Diesel Cec System - W/external EGR Buick Electra Estate Wagon

DESCRIPTION

The Diesel Electronic Control (DEC) system is used on 4.3L V6 and 5.7L V8 diesel engines (except 4.3L V6 diesel with internal EGR). A different DEC system is used on the 4.3L V6 diesel engine equipped with internal EGR. See the DIESEL INT/EGR SYSTEM article in this section (if applicable).

The DEC system monitors various engine functions to electronically control Exhaust Gas Recirculation (EGR) system, injection pump timing, reduce emissions and improve driveability at different altitudes. The diesel Electronic Control Module (ECM) is the "brain" of the system.

EGR Valve Location on 4.3L V6 Diesel. Scheme 394

SYSTEM OPERATION

The DEC system is designed to help lower exhaust emissions and improve engine performance. It consists of the following sub-systems: ECM, ECM input sensors and output emission control components, electronic EGR control, and diagnostic system.

EGR Valve Location on 5.7L V8 Diesel. Scheme 395

ECM TABLE OPERATION

The ECM analyzes signals sent from various input sensors and operates systems to control emissions and engine performance. ECM has 2 parts: controller, and Programmable Read Only Memory (PROM), which fits inside controller.

PROM contains factory programmed information including specific engine calibration data, transmission axle ratio, vehicle weight and other important data. Although a controller may be used for many different vehicles, PROM may not because of its specific data.

A/C On Signal (4.3L V6 Only)

When A/C is turned on, a 12 volt signal is sent to ECM by A/C clutch. ECM will then energize fast idle solenoid to compensate for A/C clutch load.

Brake Switch Signal (5.7L V8 Only)

When brakes are applied, brake light switch sends ECM a 12 volt signal. ECM will then delay Exhaust Pressure Regulation (EPR) to improve driveability.

Engine Coolant Temperature (ECT) Sensor

Engine temperature affects most systems controlled by ECM. Coolant sensor is a thermistor which changes resistance values with temperature. Low coolant temperature produces high thermistor resistance, and high coolant temperature produces low thermistor resistance.

ECM supplies 5 volt signal to thermistor through a resistor inside ECM. Failure in coolant sensor circuit should set code 14 or code 15.

Engine Rpm Signal

ECM monitors engine RPM through tachometer lead on alternator Terminal "R". As engine speed increases, frequency of voltage pulses from alternator increases and allows ECM to compare other input signals to determine output commands.

Manifold Absolute Pressure (MAP) Sensor

ECM uses MAP sensor to monitor vacuum in EGR circuit and to measure barometric pressure for altitude compensation. During normal engine operation, ECM compares input signal from MAP sensor with output signal (duty cycle) to EGR. If EGR control vacuum and MAP sensor signal differs more than 2.4 in. Hg, ECM will set code 53 after 10 seconds.

Metering Valve Sensor (MVS)

MVS is a variable resistor that sends metering valve position to ECM. Output of MVS is used by ECM to determine amount of EGR. Resistance of MVS is highest at closed throttle and lowest at wide open throttle.

MVS is connected to reference voltage and its output should be near 5 volts when throttle is wide open (lowest resistance). MVS failure will set check engine light and set code 21 for low MVS signal, or code 22 for high MVS signal.

Vehicle Speed Sensor (VSS)

A 12 volt signal fed to VSS by ECM is turned on and off as rear wheels turn. On and off rate of VSS varies proportionally with vehicle speed and allows ECM to control output emission control components.

3rd & 4th Gear Switch (If Equipped)

Some automatic transmissions use a switch that tells ECM when transmission is in 3rd or 4th gear. ECM uses 3rd & 4th gear switch signal to determine when to engage TCC. Transmissions equipped with 3rd & 4th gear switch can be identified by a 3 or 4 wire Torque Converter Clutch (TCC) solenoid connector.

Altitude Fuel Limiter (AFL)

The ECM uses the AFL to limit travel of metering valve at wide open throttle. AFL is mounted through end of injection pump housing and energizes whenever altitude compensation is needed. AFL receives its current from ignition and is energized by grounding circuit inside ECM.

EGR Solenoid

Vacuum to EGR valve is controlled by EGR solenoid. ECM determines on and off time of EGR solenoid. Cycling EGR solenoid on and off determines amount of EGR in engine. Supply voltage for EGR solenoid comes from ignition, and ECM energizes EGR solenoid by grounding circuit inside ECM.

If ECM recognizes a vacuum error, it turns EGR off, turns check engine light on and sets code 53. If problem is corrected, check engine light will not go off until key is turned off.

