2.8l Pfi Tests W/codes Pontiac 6000 I

MODEL IDENTIFICATION

Repair procedures in this article are sometimes identified by a specific body code. The following table lists GM division, model name, and body types that apply to the body codes.

MODEL IDENTIFICATION

DESCRIPTION

The computerized engine control system monitors as many as 19 engine/vehicle functions. (Scheme 316) This system controls engine operation and lowers exhaust emissions while maintaining fuel economy and driveability. The Electronic Control Module (ECM) is the "brain" of the CCC system.

The computerized engine control system is primarily an emission control system, designed to maintain a 14.7:1 air/fuel ratio under all operating conditions. When the ideal air/fuel ratio is maintained, the 3-way catalytic converter can control Oxides of Nitrogen (NOx), Hydrocarbon (HC) and Carbon Monoxide (CO) emissions.

ECM Conditions Sensed & Systems Controlled. Scheme 316

CONDITIONS TO BE MET BEFORE TESTING

Note. The following conditions must be met before testing

1. Engine at operating temperature.
2. Engine in closed loop operation.
3. Engine idling ("Engine Run" column).
4. Test terminal NOT grounded.
5. Scanner or ALDL tool NOT installed.

OPERATION

The ECM of computerized engine control system is equipped with a self-diagnostic system which detects system failures or abnormalities. As a bulb and system check, "SERVICE ENGINE SOON" light will glow when ignition switch is turned to "ON" position and engine is not running. When engine is started, light should go out. If not, a malfunction has been detected in the computerized engine control system or "SERVICE ENGINE SOON" light circuit is faulty.

When a malfunction occurs, ECM will illuminate the "SERVICE ENGINE SOON" light located on instrument panel. When malfunction is detected and light is turned on, a corresponding trouble code will be stored in ECM memory. Malfunctions are recorded as "hard failures" or as "intermittent failures".

"HARD FAILURES"

Hard failures cause "SERVICE ENGINE SOON" light to glow and remain on until the malfunction is repaired. If light comes on and remains on during vehicle operation, cause of malfunction must be determined using diagnostic charts. If a sensor fails, ECM will use a substitute value in its calculations to continue engine operation. In this condition, vehicle is driveable, but loss of good driveability will most likely be encountered.

"INTERMITTENT FAILURES"

Intermittent failures cause "SERVICE ENGINE SOON" light to flicker or illuminate and go out about 10 seconds after the intermittent fault goes away. The corresponding trouble code, however, will be retained in ECM memory. If related fault does not reoccur within 50 engine restarts, related trouble code will be erased from ECM memory. Intermittent failures may be caused by sensor, connector or wiring related problems. See INTERMITTENTS in the CEC TESTS W/O CODES article in this section.

Note. Trouble codes will be recorded at various operating times. Some codes require operation of that sensor or switch for 5 seconds. Others may require operation for 5 minutes or longer under engine load.

BASIC DIAGNOSTIC PROCEDURE

Note. Most computerized engine control problems are the result of mechanical breakdowns, poor electrical connections or damaged vacuum hoses. Before considering the computer system as a possible cause of problems, ignition high tension wires, fuel supply, electrical connections and vacuum hoses should be checked. Failure to do so may result in lost diagnostic time.

Diagnosis of the computerized engine control system should be performed in the following order

1. Make sure that all engine systems not related to the computer system are operating properly. Do not proceed with testing unless all other problems have been repaired.
2. Perform appropriate DIAGNOSTIC CIRCUIT CHECK for that system. If trouble codes were displayed (other than Code 12), decide whether codes are "hard" or "intermittent" trouble codes. "Hard" codes will cause the "SERVICE ENGINE SOON" light to illuminate continuously while engine is running. See ECM TROUBLE CODE DEFINITIONS table in this article.
3. If no trouble codes were displayed, perform FIELD SERVICE MODE CHECK procedures.
4. If no trouble is indicated by the FIELD SERVICE MODE check and/or a driveability problem exists, refer to SYMPTOM DIAGNOSIS and/or SCAN TESTER USAGE in the TROUBLE SHOOTING procedures in the CEC TESTS W/O CODES article in this section. The comments there will send you to the proper component charts or tell you the most likely system/component to check.
5. After any repairs are made, clear any trouble codes and perform FIELD SERVICE MODE check again.

Scheme 317

1. Turn ignition on. Do not start engine. "SERVICE ENGINE SOON" light should glow. Locate Assembly Line Data Link (ALDL) connector attached to ECM wiring harness under instrument panel, left or right of steering column (under cigar lighter plate in center console on Fiero). Insert jumper wire across terminal "B", "DIAGNOSTIC TERMINAL" and terminal "A", "GROUND". (Scheme 317) CAUTION: Inserting spade lug (jumper lead) into terminals of ALDL connector grounds "DIAGNOSTIC TERMINAL". Do not ground ALDL connector until after ignition is on (engine not running). (Scheme 317): ALDL Connector Terminal Identification NOTE: In some of the diagnostic and trouble shooting charts the Assembly Line Data Link (ALDL) may also be referred to as the Assembly Line Communication Link (ALCL). These are referring to the same connector. It is also the test point for connection of aftermarket "Scan" testers.
2. "SERVICE ENGINE SOON" light should flash Code "12". Code "12" consists of "FLASH", pause, "FLASH", "FLASH" followed by a longer pause. Trouble Code "12" will be repeated 2 more times. If any other trouble codes are stored in ECM memory, they will be displayed in the same manner.
3. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.

READING TROUBLE CODES

The ECM stores component failure information for the CCC system under a related trouble code which can be recalled for diagnosis and repair. Trouble codes may be read by counting flashes of the "SERVICE ENGINE SOON" light, or by reading the output of a diagnostic "Scan" tester connected to the ALDL connector. The tester is faster, more accurate, and capable of reading information which otherwise would necessitate testing individual ECM and sensor/solenoid connector terminals with a volt/ohmmeter. See SCAN TESTER - TEST DATA PARAMETERS table and SCAN TESTER USAGE in this article.

If "Scan" tester is not available, it is possible to read flashes of the dashboard "SERVICE ENGINE SOON" light by grounding the diagnostic terminal of the ALDL with ignition on and engine off. For example, "FLASH", "FLASH", pause, "FLASH", longer pause, identifies "21". The first series of flashes are the first digit of trouble code; second series of flashes are the second digit of trouble code. Trouble codes are displayed starting with the lowest numbered code. Each code is displayed 3 times. Codes will continue to repeat as long as ALDL "DIAGNOSTIC TERMINAL" is grounded.

Note. Trouble codes will be recorded at various operating times. Some codes require operation of that sensor or switch for 5 seconds; others may require operation for 5 minutes or longer at normal operating temperature, road speed and load. Therefore, some codes may not set in a service bay operational mode.

ECM TROUBLE CODE DEFINITIONS

ECM TROUBLE CODE DEFINITION

Note. Trouble code charts should only be used if "SERVICE ENGINE SOON" light is illuminated (indicating a current problem exists). Exceptions are Code 13, 15, 24, 44 and 45 charts, which may be used to help diagnose intermittent codes.

Note. Any time Codes 51, 52, 54 or 55 are displayed with another code, start with "50-series" code first, then proceed to low profile numbered code.

