engine function control.

GMC Rally G2500 1990 - DIAGNOSTIC PROCEDURE PRELIMINARY INSPECTION

1. Check all vacuum hoses for correct routing, restrictions, cuts or other damage. Inspect difficult to see vacuum hoses beneath the air cleaner assembly and other engine components.
2. Inspect all engine compartment wiring for proper connections. Also check wires for pinched or chaffed spots, as well as contact with sharp edges or exhaust manifolds.
3. The preliminary inspection is very important and should be performed CAREFULLY and THOROUGHLY as it can often lead to fixing a problem without further diagnosis.

GMC Rally G2500 1990 - DIAGNOSTIC PROCEDURE

1. Ensure all engine systems not related to the Diesel Electronic Control (DEC) system are operating properly. DO NOT proceed with testing unless all other non-DEC system problems are repaired.
2. The DEC system diagnosis should ALWAYS begin with the DIAGNOSTIC CIRCUIT CHECK chart. This determines if the DEC system and ECM are working properly. If trouble codes are displayed (other than Code 12), determine if codes are "hard" or "intermittent" trouble codes.
3. A "hard" code is one which is present when working on vehicle and the condition still exists. "Hard" codes will cause the SERVICE ENGINE SOON light to come on.
4. An "intermittent" code is one which does not reset itself and is NOT present while working on vehicle. Intermittent codes are often caused by loose connections. The SERVICE ENGINE SOON light will go out 10 seconds after fault goes away. For intermittent diagnosis procedures, see H - TESTING W/O CODES article in ENGINE PERFORMANCE.

GMC Rally G2500 1990 - ENTERING OR EXITING DIAGNOSTIC MODE

1. With key on and engine off, connect a jumper wire between ALDL terminal "B" (diagnostic terminal) and terminal "A" (ground). See Fig. 1. The Diesel Electronic Control (DEC) system will enter the diagnostic mode.
2. In this mode, the ECM will display Code 12, by flashing the SERVICE ENGINE SOON light. Code 12 consists of one (1) flash, followed by a short pause, then 2 flashes in quick succession.
3. Code 12 will be displayed 3 times. If no other codes are stored, Code 12 will continue to flash until the diagnostic terminal is ungrounded. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.

Fig. 1: GMC Rally G2500 1990 - Component Locations - ALDL Connector Terminal Identification

GMC Rally G2500 1990 - TROUBLE CODE IDENTIFICATION

1. The DEC system codes indicate a failure of a specific sensor and/or circuit. Sensor/circuit diagnosis may indicate replacement of the ECM. Code 51 indicates that PROM is improperly installed or has failed.
2. Code 52 indicates that the ECM has failed and must be replaced. If the ECM is replaced and the condition was not corrected, the following may be the cause:
   • An incorrect ECM or PROM application may cause a malfunction and may, or may not, set a code.
   • The ECM connector may be the problem. Connector terminals may have to be removed from the connector to properly check them.
   • PROM failure. Although the PROM rarely fails, it could be the cause of the problem.
   • Replacement ECM may be faulty.
   • Intermittent problem. Make a careful physical inspection of affected sensor/circuit.
   • A shorted solenoid, coil relay, or harness may cause an ECM to fail and a replacement ECM to fail when it is installed. Use Short Circuit Tester (J-34636), to check for short circuits.

GMC Rally G2500 1990 TROUBLE CODE IDENTIFICATION

Code (Affected Circuit)	Probable Cause
12 (Engine Speed Sensor)	No engine speed sensor reference pulse to ECM.
14 (CTS High Temp.)	Sensor or signal line grounded.
15 (CTS Low Temp.)	Sensor, connections, or wires open.
21 (TPS Voltage High)	Open circuit or misadjusted TPS.
22 (TPS Voltage Low)	Circuit grounded.
23 (TPS Not Calibrated)	Voltage not .25-1.3 volts at curb idle.
24 (Vehicle Speed Sensor)	VSS circuit open or grounded.
31 (MAP Voltage Low)	Open or short to ground.
32 (EGR Loop Error)	Improper EGR vacuum signal.
33 (MAP Voltage High)	Open or short to ground.
51 (PROM Fault)	Improperly installed or faulty PROM.
52 (ECM Fault)	Fault in ECM circuit.
53 (5-Volt Reference Overload)	Grounded 5-volt reference signal.