GM 4.3L V6 Diesel EGR & EPR Solenoids. Scheme 396

EPR Solenoid & Valve

EPR valve partially restricts exhaust to provide backpressure necessary for proper operation of EGR. EPR solenoid controls vacuum to EPR valve. Supply voltage for EPR solenoid comes from ignition, and ECM energizes EPR solenoid by grounding circuit inside ECM.

GM 5.7L V8 Diesel EGR & EPR Solenoids. Scheme 397

EPR circuit is turned off by ECM until coolant temperature sensor sends a signal that engine temperature is above 104° F (40°C). ECM will not recognize an electrical fault in EPR circuit, but will recognize vacuum leaks preventing operation of EPR valve.

EPR Valve Location. Scheme 398

Fast Idle Solenoid

ECM energizes fast idle solenoid whenever engine coolant temperature is below 100° F (38° C), or above 248° F (120° C), and whenever A/C clutch on 4.3L V6 is engaged.

Housing Pressure Altitude Advance (HPAA) Solenoid

ECM uses HPAA solenoid to regulate pump housing pressure in proportion to altitude. Whenever MAP sensor signals ECM that vehicle is above 4000 ft. (1220 m), ECM energizes HPAA solenoid to regulate pump housing pressure.

This altitude regulated pump housing pressure will adjust injection pump timing to improve driveability and reduce emissions at high altitudes. HPCA solenoid must also be energized with HPAA solenoid to correctly adjust injection pump timing at high altitudes.

Housing Pressure Cold Advance (HPCA) Solenoid

When engine coolant temperature is below 98°F (37°C), and/or vehicle is at altitudes above 4000 ft. (1220 m), ECM energizes HPCA solenoid which causes housing pressure to drop. A drop in housing pressure results in advanced injection pump timing which improves cold starting, cold idle, and reduces emissions.

TCC Solenoid

ECM controls TCC solenoid mounted in transmission allowing torque converter to directly connect engine and transmission. This mechanical connection eliminates slippage and improves fuel economy.

ELECTRONIC EGR CONTROL OPERATION

EGR system is electronically controlled by the ECM and consists of an EGR solenoid, EGR valve, EPR solenoid, EPR valve and vacuum pump. The vacuum pump provides the system with vacuum to operate valves. The EGR and EPR solenoids are mounted at top rear of engine as an assembly.

ECM controls on and off time (duty cycle) of the EGR solenoid. This regulates amount of EGR flow into engine by controlling vacuum applied to EGR valve. The ECM calculates the amount of EGR flow required based on information supplied by following input sensors: Engine RPM, EGR control vacuum, barometric pressure, vehicle speed and metering valve position.

A Quick Vacuum Response (QVR) valve is located in vacuum line between EGR solenoid and valve, and in vacuum line between EPR solenoid and valve. Its purpose is to quicken response time when opening and closing EGR and EPR valves.

DIAGNOSTIC SYSTEM OPERATION

ECM is equipped with a self-diagnosis system which recognizes and identifies system failures. A Check Engine Light (CEL) alerts the driver of a system problem and is used to display trouble codes when in the diagnostic mode.

Check Engine Light (CEL)

Check engine light will come on when key is turned on and go off when engine is started. If check engine light remains on when engine is running, diagnostic system has recognized a problem (hard code). If problem goes away (intermittent code), check engine light should go out after 10 seconds, and trouble code will remain stored until key is turned off.

Reading Trouble Codes

Trouble code is set by ECM and describes a specific circuit fault. ECM compares each input sensor's signal with what ECM's memory has determined it should be. If sensor signal does not compare with ECM memory, the check engine light will illuminate and a trouble code will be stored.

Trouble codes may be obtained through the Assembly Line Communication Link (ALCL), which is located in passenger compartment. (Scheme 400) Terminal "B" is diagnostic test terminal, and can be connected to Terminal "A" or ground to enter diagnostic mode.

Trouble codes are indicated by flashes of the CEL. For example, FLASH, pause, FLASH, FLASH, longer pause, identifies Code 12.

ALCL Connector. Scheme 399

Diagnostic Mode

To enter the diagnostic mode, ensure key is on and engine is stopped. Ground diagnostic test terminal (Terminal "B"). System will flash a code 12, indicating that the ECM is operating properly and can recognize a system fault. Code 12 will flash until test terminal is ungrounded.

If test terminal is grounded with engine running, system will display any stored trouble codes by flashing check engine light. Each code will be flashed 3 times. Hard codes will be repeated each time key is turned off and engine restarted. Intermittent codes will disappear when key is turned off and engine is restarted.