TROUBLE CODE DETERMINATION (HARD OR INTERMITTENT)

During any diagnostic procedure, you must decide between "hard" failure codes and "intermittent" failure codes. Diagnostic charts will not usually help analyze "intermittent" codes. To determine "hard" codes and "intermittent" codes, proceed as follows

1. Manually enter diagnostic mode. Read and record all stored trouble codes. Exit diagnostic mode and clear trouble codes.
2. Apply parking brake and place transmission in Neutral (man. trans.) or "P" (auto. trans.). Block drive wheels. Start engine. "SERVICE ENGINE SOON" light should go out. Run warm engine at specified curb idle for 2 minutes. Note "SERVICE ENGINE SOON" light.
3. If "SERVICE ENGINE SOON" light comes on, enter diagnostic mode. Read and record trouble codes. This will reveal "hard failure" codes. Codes 13, 15, 24, 44, 45 and 55 may require a road test to reset "hard failure" after trouble codes were cleared.
4. If "SERVICE ENGINE SOON" light does not come on, all stored trouble codes were "intermittent failures". Exceptions are noted under DIAGNOSTIC PROCEDURE.

CLEARING TROUBLE CODES

Turn ignition switch to "ON" position and ground "DIAGNOSTIC TERMINAL" lead at ALDL connector. Turn ignition switch to "OFF" position and remove ECM fuse from fuse block for 10 seconds. Replace fuse. Remove "DIAGNOSTIC TERMINAL" ground lead.

DIAGNOSTIC MATERIALS

Note. The charts described in the following paragraphs are arranged later in this article, by engine size and fuel system type.

DIAGNOSTIC CHARTS

The diagnostic charts are used to find and repair problems which the on-car diagnostics have found. These charts include

1. Charts which test the reliability of the self-diagnostic system.
2. Charts which help fix problems which are "SERVICE ENGINE SOON" light related.
3. Charts which test the computerized fuel control system performance.
4. Charts which help fix a problem when the on-car diagnostics don't work.
5. ENGINE CRANKS BUT WON'T RUN charts. Refer to the appropriate TROUBLE SHOOTING chart in the CEC TESTS W/O CODES article in this section.
6. Charts where a stored trouble code leads you to a particular problem. See ECM TROUBLE CODE DEFINITION and DIAGNOSTIC AIDS in this article. Charts which are used because the FIELD SERVICE MODE CHECK found a problem.

Note. Although there are many charts connected with computer diagnosis, only 2 charts are needed to prove system is operating properly. Normally, only 3 charts are necessary to find a problem, if one exists.

DIAGNOSTIC AIDS

Diagnostic aids (located in each "trouble code" chart box for each system) are additional tips used to help diagnose trouble codes when inspected circuit checks out okay. Diagnostic aids may help lead to a definitive solution to that trouble code problem.

FIELD SERVICE MODE CHECK (FUEL INJECTED MODELS)

On fuel injected models, "SERVICE ENGINE SOON" light will indicate operational mode of engine if ALDL is grounded while engine is running. In closed loop mode, "SERVICE ENGINE SOON" light will flash at a rate of one flash per second. In open loop, light will flash at a rate of 2.5 flashes per second. If light is off all or most of the time, a lean exhaust is indicated. If light is on all or most of the time, a rich exhaust is indicated.

This test confirms proper operation of fuel system and verifies closed loop operation. Clear codes and perform this test after any repair is completed. When performing this check, always engage parking brake and block DRIVE wheels. Parking brake on front-wheel drive models does NOT hold drive wheels.

Note. On some engines, oxygen sensor will cool off after only a short period of time while engine is idling. This will cause engine to go into open loop. To restore closed loop mode, run engine at part throttle several minutes and accelerate from idle to part throttle several times.

SPECIAL DIAGNOSTIC TOOLS

Note. 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 (terminal "E" on EFI and terminal "M" on EFI with P-4 systems). These testers can save a great deal of time. For additional information see SCAN TESTER USAGE and SCAN TESTER - TEST DATA PARAMETERS table in this article.

The computerized engine control system is most easily diagnosed using a "Scan" tester, however, other tools may aid in diagnosing problems if a "Scan" tester is unavailable. These are: tachometer, test light, ohmmeter, digital voltmeter with 10-megohm impedance (minimum), vacuum pump, vacuum gauge, fuel injector test lights and 6 jumper wires 6" long (one wire with female connectors at both ends, one wire with male connector at both ends and 4 wires with male and female connectors at opposite ends). A test light, rather than a voltmeter, must be used when indicated by a diagnostic chart.

SCAN TESTER USAGE

Note. Prior to connection of scan tester to vehicle, diagnostic system should be checked to determine if system is operating properly and if information received by scan tester will be accurate. This is done by performing appropriate DIAGNOSTIC CIRCUIT CHECK for that system. If vehicle does not pass diagnostic circuit check, information received by scan tester may be invalid.

CCC Scan tester is a specialized tester which, when plugged into ALDL, can be used to diagnose on-board computer control stems by providing instant access to circuit voltage information without need to crawl under dash or hood to back-probe sensors and connectors.

Scan testers cut down diagnostic time dramatically by furnishing input data (voltage signals) which can be compared to specification parameters. See SCAN TESTER - TEST DATA PARAMETERS table. They also furnish information on output device (solenoids and motors) status. Status parameters, however, are only an indication that output signals have been sent to devices by the ECM. It does not indicate if devices have responded properly to that signal. This will need to be verified at output device using a voltmeter or test light.

Note. Code 12 should always exist when ALDL is grounded with key on and engine not running but may not be indicated by all makes of scan tester.

If trouble codes are not present, this is not an indication that there is not a problem. CCC related problems are about 20 percent codes and 80 percent driveability. Sensors that are out of specification WILL NOT set a trouble code but WILL cause driveability problems. Use of a scan tester is easiest method of checking sensor specifications and other data parameters. Tester is also useful in finding intermittent wiring problems by wiggling wiring harnesses and connections (key on, engine off) while observing data parameters. See the SCAN TESTER - TEST DATA PARAMETERS table below.

Note. Information obtained by scan tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, it will be necessary to verify tester information using a digital voltmeter and wiring schematic. If non-existent codes are evidenced, turn ignition off, remove tester, turn ignition on. Ground ALDL "DIAGNOSTIC TERMINAL". If same codes are not flashed by "SERVICE ENGINE SOON" light that were indicated by scan tester, tester cannot be used on vehicle as information obtained by it may not be accurate.

SCAN TESTER - TEST DATA PARAMETERS

Note. Information in the following tables is typical readings taken on vehicle with engine idling, upper radiator hose hot, closed throttle, transmission in Park or Neutral, "closed loop" status achieved and all accessories off (except as noted in tables). Data parameters are updated every 1 1/4 seconds. On systems using P-4 computers, parameter updates are virtually instantaneous. Not all devices & systems are used on all models.

PORT FUEL INJECTION TEST DATA

Note. This ECM voltage chart can be used with a digital voltmeter to help save time in diagnosis. Voltages on the car being tested my vary slightly from these due to battery or alternator charging level.

DIAGNOSTIC CIRCUIT CHECK

The diagnostic circuit check is an organized approach for identifying a problem caused by the fuel injection system. Driver comments normally fall into one of the following areas: steady "SERVICE ENGINE SOON" light, driveability problem and engine "CRANKS BUT WON'T RUN".