GMC Rally G2500 1990 - CLEARING TROUBLE CODES

Trouble codes should be cleared after repairs have been completed. Also, some diagnostic charts require that codes be cleared before using diagnostic chart. To clear codes, remove ECM memory battery voltage for 30 seconds.

If no hard fault codes are present (only intermittent codes exist), proceed to H - TESING W/O CODES article for intermittent diagnostic procedures.

GMC Rally G2500 1990 - ECM LOCATION

The ECM is located under the dash, behind the glove box.

GMC Rally G2500 1990 - SPECIAL TOOLS (DIAGNOSTIC)

Special "Scan" testers plugged into the ALDL may be used to read trouble codes and check voltages in the system on the serial data line. These testers can save a great deal of time. For additional information, see owners manual included with tester.

GMC Rally G2500 1990 - TEST EQUIPMENT

A tachometer, test light, Digital Volt/Ohmmeter (DVOM) with a minimum 10-megohm input impedance, a vacuum gauge, and jumper wires are required to test and diagnose DEC system. A "Scan" tester may also be used to access data parameters. Tester will supply a visual reading of most inputs to the ECM and some outputs.

GMC Rally G2500 1990 - DIAGNOSTIC CHARTS

The following diagnostic flow-charts and circuit schematics are provided courtesy of General Motors Corp.

GMC Rally G2500 1990 - DIAGNOSTIC CIRCUIT CHECK

The ECM provides diagnostic logic to detect faults in the Diesel Electronic Control (DEC) system. When the ECM recognizes a fault, it can turn on SERVICE ENGINE SOON light and store codes. If the condition is corrected, the SERVICE ENGINE SOON light will turn off. The ECM recognizes errors in engine speed and EGR vacuum, as well as electrical faults involving the 5-volt reference circuit. The ECM controls the following:

• Exhaust Gas Recirculation (EGR)
• Exhaust Pressure Regulation (EPR)
• Torque Converter Clutch (TCC)
• System Diagnosis
• Cold Advance & Glow Plugs

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

• Engine RPM
• Manifold Absolute Pressure (MAP)
• Throttle Position Sensor (TPS)
• Vehicle Speed Sensor (VSS)
• Coolant Temperature Sensor (CTS)

All diagnosis begins with DIAGNOSTIC CIRCUIT CHECK chart. After any DEC system repair, repeat the diagnostic circuit check.

NOTE:

Test numbers refer to test numbers on diagnostic chart.

1. This tests SERVICE ENGINE SOON light operation. With ignition on and engine off, the light should be on.
2. Grounding the diagnostic terminal allows 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.

Fig. 2: GMC Rally G2500 1990 - Component Locations - Diagnostic Circuit Check Diagram

Fig. 3: GMC Rally G2500 1990 - Component Locations - Diagnostic Circuit Check Flow Chart

GMC Rally G2500 1990 - 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.

Fig. 4: GMC Rally G2500 1990 - Component Locations - Service Engine Soon Light Inop. Circuit Diagram

Fig. 5: GMC Rally G2500 1990 - Component Locations - Flow Chart Service Engine Soon Light Inop.

GMC Rally G2500 1990 - 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.

Fig. 6: GMC Rally G2500 1990 - Component Locations - Ckt Diag, ECM Check, Svc Eng Soon Lt On all Times or Won't Flash Code 12

Fig. 7: GMC Rally G2500 1990 - Component Locations - Flow Chart ECM Check, Svc Eng Soon Lt On all Times or Won't Flash Code 12

GMC Rally G2500 1990 - 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.

Fig. 8: GMC Rally G2500 1990 - Component Locations - Flow Chart EPR Solenoid Electrical Check

GMC Rally G2500 1990 - 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.

Fig. 9: GMC Rally G2500 1990 - Component Locations - Flow Chart EPR Vacuum Circuit Check

GMC Rally G2500 1990 - 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.

Fig. 10: GMC Rally G2500 1990 - Component Locations - Code 12: Circuit Diagram & Flow Chart No Reference Pulse

GMC Rally G2500 1990 - 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.