REMOVAL & INSTALLATION PRECAUTION

Note. Power should never be removed or applied to ECM with the ignition turned on. Before removing or connecting battery cables, fuses, or ECM connectors, always turn ignition off to prevent damage to module.

ECM

ECM is located in passenger compartment on right kick panel. If diagnostic procedures call for replacement of ECM, check first to see if ECM and PROM are the correct parts. A replacement ECM will not contain a PROM.

Removal

Turn ignition key off and remove right hand sound insulator. Remove 2 screws from lower ECM bracket and pull ECM down. Remove ECM connectors and remove ECM.

Installation

Install both ECM connectors. Install ECM so both locator pins go through upper ECM bracket. Install sound insulator.

PROM

If code 51 indicates a faulty PROM, check to see if PROM has bent pins, is fully seated in ECM, and is correctly aligned according to locating marks.

ECM Terminal Identification For 4.3L V6 & 5.7L V8 Diesel. Scheme 400

4.3L V6 W/External EGR & 5.7L V8 Diesel Wiring Diagram. Scheme 401

BASIC DIAGNOSTIC PROCEDURE

The diagnostic charts at the end of article provide a logical sequence for determining system failures. Any diagnosis of the DEC system should be performed in the following order

1. Ensure that all engine systems not related to the DEC system are operating properly. This should include a careful visual underhood inspection including: all vacuum hoses for correct routing, pinches, cuts, or disconnects; all wires in engine compartment for correct and good connections, burned or chaffed spots, pinched wires, or contact with sharp edges or hot exhaust manifolds.
2. Start with "DIAGNOSTIC CIRCUIT CHECK" chart and continue diagnosis as indicated in chart. This check will either verify proper DEC system and ECM operation, or direct you to a specific trouble code chart(s) or another DEC system chart.

Note. When measuring DEC circuit voltages, use a Digital Volt/Ohmmeter (DVM) with 10 megohm impedance.

TROUBLE CODE IDENTIFICATION

Check engine light will go on approximately 10 seconds after the ECM recognizes a problem. If malfunction clears, check engine light will go out and trouble code will be set in ECM. Code 53 will clear only after key is turned off. Code 12 does not store in memory.

If check engine light goes on intermittently, and does not store any codes after key is turned off and engine is restarted, see INTERMITTENT CHECK ENGINE LIGHT in TROUBLE SHOOTING section of this article.

ECM TROUBLE CODE IDENTIFICATION

EGR VALVE

1. With engine at idle speed and air cleaner removed, apply 21 in. Hg to EGR valve port. When valve is open, exhaust gases passing through EGR valve and into air crossover should be heard.
2. When vacuum is bled off, and valve closed, there should be a noticeable reduction of sound. If a sound difference is not heard, remove EGR valve and inspect valve and valve seat. If valve cannot be corrected, replacement is necessary.

EPR VALVE

Connect vacuum pump to EPR valve and apply 7 in. Hg. Check that there is no leakage and that valve is open. Apply 12 in. Hg to EPR valve and check that valve is fully closed. Check that valve shaft does not bind in bushings.

QVR VALVE

1. Install vacuum gauge in vacuum line between QVR valve and EPR valve.
2. Disconnect hose from inlet port "S" and connect vacuum pump to valve. Apply 22 in. Hg and check that vacuum gauge reads 20.7 in. Hg within 1.7 seconds.
3. Reduce vacuum at vacuum pump to 1 in. Hg and check that other vacuum gauge reads the same within 1/2 second.

Testing Quick Vacuum Response (QVR) Valve. Scheme 402

INTERMITTENT CHECK ENGINE LIGHT

An intermittent check engine light comes on at times, but does not stay on. A trouble code may be stored, but will erase after key is turned off and engine is restarted. Do not use trouble code charts to trouble shoot intermittent check engine light faults. This may result in replacement of good parts.

Use the following procedure to trouble shoot intermittent check engine light faults

1. Perform careful visual inspection of vacuum hoses, checking for splits, kinks, deterioration, and proper connections as shown on emission control information label. Check fuel injection system for leaks.
2. Check for backed out connector halves (not fully seated), damaged or collapsed terminal connectors, and broken wires.
3. If visual examination does not reveal problem, connect voltmeter to suspected circuit and ground. Drive car and watch voltmeter to see if it changes at same moment that problem is noticed.
4. Check for intermittent ground in circuit between check engine light wire and ECM, between diagnostic test terminal on ALCL and ECM, and between ECM Terminal "K" and engine ground. Also see Chart A1, "CHECK ENGINE" LIGHT MALFUNCTION in this article.
5. Check for electrical system interference caused by defective relay, ECM driven solenoid or switch.
6. Check for improper installation of electrical options, including lights and 2-way radios.
7. Check for open diode or resistor across A/C compressor clutch. Check wiring diagram for other open diodes.