DIAGNOSTIC CIRCUIT CHECK PROCEDURES

1. Steady "SERVICE ENGINE SOON" light with the ignition on and engine not running confirms battery voltage to the ECM.
2. Code 12 should flash 3 times, followed by any other trouble codes stored in memory.
3. Record all stored trouble codes (except Code 12).
4. With the engine running and the diagnostic terminal grounded, the ECM will respond to the oxygen sensor signal voltage and use the "SERVICE ENGINE SOON" light to display this information as follows: Closed loop confirms the O2 sensor signal is being used by the ECM to control fuel delivery, and the system is working normally. Signal voltage will vary from below .35 to above .55 volts. Open loop indicates that oxygen sensor voltage is unusable to the ECM. Signal voltage is a constant value between .35 and .55 volts. System will flash open loop from 30 seconds to 2 minutes after engine starts or until sensor reaches normal operating temperature. If system fails to go to closed loop, see Code 13. "SERVICE ENGINE SOON" light out indicates lean exhaust. Oxygen sensor signal voltage will be less than .35 volts and steady. "SERVICE ENGINE SOON" light on steady indicates rich exhaust. O2 sensor signal voltage will be greater than .55 volts and steady.
5. Road test of the vehicle using field service mode must be done at steady speeds. In this mode, the following conditions may be observed and are considered normal: light on too long under acceleration, light off too long under deceleration, light on too long with idle below 1200 RPM.
6. Clearing codes. Ignition should be off. Remove continuous battery fuse for 30 seconds. Fuse and holder are located near battery.

Flow Chart, Diagnostic Circuit Check. Scheme 318

Flow Chart, Diagnostic Circuit Check. Scheme 319

"SCAN" DIAGNOSTIC CIRCUIT CHECK

The "SCAN" Diagnostic Circuit Check is an organized approach for identifying fuel injection problems using an Assembly Line Communication Link (ALCL). This communication link can provide diagnostic information for display on any "SCAN" device or tool designed for this purpose.

1. If the "SCAN" tool is not operating, check on another vehicle. If okay, the cigar lighter socket should be checked for 12 volts and a good ground. If the "SCAN" tool reads "no data" or "no ALCL", with the ignition "ON", check the serial data wire for an open or short to ground between ALCL terminal "E" and the ECM. Also check for an open diagnostic test terminal from ALCL terminal "B" and ECM. With ignition on, the serial data line should have a varying 2-5 volts and the diagnostic line about 5 volts.

Flow Chart, "Scan" Diagnostic Circuit Check. Scheme 320

Flow Chart, "Scan" Diagnostic Circuit Check. Scheme 321

Schematic, "Scan" Diagnostic Circuit Check. Scheme 322

CHART A1 - NO "SES" LIGHT

"SERVICE ENGINE SOON" light should be on steady when ignition is "ON" and engine is not running. Battery voltage is supplied directly to the bulb. The ECM turns the light on by grounding circuit 419 at the ECM.

If both continuous battery supply voltages are lost at terminals B1 and C16 or the ignition feed to terminal A6 is not present, the check engine light will not come on.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Solenoids are turned on or off by the ECM, using internal electronic switches called "Drivers". Each driver is part of a group of 4 called "Quad-Drivers". Failure of any driver can damage any other driver in the set. Use an ohmmeter to check resistance of solenoids or relays listed in chart.

Flow Chart A1, No "Service Engine Soon" Light. Scheme 323

Flow Chart A1, No "Service Engine Soon" Light. Scheme 324

Schematic A1, No "Service Engine Soon" Light. Scheme 325

CHART A2 - NO CODE 12 "SES" LIGHT ON STEADY

Steady "SERVICE ENGINE SOON" light should be on with ignition "ON" and engine not running. Battery voltage is supplied directly to the bulb. With the diagnostic terminal grounded the light should flash a Code 12 followed by any other trouble codes stored in memory. A steady light indicates a short to ground in the light control circuit 419, or an open in circuit 451.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. If the light goes off when the ECM connector is disconnected, then circuit 419 is not grounded. Also check the connector terminals for proper contact at this time.
2. This step checks for an open diagnostic circuit 451.
3. This step checks for a faulty ECM or PROM. If Code 51 is stored when PROM is removed, replace PROM.
4. Solenoids are turned on and off by the ECM, using internal electronic switches called "Drivers". Each driver is part of a group of 4, called "Quad-Drivers". Failure of 1 driver can damage any other driver in the set.

Chart A2, Won't Flash Code 12 "SES" Light on Steady. Scheme 326

Chart A2, Won't Flash Code 12 "SES" Light On Steady. Scheme 327

CHARTS A3A & A3B - ENGINE CRANKS, BUT WON'T RUN

Battery condition and engine cranking speed are okay, and there is adequate fuel in the tank. If engine starts but immediately stalls, SEE TROUBLE SHOOTING charts in the CEC TESTS W/O CODES article in this section.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. "SERVICE ENGINE SOON" light on shows ignition and battery supply to the ECM.
2. Fuel pump fuse should be checked at this time and if blown, repair fuel pump circuit before proceeding. Secondary voltage is checked using tool (ST-125). No spark indicates a basic HEI problem.
3. This test checks if ECM is receiving reference signal and is controlling the injectors. If the test light blinks while cranking, the ECM should be considered okay. Be sure to check both sides of engine and that the test light makes good contact between connector and terminals during test. Use test light (J-34730-2) or equivalent.
4. This test checks fuel pressure using Pressure Gauge J-34730-1). Wrap shop towel around fuel pressure tap to absorb fuel leakage when installing gauge.
5. The following can cause a "No Start" condition: An EGR valve sticking open, fouled spark plugs, TPS binding in the wide open position, defective cold start circuit or water in fuel, or a defective MAF Sensor. If all okay, SEE TROUBLE SHOOTING procedures in the CEC TESTS W/O CODES article in this section.

Flow Chart A3A, Engine Cranks But Won't Run. Scheme 328

Flow Chart A3A, Engine Cranks But Won't Run. Scheme 329

Schematic A3A, Engine Cranks But Won't Run. Scheme 330

A3B - ENGINE CRANKS, BUT WON'T RUN (CONT.)

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. 6) This test checks for 12 volt supply to injectors.
2. 7) This test determines if the distributor module is not generating the reference pulse or if the wiring or ECM are at fault. By touching circuit 430 with a test light, a reference signal is being generated. If the test light (J-34730-2) blinks a the injector, then the ECM and wiring are okay.
3. 8) If the ECM is seeing high TPS voltage it may be in the clear flood mode. Disconnecting the TPS will allow the ECM to use a default value and turn the injectors on. If the TPS ground wire and connections are okay, the TPS should be replaced.
4. 9) Each time the test light touches circuit 430, the ECM should turn the fuel pump on for 2 seconds.
5. 10) All checks made to this point would indicate that the ECM is at fault. However, there is a possibility of circuit 467 or 468 being shorted to a voltage source either in the engine harness or in the injector harness.
6. To test for this condition, disconnect the ECM connectors and injectors. Turn ignition "ON". Probe circuits 467 and 468 with a test light connected to ground. There should be no light. If light is present, repair short. If okay, check harness for terminals shorted together. Check each injector resistance. Resistance should be more than 10 ohms. If all checks okay, check ECM ground circuits before replacing the ECM.

Note. EFI system under pressure. To avoid spillage, refer to field service procedures for testing or making repairs requiring disassembly of fuel lines or fittings.