GMC Rally G2500 1990 - Diagnostic Aids

Check circuit No. 410 routing for a potential short to circuit No. 452 or ground. SCAN tester displays engine temperature in degrees Celsius. After tarting engine, temperature should rise to about 90?C and stabilize when thermostat opens. Use COOLANT SENSOR TEMPERATURE TO RESISTANCE VALUES chart to test coolant sensor at various temperature levels. An out-of-calibration sensor can result in poor driveability.

Fig. 11: GMC Rally G2500 1990 - Component Locations - Code 14: Circuit Diagram CTS Sensor Signal Voltage Low

Fig. 12: GMC Rally G2500 1990 - Component Locations - Code 14: Flow Chart CTS Sensor Signal Voltage Low

GMC Rally G2500 1990 - 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).

GMC Rally G2500 1990 - Diagnostic Aids

SCAN tester displays engine temperature in degrees Celsius. After starting engine, temperature should rise to about 90?C and stabilize when thermostat opens. If Code 12 or 21 are also set, check circuit No. 452 for faulty wiring or connections. Check terminals at sensor connector.

Use COOLANT SENSOR TEMPERATURE TO RESISTANCE VALUES chart to test coolant sensor at various temperature levels. An out-of-calibration sensor can result in poor driveability.

Fig. 13: GMC Rally G2500 1990 - Component Locations - Code 15: Circuit Diagram CTS Sensor Signal Voltage High

Fig. 14: GMC Rally G2500 1990 - Component Locations - Code 15: Flow Chart CTS Sensor Signal Voltage High

GMC Rally G2500 1990 - 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.

Fig. 15: GMC Rally G2500 1990 - Component Locations - Code 21: Circuit Diagram & Flow Chart TPS Signal Voltage High

GMC Rally G2500 1990 - 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.

Fig. 16: GMC Rally G2500 1990 - Component Locations - Code 22: Circuit Diagram TPS Signal Voltage High

Fig. 17: GMC Rally G2500 1990 - Component Locations - Code 22: Flow Chart TPS Signal Voltage High

GMC Rally G2500 1990 - 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.

GMC Rally G2500 1990 - Diagnostic Aids

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

Fig. 18: GMC Rally G2500 1990 - Component Locations - Code 23: Circuit Diagram & Flow Chart TPS Misadjusted

GMC Rally G2500 1990 - 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.

GMC Rally G2500 1990 - Diagnostic Aids

With drive wheels turning, SCAN tester and speedometer reading should be close.

Fig. 19: GMC Rally G2500 1990 - Component Locations - Code 24: Circuit Diagram VSS Circuit

Fig. 20: GMC Rally G2500 1990 - Component Locations - Code 24: Flow Chart VSS Circuit

GMC Rally G2500 1990 - 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.

Fig. 21: GMC Rally G2500 1990 - Component Locations - Code 31: Circuit Diagram MAP Sensor Signal Voltage Low

Fig. 22: GMC Rally G2500 1990 - Component Locations - Code 31: Flow Chart MAP Sensor Signal Voltage Low

GMC Rally G2500 1990 - 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.

GMC Rally G2500 1990 - Diagnostic Aids

A vacuum leak may cause a Code 32. Check all vacuum hoses and components connected to vacuum hoses for leaks. Also check cruise control and A/C systems vacuum hoses and components.

Fig. 23: GMC Rally G2500 1990 - Component Locations - Code 32: Circuit Diagram EGR Circuit Loop Error

Fig. 24: GMC Rally G2500 1990 - Component Locations - Code 32: Flow Chart EGR Circuit Loop Error

GMC Rally G2500 1990 - 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.

Fig. 25: GMC Rally G2500 1990 - Component Locations - Code 33: Circuit Diagram MAP Sensor Signal Voltage High

Fig. 26: GMC Rally G2500 1990 - Component Locations - Code 33: Flow Chart MAP Sensor Signal Voltage High

GMC Rally G2500 1990 - 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.

GMC Rally G2500 1990 - 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.

GMC Rally G2500 1990 - 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.

Fig. 27: GMC Rally G2500 1990 - Component Locations - Code 53: Voltage Reference Overload