DIAGNOSTIC CIRCUIT CHECK

The Diagnostic Circuit Check ensures "CHECK ENGINE" light works, ECM is operating and can recognize a fault, and determines if any trouble codes are stored. Memory is cleared on the DEC system any time ignition switch is turned off. Therefore, unless the engine is left running when vehicle is brought in for complaint or code is reset during the diagnostic circuit check, the problem should be considered intermittent. This is the starting point for any diagnosis.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. To check operation of "CHECK ENGINE" light, turn key to "ON", engine not running. The light should be on steady.
2. Ground the "Test" terminal. This will flash a Code 12 and any stored trouble 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. See Chart A1 "CHECK ENGINE" LIGHT (CEL) MALFUNCTION in this case.
3. This step determines if fault is still present or was intermittent and is no longer there. Memory is cleared when ignition is cycled off and engine is run for 30 seconds to see if trouble code(s) will reset.
4. If light is on, fault is still present. Go to applicable trouble code chart.
5. If light is off, fault is either intermittent, or is a code that cannot be set in the stall (24 or 41). For codes that cannot be set in the stall during the Diagnostic Circuit Check, the vehicle may be run on hoist or road tested.

Flow Chart - Diagnostic Circuit Check. Scheme 403

Flow Chart - Diagnostic Circuit Check. Scheme 404

CHART A1, "CHECK ENGINE" LIGHT (CEL) MALFUNCTION

The "CHECK ENGINE" light (CEL) is used to indicate the ECM has recognized a fault, and to aid in diagnosis of the fault by displaying codes. It is supplied 12 volts by the gauges fuse, and grounded by the ECM to turn the light on. The light should be on with key "ON", and should go off when engine speed exceeds 400 RPM.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. If CEL is off, system problem is indicated.
2. CEL should light when ECM terminal "E" is grounded. If it does not, an open in CEL circuit is indicated.
3. Check for open circuit to ECM. Ensure voltage to ECM at terminal "E" battery voltage (10 volts minimum).
4. Determines if problem is faulty ECM, connections or poor ground.
5. If CEL does not flash when ALCL "Test" terminal is grounded, ECM's memory cannot be accessed for fault codes.
6. Determines if fault is grounded CEL circuit, open ALCL "Test" terminal, or faulty ECM.
7. Fault is either grounded ALCL "Test" terminal circuit or faulty ECM.

Chart A1, "CHECK ENGINE" Light (CEL) Malfunction. Scheme 405

Chart A1, "CHECK ENGINE" Light (CEL) Malfunction. Scheme 406

CODE 12, NO RPM SIGNAL-VEHICLE STANDING

Code 12 means ECM is on and sees no RPM signal from the alternator. This is a normal code 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 either an open or ground in tach signal circuit. Code 41 will appear if engine is running with no RPM signal and vehicle is moving. Code 41 will store, even if the problem clears.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. This step checks for voltage output at alternator "R" terminal which is source of RPM signal. Check for voltage increase as engine speed is increased.
2. This step determines if a RPM signal is being sent to ECM. The voltage reading should be approximately the same as that noted in step 1). A voltage increase indicates signal is being generated by the alternator and fault is a bad connection at the ECM, or faulty ECM. To check ECM connection, the terminal must be removed from connector.
3. With an open circuit, there is still a small amount of voltage at the ECM. It will not increase when throttle is opened.

Code 12, No RPM Signal. Scheme 407

Code 12, No RPM Signal. Scheme 408

COOLANT TEMPERATURE SENSOR (CTS) CIRCUIT - VOLTAGE LOW

Code 14 indicates ECM has read low resistance of the Coolant Temperature Sensor (CTS) circuit for a time longer than specified. Causes may be low resistance (high engine temperature), low voltage (at ECM terminal "W"), or grounded Coolant Sensor Circuit.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. This step determines whether fault is at Coolant Sensor or elsewhere in circuit. Normal voltage should be about 4 volts or over in circuit.
2. This step checks for grounded circuit between ECM and Coolant Sensor. Test light to battery positive terminal should be off in an ungrounded circuit. Coolant Sensor is not grounded during test.