Chart A3B, Cranks, But Won't Run (Cont.). Scheme 331

Chart A3B, Cranks, But Won't Run (Cont.). Scheme 332

CHART A7A - FUEL SYSTEM DIAGNOSIS

The ECM turns the in-tank fuel pump on for 2 seconds when the ignition switch is turned "ON". It will turn it on again during cranking and will keep it on as long as the engine is running or cranking and the ECM is receiving HEI distributor reference pulses. If there are no reference pulses, the ECM will shut the fuel pump off within 2 seconds after the key is turned "ON" or engine stopped. The fuel pump will deliver fuel to the fuel rail and injectors, then to the pressure regulator, where the system pressure is controlled to 34-47 psi with the ignition on, engine stopped or during WOT. Excess fuel is returned to the fuel tank.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Use pressure gauge (J-34730-1). Wrap a shop towel around fuel pressure tap to absorb any fuel spillage when installing gauge. Ignition "ON" pump pressure should be 40.5-47 psi. This pressure is controlled by spring pressure within the regulator assembly.
2. When the engine is idling, the manifold pressure is low (high vacuum) and is applied to the fuel regulator diaphragm, this will offset the spring and result in a lower fuel pressure. The idle pressure will vary slightly depending on barometric pressure, however, the pressure idling is less indicating pressure regulator control.
3. Pressure that continues to fall is caused by one of the following: In-tank fuel pump check valve not holding, pump coupling hose leaking, pressure regulator valve leaking or an injector that is sticking open.
4. An injector sticking open can best be determined by checking for a fouled or saturated spark plug(s). If a leaking injector can not be determined by a fouled or saturated spark plug the following procedure should be used. Remove plenum, cold start valve and remove fuel rail bolts. Reconnect cold start valve. Connect a hose to valve nozzle and insert into a gasoline container. Lift fuel rail out just enough to leave injector nozzles in the ports.

Pressurize the fuel system. Lift each side of rail up and observe for injectors leaking.

Flow Chart A7A, Fuel System Diagnosis. Scheme 333

Flow Chart A7A, Fuel System Diagnosis. Scheme 334

CHART A7B - FUEL SYSTEM DIAGNOSIS

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Pressure, but less than 40.5 psi. falls into two areas: Regulated pressure, but less than 40.5 psi. Amount of fuel to injectors okay but pressure is too low. System will be lean running and may set Code 44. Can also be hard starting cold and poor overall performance. Restricted flow causing pressure drop. Normally, a vehicle with a fuel pressure of less than 24 psi. at idle will not be driveable. If the pressure drop occurs only while driving, the engine will surge and then stop as pressure drops rapidly.
2. Restricting the fuel return line allows the fuel pump to develop its maximum pressure. When battery voltage is applied to the ALCL pump test terminal "G", pressure should be approximately 60 psi.
3. This test determines if the high fuel pressure is due to a restricted fuel return line or a pressure regulator problem.

Flow Chart A7B, Fuel System Diagnosis. Scheme 335

Flow Chart A7B, Fuel System Diagnosis. Scheme 336

CHART A9 - COLD START VALVE

The cold start valve is used to provide additional fuel during the crank mode to improve cold starts. This circuit is activated only in the crank mode. The system is controlled by a thermal time switch which provides a ground path for the valve during cranking when coolant temperature is below 95°F (35°C).

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Disconnecting the distributor 4-way connector will disable the other injectors. The amount of pressure drop depends on the temperature of the engine.
2. This test will determine the continuity through the thermal switch to ground.

Flow Chart A9, Cold Start Valve. Scheme 337

Flow Chart A9, Cold Start Valve (1 Of 2). Scheme 338

Flow Chart A9, Cold Start Valve (1 Of 2). Scheme 339

Schematic A9, Cold Start Valve. Scheme 340

CODE 13 - OPEN OXYGEN (O2) SENSOR CIRCUIT

Code 13 will set with the engine operating temperature at least 2 minutes after engine start. The ECM must see: O2 signal voltage steady between .35 and .55 volts for more than 1 minute, or see TPS signal indicating not at idle. The ECM supplies a voltage of about .45 volt between terminals "D7" and "D8". (If measured with a 10 megohm digital voltmeter, this may read as low as .32 volts). The O2 sensor varies the voltage within a range of about 1 volt if the exhaust is rich, down to about .10 volt if the exhaust is lean.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Grounding the diagnostic terminal with the engine running enables the "Field Service Mode", allowing the ECM to confirm either open or closed loop operation using the "SERVICE ENGINE SOON" light.
2. This step verifies no additional codes stored, and that Code 13 is intermittent. SEE TROUBLE SHOOTING charts in the CEC TESTS W/O CODES article in this section.
3. This step simulates a lean exhaust. If the ECM and wiring are okay, the ECM will see the lean condition and turn the "SERVICE ENGINE SOON" light off at least 30 seconds after engine start and then flash open loop. It is normal for the light to remain off for a longer period of time before flashing open loop.

Code 13 Flow Chart, Open O2 Sensor Circuit. Scheme 341

Code 13 Flow Chart, Open O2 Sensor Circuit. Scheme 342

Code 13 Schematic, Open O2 Sensor Circuit. Scheme 343

CODE 14 - CTS SIGNAL VOLTAGE LOW

The CTS uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage on circuit 410 to the sensor. When the engine is cold the sensor resistance is high, therefore the ECM will see a high signal voltage. As the engine warms, the sensor resistance becomes less and the signal voltage drops. At normal operating temperature the voltage will measure about 1.5-2.0 volts at the ECM terminal "C10". Coolant temperature is one of the inputs used to control: Fuel delivery, spark timing (EST), idle (IAC), convertor clutch (TCC), canister purge (CCP), air management (AIR) (man. trans.), EGR and cooling fan. Code 14 will set if signal voltage indicates a coolant temperature above 275°F (134°C) for 3 seconds.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Checks to see if code was set as a result of hard failure or intermittent condition.
2. This test simulates a Code 15. If the ECM recognizes the open circuit (high voltage) and sets Code 15, then the ECM and wiring are okay.
3. If a Code 14 repeats, then circuit 410 is shorted to ground or the ECM is faulty.

Code 14 Flow Chart, CTS Signal Voltage Low. Scheme 344

Code 14 Flow Chart, CTS Signal Voltage Low. Scheme 345

Coolant Sensor Temperature To Resistance Values. Scheme 346

Code 14 Schematic, CTS Signal Voltage Low. Scheme 347

CODE 15 - CTS SIGNAL VOLTAGE HIGH

The CTS uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage (5-6 volts) on circuit 410 to the sensor. When the engine is cold, the sensor resistance is high. Therefore, the ECM sees a high signal voltage. As the engine warms, sensor resistance becomes less and the signal voltage drops. At normal operating temperature the voltage will measure about 1.5-2.0 volts at the ECM terminal "C10". Coolant temperature is one of the inputs used to control: Fuel delivery, spark timing (EST), idle (IAC), convertor clutch (TCC), canister purge (CCP), air management (AIR)(man. trans.), EGR and cooling fan. Code 14 will set if signal voltage indicates a coolant temperature below B35°C (B31°F) for 3 seconds.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Checks to see if code was set as a result of hard failure or intermittent condition.
2. This test simulates a Code 14. If the ECM recognizes the low voltage and sets Code 15, then the ECM and wiring are okay.
3. If a Code 15 repeats, then the problem is an open circuit 410, 452, a poor connection at ECM or sensor, or faulty ECM.