Code 14, Coolant Sensor Circuit. Scheme 409

Code 14, Coolant Sensor Circuit. Scheme 410

COOLANT TEMPERATURE SENSOR (CTS) CIRCUIT - VOLTAGE HIGH

Code 15 indicates the ECM has detected high resistance in the Coolant Sensor circuit for a time longer than specified. Causes may be high resistance (cold engine temperature), high voltage (at ECM terminal "W"), or open Coolant Sensor circuit.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. This step determines whether the fault is at Coolant Sensor or elsewhere in the circuit. Normal voltage should be about 4 volts or over in the circuit.
2. This step checks for an open circuit between ECM terminal "J" and Coolant Sensor. Coolant Sensor is not connected during test.
3. Determines if problem is in CTS circuit to ECM terminal "W" or ECM. A voltage reading of under 4 volts would indicate poor contact at ECM terminal "W" or faulty ECM. A reading of 4 volts or over would indicate an open Coolant Sensor signal lead to ECM terminal "W".

Code 15, Coolant Sensor Circuit. Scheme 411

Code 15, Coolant Sensor Circuit. Scheme 412

CODE 21, METERING VALVE SENSOR (MVS)

Code 21 indicates ECM has read either low voltage at ECM terminal "R" grounded MVS signal circuit for a time in excess of 10 seconds.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. Checks for reference voltage at MVS harness connector. Normal reading should be 4 volts or above.
2. Determines if low voltage in step 1) was due to open in sensor return or low 5 volt reference voltage supply at MVS terminal "D".
3. Checks if low reference voltage at MVS terminal "D" is due to grounded wire pulling reference voltage low or open circuit (including ECM).
4. Separates an electrical circuit problem from a faulty MVS. If the circuit is good, normal voltage reading will be above 4 volts.

Code 21, Metering Valve Sensor (MVS). Scheme 413

Code 21, Metering Valve Sensor (MVS). Scheme 414

CODE 22, METERING VALVE SENSOR (MVS)

Code 22 indicates ECM has read a high MVS voltage below a specified RPM, below a specified engine load, or for a time in excess of 10 seconds. Due to the pull-up resistor between terminals "C" and "R" within ECM, an open in MVS circuit will place about 5 volts (high MVS signal) at terminal "R" of ECM.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. Checks circuit from MVS connector back to ECM terminal "C". Voltage should be above 4 volts because of pull-up resistor in ECM. CAUTION: A 10 Meg. ohm resistance ohmmeter must be used. A lower resistance voltmeter would read virtually zero.
2. Checks to see if low voltage at MVS connector is an open in the circuit or faulty ECM. A normal reading at ECM is above 4 volts.
3. This tests resistance of MVS itself. A normal reading is under 20,000 ohms.

Code 22, Metering Valve Sensor (MVS). Scheme 415

Code 22, Metering Valve Sensor (MVS). Scheme 416

CODE 24, VEHICLE SPEED SENSOR (VSS)

The ECM applies and monitors 12 volts on circuit 437. Circuit 437 connects to the VSS which alternately grounds circuit 437 when drive wheels are turning. This pulsing action takes place about 2000 times per mile and the ECM will calculate vehicle speed based on time between pulses.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. This test monitors ECM voltage on circuit 437. With wheels turning, a varying voltage will be present. Variation will be greater at low wheel speeds to an average of 4-6 volts at about 20 MPH.
2. Less than 1 volt at ECM connector indicates circuit 437 wire is shorted to ground. Disconnect circuit 437 at VSS. If voltage now above 10 volts, VSS is faulty. If less than 10 volts, then circuit 437 is grounded.
3. A steady 8-12 volts at ECM connector indicates circuit 437 is open or a faulty VSS.
4. This normal voltage and indicates possible intermittent condition.

Code 24, Vehicle Speed Sensor. Scheme 417

Code 24, Vehicle Speed Sensor. Scheme 418

CODE 41, NO RPM SIGNAL - VEHICLE IN MOTION

Code 41 indicates ECM is on and reads no RPM signal from alternator. Code 41 could mean an open or ground in tachometer signal circuit, if engine is running with no RPM signal and vehicle is moving. If problem clears, Code 41 will store.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. This step checks for voltage output at alternator "R" terminal which is source of RPM signal. Check for voltage increase as engine speed is increased.
2. This step indicates if RPM signal is being sent to ECM. Voltage reading should be approximately same as was noted in step 1). A voltage increase indicates signal is being generated by alternator and fault is bad connection at the ECM, or faulty ECM. To check connection at ECM, terminal must be removed from connector.
3. With an open circuit, there is still a small amount of voltage at ECM. It will not increase as throttle is opened.