Code 15 Flow Chart, CTS Signal Voltage High. Scheme 348

Code 15 Flow Chart, CTS Signal Voltage High. Scheme 349

CODE 21 - TPS SIGNAL VOLTAGE HIGH

The TPS provides a voltage signal that changes relative to the throttle blade. Signal voltage will vary from about .5 volt at idle to 4.5 volts at wide open throttle. Code 21 will set if: TPS signal voltage is greater than 2.5 volts for 3 seconds, air flow less than 12 gm/sec., engine speed less than 1200 RPM or TPS voltage over 4.5 volts with ignition "ON" or engine running.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Confirms Code 21, and that fault is present.
2. Simulates Code 22: If the ECM recognizes the low signal voltage and sets Code 22, the ECM and wiring are okay.

Code 21 Flow Chart, TPS Signal Voltage High. Scheme 350

Code 21 Flow Chart, TPS Signal Voltage High. Scheme 351

Code 21 Schematic, TPS Signal Voltage High. Scheme 352

CODE 22 - TPS SIGNAL VOLTAGE LOW

The TPS provides a voltage signal that changes relative to the throttle blade position. Signal voltage will vary from about .5 volt at idle to 4.5 volts at wide open throttle. Code 22 will set if: TPS signal voltage is less than about .2 volt for 3 seconds and the engine is running.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Confirms Code 22, and that fault is present.
2. Simulates Code 21: If the ECM recognizes the high signal voltage and sets Code 21, the ECM and wiring are okay.
3. This test checks for reference voltage from the ECM. To prevent damage to ECM, be sure to disconnect CD connector when checking circuit wiring for open or shorts to ground.

Code 22 Flow Chart, TPS Signal Voltage Low. Scheme 353

Code 22 Flow Chart, TPS Signal Voltage Low. Scheme 354

CODE 23 - MAT SENSOR SIGNAL VOLTAGE HIGH

The MAT sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies 4-6 volts on circuit 472 to the sensor. When the air is cold the sensor resistance is high, therefore the ECM will see a high signal voltage. If the air is warm the sensor resistance is low, therefore the ECM will see a low voltage. Code 23 will set if: Signal voltage indicates manifold air temperature below B35°C (B31°F), time since engine start is 8 minutes or longer or no VSS. Due to the conditions necessary to set a Code 23, the "SERVICE ENGINE SOON" light will only stay on when all 3 conditions are met.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. A Code 23 will set due to an open sensor, wire or connection. This test will determine if the wiring and ECM are okay.
2. If the resistance is greater than 25,000 ohms, replace the MAT sensor.

Code 23 Flow Chart, MAT Sensor Voltage High. Scheme 355

Code 23 Flow Chart, MAT Sensor Voltage High. Scheme 356

MAT Sensor Temperature To Resistance Values. Scheme 357

Code 23 Schematic, MAT Sensor Voltage High. Scheme 358

CODE 24 - VEHICLE SPEED SENSOR (VSS)

The VSS gives the ECM the car speed information. The ECM applies and monitors 12 volts on circuit 437. circuit 437 connects to the VSS which alternately grounds circuit 437 when the drive wheels are turning. This "pulsing" action takes place about 2000 times per mile and the ECM calculates vehicle speed based on the time between "pulses". Code 24 will set if: circuit 437 voltage is constant, engine speed is between 1400-3600 RPM, less than 2 percent throttle opening, low load condition (low air flow) and not in park or neutral. All conditions must be met for 4 seconds.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. This test monitors the ECM voltage on circuit 437. With the drive wheels turning, the "pulsing" action will result in a varying voltage. This variation will be greater at low speeds to an average of 4-6 volts at about 20 MPH.
2. Voltage less than 1 volt at the ECM connector indicates that the circuit 437 wire is shorted to ground. Disconnect circuit 437 at the VSS, if voltage now reads about 10 volts, the VSS is faulty. If voltage remains less than 10 volts, then circuit 437 wire is grounded. If circuit 437 is not grounded, check for a faulty ECM connector ECM.
3. A steady 8-12 volts at the ECM connector indicates circuit 437 is open or a faulty VSS.
4. This is normal voltage which indicates a possible intermittent problem.
5. If SCAN displays vehicle speed, check P/N switch chart C1A or vehicle with auto trans. If switch is okay check for intermittents.

Code 24 Flow Chart, Vehicle Speed Sensor (VSS). Scheme 359

Code 24 Flow Chart, Vehicle Speed Sensor (VSS). Scheme 360

Code 24 Schematic, Vehicle Speed Sensor (VSS). Scheme 361

CODE 25 - MAT SENSOR SIGNAL VOLTAGE LOW

The MAT sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies a voltage on circuit 996 to the sensor. When manifold air is cold the sensor resistance is high, therefore the ECM will see a high signal voltage. As the air warms, the sensor resistance becomes less, and the voltage drops. Code 25 will set if: Signal voltage indicates a manifold air temperature greater than 275°F (134°C) for 3 seconds or time since engine start is 8 minutes or longer. Due to the conditions necessary to set a Code 25, the "SERVICE ENGINE SOON" light will only remain on while the signal is low and vehicle speed is present.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. If voltage is above 4 volts, the ECM and wiring are okay.
2. If resistance is less than 185 ohms, replace the sensor.

Code 25 Flow Chart, MAT Sensor Voltage Low. Scheme 362

Code 25 Flow Chart, MAT Sensor Voltage Low. Scheme 363

CODE 32 - EGR SYSTEM FAILURE

Code 32 means that the EGR diagnostic switch was closed during start-up or that the switch was not detected closed under the following conditions: Coolant temperature greater than 176°F (80°C), EGR duty cycle commanded by the ECM is greater than 55 percent and TPS at less than half throttle, but not idle. All conditions must be met for about 5 minutes.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. If the first step caused Code 32 to set, then the ECM has recognized a closed vacuum switch on start-up. This test determines whether the EGR control valve, the wiring or the ECM is the cause.
2. With the ignition on, the solenoid should not be energized and vacuum should not pass to the EGR valve.
3. To this point, the EGR solenoid and valve are okay and the following step will check the diagnostic vacuum portion of the system.
4. The diagnostic switch should close at about 2" of vacuum. With vacuum applied, the switch should close and resistance go to near zero ohms. The switch should also hold vacuum or it is defective.

Note. Before using this chart, check for ported vacuum to EGR solenoid, also check hoses for leaks or restrictions, should be at least 23.64 kPa (7" Hg) vacuum at 2000 RPM.

Code 32 Flow Chart, EGR System Failure. Scheme 364

Code 32 Flow Chart, EGR System Failure (1 Of 2). Scheme 365

Code 32 Flow Chart, EGR System Failure (1 Of 2). Scheme 366

Code 32 Schematic, EGR System Failure. Scheme 367

CODE 33 - MAF SIGNAL VOLTAGE HIGH

The MAF sensor is used to measure the amount of air entering the engine. The sensor produces a frequency output between 32-150 hertz. A large quantity (high frequency) indicates acceleration and a small quantity (low frequency) indicates deceleration or idle. The information is used by the ECM for fuel control and is converted by a SCAN tool to read out the air flow in grams per second. A normal reading is about 5-8 grams per second at idle.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Checks to see if the cause of Code 33 still exists.
2. Inspect wire routing of high voltage wires such as spark plug wires. Such wires routed too closely to MAF wiring harness could possibly cause an intermittent Code 33.

Code 33 Flow Chart, MAF Signal Voltage High. Scheme 368

Code 33 Flow Chart, MAF Signal Voltage High. Scheme 369

Code 33 Schematic, MAF Signal Voltage High. Scheme 370

CODE 34 - MAF SENSOR SIGNAL VOLTAGE LOW

The MAF sensor measures the flow of air entering the engine. The sensor produces a frequency output between 32-150 hertz. A high frequency indicates acceleration and a low frequency indicates deceleration or idle. This information is used by the ECM for fuel control. Code 34 will set if: Engine running, MAF sensor disconnected, faulty relay or MAF signal circuit shorted or grounded, or air flow less than 2 grams per second (low frequency).

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. A loose or damaged air duct or large vacuum leak could set a Code 34.
2. Checks to see if the ECM recognizes a problem. A light off at this point indicates an intermittent problem.
3. Checks to see if the 5 volt reference signal from ECM is at MAF sensor harness connector. Less than 1 volt indicates a faulty circuit 528, wiring connector or ECM.
4. Checks continuity of electrical circuit at MAF sensor.
5. Checks for open in 12 volt supply.

Code 34 Flow Chart, MAF Sensor Signal Voltage Low. Scheme 371

Code 34 Flow Chart, MAF Sensor Signal Voltage Low. Scheme 372

CODE 34 - MAP SENSOR SIGNAL VOLT LO (P-BODY)

The Manifold Absolute Pressure sensor (MAP) responds to changes in manifold pressure (vacuum). The ECM receives this information as a signal voltage that will vary from about 1-1.5 volts at idle to 4-4.5 volts at wide open throttle. If MAP sensor fails the ECM will substitute a fixed MAP value and use the TPS to control fuel delivery. Code 34 will set when signal reading is too low and ignition is turned on.

1. This test confirms Code 34, and that fault is present.
2. If the ECM recognizes and sets Code 33, high MAP signal, the ECM and wiring are okay.

Code 34 Flow Chart: Map Sensor Signal Voltage Low. Scheme 373

Code 34 Flow Chart: MAP Sensor Signal Voltage Low. Scheme 374

Code 34 Schematic: Map Sensor Signal Voltage Low. Scheme 375

CODE 41 - CYLINDER SELECT ERROR

Due to the same ECM being used for different engines, it is necessary for the engine application to be selected. This is done by leaving terminals "B12" and "D-3" open for six cylinder applications. Code 41 will set if the reference pulses are not equal to a value selected within the PROM when engine RPM is below 2000.

Code 41 Flow Chart, Cylinder Select Error. Scheme 376

Code 41 Flow Chart, Cylinder Select Error. Scheme 377

Code 41 Schematic, Cylinder Select Error. Scheme 378

CODE 42 - ELECTRONIC SPARK TIMING

Code 42 means the ECM has seen an open or short to ground in the EST or bypass circuit.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Confirms Code 42 and that the fault causing the code is still present.
2. Checks for a normal EST ground path through ignition module. An EST circuit 423 shorted to ground will read less than 500 ohms.
3. As the test light voltage touches terminal "D5", the module should switch changing the ohmmeter reading to over 5000 ohms. The important thing is that the module "switched".
4. Check for the EST circuit 423 shorted to ground, bypass circuit 424 open or a faulty ignition module connection or module.
5. Confirms that Code 42 is a faulty ECM and not an intermittent in circuits 423 or 424.

Code 42 Flow Chart, Electronic Spark Timing. Scheme 379

Code 42 Flow Chart, Electronic Spark Timing. Scheme 380

Code 42 Schematic, Electronic Spark Timing. Scheme 381

CODE 44 - LEAN EXHAUST INDICATION

The ECM supplies approximately .45 volt between terminals "D6" and "D7". (This may read as low as .32 volt, if measured with a 10 megohm digital voltmeter). The O2 sensor varies the voltage within a range of about 1 volt if the exhaust is rich, down to about .10 volt if the exhaust is lean. The O2 sensor is like an open circuit and produces no voltage when it is below about 315°C (600°F). An open sensor circuit or cold sensor causes open loop operation. Code 44 is set when the O2 sensor signal voltage at the ECM terminal "D7" remains below .2 volt for 20 seconds or more and the system is in closed loop.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Grounding the diagnostic terminal with the engine running enables the "Field Service Mode" and allows the ECM to confirm either open or closed loop operation using the "SERVICE ENGINE SOON" light.
2. A light out or "open loop" indicates the presence of a fault. Disconnecting the O2 sensor will raise the signal voltage above .2 volt. If the ECM and wiring are okay, the ECM should recognize the higher voltage, .35 to .55 volt, and flash open loop when the engine is started.
3. Check the following if Code 44 is set: MAF and O2 sensors, fuel contamination, EGR, low fuel pressure, lean injectors and exhaust leaks ahead of O2 sensors.

Code 44 Flow Chart, Lean Exhaust Indication. Scheme 382

Code 44 Flow Chart, Lean Exhaust Indication. Scheme 383

CODE 45 - RICH EXHAUST INDICATION

The ECM supplies approximately .45 volt between terminals "D6" and "D7". (This may read as low as .32 volt, if measured with a 10 megohm digital voltmeter). The O2 sensor varies the voltage within a range of about 1 volt if the exhaust is rich, down to about .10 volt if the exhaust is lean. The O2 sensor is like an open circuit and produces no voltage when it is below about 315°C (600°F). An open sensor circuit or cold sensor causes open loop operation. Code 45 is set when the O2 sensor signal voltage at the ECM terminal "D7" remains below .7 volt for 1 minute or more and the system is in closed loop.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Grounding the diagnostic terminal with the engine running enables the "Field Service Mode" and allows the ECM to confirm either open or closed loop operation using the "SERVICE ENGINE SOON" light.
2. A steady light or "open loop" indicates the presence of a fault. Grounding circuit 412 causes a low O2 sensor signal voltage. If the ECM and wiring are OK, the ECM should recognize the low voltage and confirm the lean signal by turning off the "SERVICE ENGINE SOON" light for at least 15 seconds.
3. Check the following if a Code 45 is set: MAF and O2 sensors, high fuel pressure, leaking injectors, canister purge, leaking fuel pressure regulator diaphragm or the TPS. Perform the injector balance test.

Code 45 Flow Chart, Rich Exhaust Indication. Scheme 384

Code 45 Flow Chart, Rich Exhaust Indication. Scheme 385

CODE 51 - FAULTY MEM-CAL

Check that all pins are fully inserted into socket. If okay, replace PROM, clear memory, and recheck. If Code 51 reappears, replace the ECM.

CODE 53 - SYSTEM OVER VOLTAGE

This code indicates there is a basic generator problem. Code 53 will set if voltage at ECM terminal B2 is greater than 17.1 volts for 2 seconds. Check and repair charging system.

CODE 54 - FUEL PUMP CIRCUIT LOW VOLTAGE

Fuel pump circuit 120 is monitored by the ECM at terminal "B2" and is used to compensate fuel delivery based on system voltage. There should be about 12 volts on circuit 120 for 2 seconds after the ignition is turned or any time reference pulses are being received by the ECM. Code 54 will set if the voltage at terminal "B2" is less than 2 volts for 1.5 seconds since the last reference pulse was received. If the fault is detected during start-up, the "SERVICE ENGINE SOON" light will remain on until the key is cycled off. If the voltage is detected below 2 volts with the engine running, the light will only remain on while the condition exists.