Code 41, No RPM Signal-Vehicle In Motion. Scheme 419

Code 41, No RPM Signal-Vehicle In Motion. Scheme 420

CODE 51, PROM ERROR

Code 51 sets if there is a faulty PROM, PROM is improperly installed, or some PROM pins are not making good contact. Ensure PROM is inserted properly into the ECM. Ensure PROM is installed in the correct direction. The PROM may not set a code if installed backwards.

Code 51, PROM Error - Location of PROM. Scheme 421

CODE 53, EGR CONTROL ERROR

During normal operation, ECM compares its EGR duty cycle signal with the MAP absolute pressure signal and makes corrections in the duty cycle accordingly. If actual EGR control pressure (line vacuum) varies more than 2.4 in. Hg from what ECM has previously determined pressure should be, and variance continues for more than 10, Code 53 will be set. ECM will shut down EGR. The "CHECK ENGINE" light will remain on and EGR off until ignition switch is cycled off.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. Checks if adequate control signal vacuum is available to MAP sensor.
2. Tests MAP sensor's ability detect a variance in control signal vacuum and adjust voltage output accordingly to ECM.
3. Determines if EGR control error was detected and Code 53 set due to EGR Solenoid malfunction or MAP Sensor problem.

Code 53, EGR Control Error. Scheme 422

Code 53, EGR Control Error. Scheme 423

CODE 55, 5 VOLT REFERENCE CIRCUIT

The ECM regulates ignition supplied voltage from ECM terminal "A" down to 5 volts, which it supplies to MAP Sensor and Metering Valve Sensor (MVS) through ECM terminal "C". To aid in diagnosis, a pull-up resistor is connected between 5 volt reference terminal "C" and MVS terminal "R" inside ECM. If MVS circuit from ECM terminal "R" is open, voltage reading at terminal "R" will be about 5 volts. Code 55 indicates ECM reads voltage that is too high or too low at ECM terminal "C".

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. Checks ECM terminal "C" voltage with ignition on and engine stopped. Voltage should read 4-6 volts.
2. Determines if high voltage is result of short to battery voltage to or faulty ECM.
3. Determines if low voltage is due to grounded circuit, MAP sensor, MVS, or faulty ECM.
4. Since voltage reading in step 1) was normal, intermittent short or ground in ECM terminal "C" circuits may be at fault.

Code 55, 5-Volt Reference Circuit. Scheme 424

Code 55, 5-Volt Reference Circuit. Scheme 425

CHART C2, MAP SENSOR CHECK

A Manifold Absolute Pressure (MAP) sensor is used to monitor vacuum in the EGR vacuum circuit. It also sends the ECM a barometric pressure (altitude) signal when the ignition switch is on without the engine running or with the engine running when EGR is not in operation. This signal allows the ECM to control altitude related injection pump timing and metering valve travel at wide open throttle. The barometric signal is also used by the ECM along with absolute EGR control pressure, coolant temperature, metering valve position, TCC state, vehicle MPH, and engine RPM to determine and control EGR Pulse Width Modulated (PWM) duty cycle.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. Checks for reference voltage at MAP sensor harness. Normal voltage at terminal "C" of MAP harness should be about 5 volts.
2. Checks for an open ground in the MAP circuit to ECM. Normal voltage is about 5 volts with a good ground.
3. Checks for proper circuit from the MAP signal wire to ECM terminal "P". Incorrect voltage is an open or grounded wire, or faulty MAP sensor. Normal voltage is shown on table at bottom of chart.

Chart C2, Map Sensor Check. Scheme 426

Chart C2, MAP Sensor Check. Scheme 427

CHART C3, EGR SOLENOID CHECK

The EGR solenoid controls vacuum to the EGR valve. The signal from the ECM is Pulse Width Modulated (PWM), which changes the cycle (on time) from 0% to 100%. As the EGR solenoid cycles, control vacuum to the EGR valve is metered. The solenoid operates on 12 volts, supplied by the ignition. The ground is completed by the ECM to turn the solenoid on, allowing vacuum to the EGR valve. A vacuum error will set Code 53 and could be the result of a faulty EGR solenoid. When a vacuum error is sensed by the ECM, EGR is turned off. It takes approximately 10 seconds for the ECM to sense a vacuum error, and the CEL will not go off until the key is turned off.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. 1) Checks for ignition voltage to terminal "G" of ECM. Normal reading will be 10 volts or more.
2. 2) Checks for ignition voltage to EGR solenoid. Should normally be battery voltage.
3. 3) This checks for EGR control by the ECM. With the engine at idle, voltage will drop below battery voltage as PWM solenoid cycles. If voltage falls for 10 seconds, then goes back to battery voltage, ECM has recognized a vacuum error and has shut the EGR off. Constant battery voltage indicates no ECM control. Normal voltage is battery voltage with key "ON", and below battery voltage and varying, with engine at idle.
4. 4) Checks for a grounded wire between ECM and EGR solenoid. Test light should normally be off.
5. 5 & 5A) Checks for proper resistance of EGR and EPR solenoids. Normal for a good solenoid is 20-40 ohms. If less than 20 ohms, ECM damage will result, and ECM must be replaced with the solenoid. If resistance is 20 ohms or above, no ECM damage will occur.