Code 54 Flow Chart, Fuel Pump Ckt Voltage Low. Scheme 386

Code 54 Flow Chart, Fuel Pump Ckt Voltage Low (1 Of 2). Scheme 387

Code 54 Flow Chart, Fuel Pump Ckt Voltage Low (2 Of 2). Scheme 388

Code 54 Schematic, Fuel Pump Ckt Voltage Low. Scheme 389

CHART C1 - ECM REPLACEMENT CHECK CHART

In order to reduce incidents of repeat ECM failure, a revised ECM diagnostic procedure is available. Beginning in 1982, most ECMs are equipped with Integrated Circuits (IC) in place of separate transistors to operate various controlled components.

These ICs, called Quad-Drivers (QDR), have 4 separate outputs, meaning that each QDR can operate up to 4 different components. An inoperative QDR can result in ECM output becoming open or shorted to ground. Often, all 4 outputs of a QDR will fail, even if just one QDR circuit is faulty.

Refer to the following tables to determine which ECMs contain QDRs. Since this procedure is not applicable to ECMs which do not contain QDRs, those ECMs are not listed.

Performing the diagnostic flow chart will identify an inoperative QDR. Once the circuit is identified, it must be repaired to eliminate repeat ECM failure. This diagnostic procedure must be used when "Replace ECM" is the conclusion of any procedure.

ECM QDR IDENTIFICATION (PFI)

Scheme 390

CHART C1A - PARK NEUTRAL SWITCH

Park/Neutral switch contacts are a part of the Neutral Start switch and are closed to ground in park or neutral, and open in drive. The ECM supplies ignition voltage through a current limiting resistor to circuit 434 and senses a closed switch when the voltage on circuit 434 drops to less than 1 volt. The ECM uses the P/N signal as one of the inputs to control, Idle Air Control and VSS Diagnostics.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. This test checks for a closed switch to ground in park position. If using an ohmmeter instead of an test light, the resistance will be low, indicating continuity to ground.
2. This test checks for an open switch in drive range. If using an ohmmeter instead of a test light to 12 volts, the resistance will be high or infinity, indicating an open switch.
3. Checks to this point indicate the P/N switch and wiring are okay, however, the ECM signal voltage on circuit 434 may be mission. To check, reconnect ECM. Either back probe ECM connector circuit 434 with selector in drive or disconnect P/N switch and probe harness connector circuit 434 with a voltmeter to ground.

Flow Chart C1A, Park Neutral Switch. Scheme 391

Flow Chart C1A, Park Neutral Switch. Scheme 392

Schematic C1A, Park Neutral Switch. Scheme 393

CHART C1E - PSPS CHECK

The power steering pressure switch opens when P/S pressure goes high, such as on a full turn in either direction. When the P/S switch opens, it turns the A/C relay off, and sends a signal to the ECM. The ECM uses this signal for idle control.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. This test checks to see that P/S pressure switch opens when pressure goes high.
2. This test checks to see that P/S pressure switch is open.
3. Less than 1 ohm indicates that the switch is closed when the power steering pressure is high. Switch is okay.

Flow Chart C1E, PSPS Check. Scheme 394

Flow Chart C1E, PSPS Check. Scheme 395

Schematic C1E, PSPS Check. Scheme 396

Flow Chart C2C, Idle Air Control. Scheme 397

Flow Chart C2C, Idle Air Control. Scheme 398

Schematic C2C, Idle Air Control. Scheme 399

The ECM will control engine idle speed by moving the Idle Air Control (IAC) valve to control air flow around the throttle plate. It does this by sending voltage pulses to the proper motor winding for each IAC motor. This will cause the motor shaft and valve to move IN or OUT of the motor a given distance for each pulse received. The ECM uses the following information to control idle speed; Battery voltage, Coolant temperature, Manifold Absolute Pressure (MAP), Throttle Position Sensor, Engine speed, A/C clutch signal. Do not apply battery voltage across the ICA motor terminals. This will permanently damage the motor windings.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Be sure to disconnect the IAC valve prior to this test. The test light will confirm the ECM signals by a steady or flashing light on all circuits.
2. Before replacing an ECM, be sure to check the resistance at the IAC motor windings. Failure to do so may result in a repeat ECM failure.

Engine idle speed can be adversely affected by the following: Faulty P/N switch, leaking injectors, vacuums leaks, binding or sticking throttle shaft or throttle position sensor, faulty EGR valve, faulty battery cables, loss of A/C compressor signal during load.

CHART C3 - CANISTER PURGE CHECK

Canister purge is controlled by a solenoid that allows manifold vacuum to purge the canister when energized. The ECM supplies a ground to energize the solenoid.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Checks to see if the solenoid is opened or closed. The solenoid is normally energized in this step. It should be closed.
2. This test checks for open or grounded solenoid circuit.
3. Completes functional check by grounding test terminal. This should normally energize the solenoid, allowing vacuum to drop.
4. Determines if ECM control circuit or solenoid is at fault.

Flow Chart C3, Canister Purge Check. Scheme 400

Flow Chart C3, Canister Purge Check. Scheme 401

Schematic C3, Canister Purge Check. Scheme 402

CHART C4B - IGNITION SYSTEM CHECK

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. 1) Two wires are checked, to ensure that an open is not present in a spark plug wire.
2. 1A) If spark occurs with EST connector disconnected, pick-up coil is too low for EST operation.
3. 2) A spark indicates that the problem is a faulty distributor cap or rotor.
4. 3) Normally, there should be battery voltage at the "C" and "+" terminal. Low voltage would indicate an open or high resistance circuit from the distributor to coil or ignition switch. If "C" terminal voltage was low, but "+" terminal voltage is 10 volts to more, circuit from "C" terminal to ignition coil or ignition coil primary winding is open.
5. 4) This test checks for a shorted module or grounded circuit from the ignition coil to the module. The distributor module should be turned off so normal voltage should be about 12 volts. If the module is turned "ON", the voltage would be low but above 1 volt. This could cause the ignition coil to fail from excessive heat. With an open ignition coil primary winding, a small mount of voltage will leak through the module from the "BAT" to the tach terminal.
6. 5) This test checks for an open module or circuit to it. Applying 1.5 to 8 volts to the terminal "P" will turn the module "ON" and the voltage should drop to about 7-9 volts.
7. 6) This should turn off the module and cause a spark. If no spark occurs, the fault is most likely in the ignition coil because most module problems would have been found before this point in the procedure. A module tester could determine which is at fault.

Flow Chart C4B, Ignition System Check. Scheme 403

Flow Chart C4B, Ignition System Check (1 Of 2). Scheme 404

Flow Chart C4B, Ignition System Check (1 Of 2). Scheme 405

Schematic C4B, Ignition System Check. Scheme 406

CHART C6A - ELECTRIC DIVERT (W/ MAN. TRANS.)

This system uses a single bed converter and Air Management is controlled by an air control valve (divert valve). When grounded by the ECM, the solenoid causes the valve to direct air to exhaust ports. When de-energized air diverts to the atmosphere. Air goes to the ports provided the valve has a ground to the ECM and good manifold vacuum.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. This is a system performance test. When vehicle goes to closed loop, air will switch from the ports to divert.
2. Tests for a grounded electric divert circuit. Normal system light will be off.
3. This test checks for an open control circuit. Grounding test terminal will energize the solenoid if ECM and circuits are normal. In this step, if test light is on, circuits are normal and fault is in valve connections or valve.