Chart C3, EGR Solenoid Check. Scheme 428

Chart C3, EGR Solenoid Check. Scheme 429

EXHAUST PRESSURE REGULATOR (EPR) SOLENOID CHECK

The EPR solenoid controls vacuum to the EPR valve. The EPR solenoid, when energized, allows engine pump vacuum to close the EPR valve. The same vacuum source is used for both EGR and EPR solenoids. Battery voltage is supplied from the ignition, and the ECM completes the ground to energize the solenoid. The ECM will not recognize an electrical fault in the EPR circuit, but will detect a vacuum leak causing no EPR. The EPR is turned off by the ECM when the coolant temperature is below 104°F (40°C). The Coolant Sensor signals the ECM that the engine is cold and EPR remains off until coolant temperature rises above that point.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. Checks for a short to ground on ECM side of EPR solenoid, in wire to ECM, or ECM itself.
2. Checks for normal solenoid on signal with engine running. Test light will normally be on if both ignition circuit to terminal "A" of solenoid is complete, and ECM completes the circuit to ground at solenoid terminal "B".
3. Checks for proper ECM control of EPR solenoid. Normal response to above 2500 RPM is to shut EPR off. The test light should normally be off above 2500 RPM.
4. Determines if fault is in EPR control lead or ECM.

Chart C4, EPR Solenoid Check. Scheme 430

Chart C4, EPR Solenoid Check. Scheme 431

CHART C5, VACUUM CONTROL CHECK

During normal operation, the ECM compares its EGR duty cycle signal with MAP absolute pressure signal and makes corrections in duty cycle accordingly. If actual EGR control pressure (line vacuum) varies more than 2.4 in. Hg from what ECM has previously determined pressure should be, and variance continues for more than 10 seconds, a Code 53 will be set and ECM will shut down the EGR. The CEL will remain on and EGR off until the ignition switch is cycled off.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. Checks if there is good source of vacuum.
2. Tests MAP sensor's ability to recognize a vacuum leak and ECM's ability to control EGR.
3. Breaks vacuum circulating into 2 parts: supply and EGR/EPR circuit. If vacuum increases when hose is pinched closed, leak is not on supply side.

Note. Vacuum readings must be observed immediately (within 10 seconds) because ECM will stop PWM signal to EGR solenoid if MAP sensor signal voltage increases sufficiently above calibrated value, indicating a vacuum leak. If leak is between EGR solenoid and EGR valve, test will not work.

Chart C5, Vacuum Control Check. Scheme 432

HOUSING PRESSURE COLD ADVANCE (HPCA) CHECK

The Housing Pressure Cold Advance (HPCA) solenoid operates in conjunction with fuel injection pump return line connector/pressure regulator. During normal operation, the HPCA will be energized by the ECM at engine coolant temperatures of 66.6°F (37°C) and below and/or at altitudes above 4000 ft. When the HPCA solenoid is energized, the HPCA solenoid plunger extends and pushes the pressure regulator ball off its seat. Housing pressure then drops which results in advanced fuel injection pump timing. At altitudes above approximately 4000 ft. housing pressure and injection pump timing are affected also by operation of ECM controlled Housing Pressure Altitude Advance (HPAA). Advanced injection pump timing is desired on a cold engine to reduce emissions, white smoke, and noise as well as improving cold starting and idle quality.

Note. This chart is used for HPCA diagnosis only.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. Checks for available ignition voltage through the HPCA solenoid and checks for ground on circuit from ECM terminal "B" to HPCA solenoid. Light should be on.
2. By disconnecting the MAP sensor, altitude is simulated and the ECM should energize the HPCA solenoid (light off).
3. Determines if problem is due to faulty ECM or open circuit.
4. Checks for available ignition voltage at HPCA solenoid, identifying source of problem as ignition feed, grounded circuit from ECM, or open solenoid.