Flow Chart C6A, Electric Divert (W/ Man. Trans.). Scheme 407

Flow Chart C6A, Electric Divert (W/ Man. Trans.). Scheme 408

Schematic C6A, Electric Divert (W/ Man. Trans.). Scheme 409

CHART C7A - EXHAUST GAS RECIRCULATION

EGR valve is controlled by a normally closed solenoid (allows vacuum to pass when energized). The ECM energizes the solenoid to turn the EGR on, and monitors vacuum to the EGR with the EGR diagnostic switch. Code 32 will detect a faulty solenoid, vacuum switch or vacuum supply.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. With ignition switch "ON" and engine stopped, the solenoid should not be energized and vacuum should not pass to the EGR valve.
2. Grounding the diagnostic terminal will energize the solenoid and allow vacuum to pass to the valve.
3. This test checks for plugged EGR passages. If passages are plugged, the engine may have severe detonation on acceleration.
4. The EGR solenoid will not be energized in Park or Neutral. This test determines if the P/N switch input is recognized by the ECM.

Flow Chart C7A, Exhaust Gas Recirculation. Scheme 410

Flow Chart C7A, Exhaust Gas Recirculation. Scheme 411

CHART C8 - TRANS. CONV. CLUTCH (ALL TRANS./TRANSAXLES)

Fused battery ignition is supplied to the TCC Solenoid through the brake switch, and transmission/transaxle third gear apply switch. The ECM will engage TCC by grounding circuit 422 to energize the solenoid. TCC sill engage when: Engine is warmed up. Vehicle speed is above calibrated Throttle Position Sensor (TPS) output not changing, indicating a steady road speed. Transaxle third gear apply switch is closed. Brake switch is closed.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Light off confirms transmission/transaxle third gear supply switch is open.
2. At 20-25 MPH the transmission/transaxle third gear switch should close. Test light will come on and confirm battery supply and closed brake switch.
3. Grounding the diagnostic terminal with engine "OFF" should energize the TCC solenoid. This test checks the capability of the ECM to control the solenoid.
4. Solenoids and relays are turned "ON" and "OFF" by the ECM internal electronic switches called "drivers". Each driver is part of a group of 4 called Quad-Drivers. Failure of one can damage any other driver within the set.

Flow Chart C8, TCC (All Transmissions/Transaxles). Scheme 412

Flow Chart C8, TCC (All Transmissions/Transaxles, 1 Of 2). Scheme 413

Flow Chart C8, TCC (All Transmissions/Transaxles, 2 Of 2). Scheme 414

Schematic C8, TCC (All Transmissions/Transaxles). Scheme 415

CHART C8A - TRANS. CONV. CLUTCH (CONT.)(125-C TRANSAXLE)

Fused battery ignition is supplied to the TCC Solenoid through the brake switch, and transmission/transaxle third gear apply switch. The ECM will engage TCC by grounding circuit 422 to energize the solenoid. TCC will engage when: Engine is warmed up. Vehicle speed is above calibrated Throttle Position Sensor (TPS) output not changing indicating a steady road speed. Transaxle third gear apply switch is closed. Brake switch is closed.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Light off confirms transmission/transaxle third gear supply switch is open.
2. At 20-25 MPH the transmission/transaxle third gear switch should close. Test light will come on and confirm battery supply and closed brake switch.
3. Grounding the diagnostic terminal with engine "Off" should energize the TCC solenoid. This test checks the capability of the ECM to control the solenoid.
4. Solenoids and relays are turned "ON" and "OFF" by the ECM internal electronic switches called "drivers". Each driver is part of a group within the set.

Flow Chart C8A, TCC (Continued for W/ 125-C Trans.). Scheme 416

Flow Chart C8A, TCC (Continued For W/ 125-C Trans.). Scheme 417

Schematic C8A, TCC (Continued for W/ 125-C Trans.). Scheme 418

CHART C10A - A/C CLUTCH CONTROL

The A/C clutch control relay is ECM controlled to Delay A/C clutch engagement .4 seconds after A/C is turned on. This allows the IAC to adjust engine RPM before the A/C clutch engages. The ECM also causes the relay to disengage the A/C clutch during WOT operation. The A/C clutch control relay is energized when the ECM provides a ground path for circuit 905.

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. Checks to see if the ECM is controlling the A/C clutch control relay.
2. This test checks operation of A/C cycling switch.
3. This test checks for grounded circuit 905 to ECM. At this point the test light should be off.
4. If the ECM recognizes a high power steering pressure the A/C clutch relay will be de-energized. The ECM will pulse circuit 905 on and off to de-energize the relay. This would cause a blinking test light when circuit 905 was tested.

If A Scan Tool Is Available, A Quick Check Of Circuit 59 Can Be Made By Observing The Tools Display When A/C Is Selected. The Scan Tool Will Display The State Of The Signal To The ECM.

Flow Chart C10A, A/C Clutch Control. Scheme 419

Flow Chart C10A, A/C Clutch Control. Scheme 420

Schematic C10A, A/C Clutch Control. Scheme 421

CHART C10B - A/C CLUTCH CONTROL

Note. The following step numbers refer to the numbers on the Diagnostic Flow Charts.

1. This test checks for battery voltage to relay through circuit 59.
2. Substitutes for relay to determine if problem is in relay or in circuit 59, A/C clutch coil, high press, switch or ground.
3. This test checks for open in circuit 59 between cycling switch and A/C fuse or open circuit 59 to relay.
4. This test checks to see that "A/C ON" signal is getting to ECM through circuit 59. A test light off at this time indicates circuit 59 is open between the cycling switch and the ECM.

Flow Chart C10B, A/C Clutch Control. Scheme 422

Flow Chart C10B, A/C Clutch Control. Scheme 423

CHART C12A - COOLANT FAN CHECK

The cooling fan is totally controlled by the ECM based on inputs from the coolant sensor and fan switch. The fan should run if coolant temperature is greater than 234°F (112°C).

Battery voltage is supplied to the fan relay on terminal "E" and ignition voltage to terminal C. Grounding circuit 335 (relay terminal "B") will energize the relay and supply battery voltage to the fan motor. The ECM will remove the ground to circuit 335 if vehicle speed is over 40 MPH unless the engine is overheating. When A/C is used, the fan control switch mounted in the A/C High pressure line will open when head pressure exceeds 233 psi. and this input causes the ECM to ground circuit 335. If coed 14 or 15 sets, the ECM will turn on the cooling fan. If the ECM fails to turn on the Fan Relay by grounding circuit 335, the fan override switch can complete the circuit.

Chart C12C, Coolant Fan Check. Scheme 424

Chart C12C, Coolant Fan Check. Scheme 425

Schematic C12C, Coolant Fan Check. Scheme 426

Component Locations (A Body). Scheme 427

Component Locations (F Body). Scheme 428

Component Locations (P Body). Scheme 429

ECM Terminal Identification (Except Fiero). Scheme 430

ECM Terminal Identification (Fiero). Scheme 431

FWD PFI CCC Wiring Diagram. Scheme 432

RWD PFI CCC Wiring Diagram. Scheme 433

Fiero PFI CCC Wiring Diagram. Scheme 434