Chart C6, HPCA Electrical Check. Scheme 433

Chart C6, HPCA Electrical Check. Scheme 434

The Housing Pressure Altitude Advance (HPAA) solenoid is located the fuel return line between the fuel injection pump and tank. timing modification according to altitude requirements. The HPAA is ECM controlled and is energized whenever the MAP sensor signals the ECM of a barometric altitude reading of about 4000 ft. or more. The HPCA must be activated with HPAA so proper housing pressure (injection timing) modification can occur.

An Altitude Fuel Limiter (AFL) is mounted through the end of the injection pump housing. The AFL is an ECM controlled solenoid with a normally extended plunger which, upon AFL activation, retracts and limits metering valve travel at wide open throttle. Both HPAA and AFL are supplied with ignition voltage whenever the ignition switch is on. Both are also energized by the same ECM terminal "L", which completes their ground circuit whenever altitude compensation is required.

HPAA/AFL ELECTRICAL CHECK

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. Checks for available ignition voltage through HPAA or AFL solenoid and checks for ground on circuit from ECM terminal "L" to solenoids. With MAP sensor connector unplugged and jumper wire installed between terminals "B" and "C", a low altitude environment is simulated and solenoids should be de-energized (extended). Test light should be on.
2. This step checks for open solenoids.
3. Removing jumper wire and having MAP sensor unplugged, simulates an altitude of over approximately 4000 ft. Both solenoids should be energized (retracted) and test light off.
4. This step determines if problem is faulty ECM, circuit, or solenoids.

Chart C7, HPAA/AFL Electrical Check. Scheme 435

Chart C7, HPAA/AFL Electrical Check (1 Of 2). Scheme 436

Chart C7, HPAA/AFL Electrical Check (2 Of 2). Scheme 437

CHART C8-1,TCC CHECK

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. Checks voltage from ignition switch through brake switch, 2nd gear apply switch (if equipped), and TCC switch. Light should have turned on by 35 MPH. Due to variations in specific TCC calibrations, it is possible to have a narrow margin between 3rd gear apply switch closing (if equipped) and ECM grounding of the TCC circuit from ECM terminal "P". Test light may turn on momentarily within this margin.
2. Checks ECM ground for TCC solenoid. Light should go off.
3. This increases throttle opening to increase MVS output. If MVS output is too low, clutch will not apply. On some applications, running free does not require enough throttle opening to allow the transmission to shift.
4. Checks for low voltage at MVS input at ECM. At wide open throttle, voltage should be approximately 5 volts. Too low a MVS output should prevent TCC from applying.
5. Checks for VSS signal at ECM. VSS signal is necessary to engage TCC.
6. Checks for open in circuits to terminal "S". ECM supplies 12 volts to this terminal through a resistor. Normally both circuits should have low voltage readings since circuits are normally closed with vehicle stopped. An open circuit will read about 12 volts.
7. Switch(es) open when transmission up shifts. Check that transmission circuit functions normally by voltage going high (near battery voltage) as switches open.

Chart C8-1, TCC Check. Scheme 438

CHART C8-2, TCC CHECK (CONT.)

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. Checks for ground in circuit to ECM terminal "D". Normally light should be off.
2. Checks for ignition voltage to terminal "A" of the transmission connector. Light should normally be on.
3. Checks for complete circuit from ignition to ground through the TCC test terminal. Normally, the light should go on if the harness is good.
4. On THM 125-C, a normally open 2nd gear apply switch is used in series in ignition feed circuits to TCC solenoid. Switch is closed on 2nd gear apply by hydraulic pressure. If this step is completed but no problem found, check continuity through 2nd gear switch with transmission operating in 2nd gear.

Chart C8-2, TCC Check (Cont.). Scheme 439

CHART C9, FAST IDLE SOLENOID ELECTRICAL CHECK

The fast idle solenoid is an ECM controlled and normally retracted (de-energized). It is extended (energized) on both 5.7L and 4.3L applications when engine coolant temperature is below 100.4°F (38°C) or above 216°F (120°C). It is also extended on 4.3L engines to maintain correct idle speed when the A/C clutch is engaged.

Note. The following step numbers refer to the numbers in the accompanying flow chart(s).

1. With engine at normal operating temperature and A/C off (4.3L) fast idle solenoid plunger should be retracted (de-energized).
2. Disconnecting coolant temperature sensor simulates a cold engine. Fast idle solenoid should extend.
3. Determines if solenoid malfunction is caused by open solenoid, open circuit, or ECM.
4. Solenoid is extended when it should be retracted. This step checks for grounded circuit, grounded solenoid, or faulty ECM.

Chart C9, Fast Idle Solenoid Electrical Check. Scheme 440

Chart C9, Fast Idle Solenoid Electrical Check. Scheme 441

WIRING DIAGRAM