5.0l Feedback Carburetor Tests W/codes Pontiac Firebird III

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 355) 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 355

DIAGNOSTIC SYSTEM OPERATION

Note. On carbureted models, a "SERVICE ENGINE SOON" light driver module is installed in wiring harness from ECM to "SERVICE ENGINE SOON" light. This driver turns on light when ignition is turned on. When vehicle starts, ECM turns light off. If ECM malfunctions or senses a malfunction, light will turn back on. On fuel injected models, lamp driver is built into ECM.

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 CCC 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, proceed to appropriate SYSTEM PERFORMANCE CHECK chart.
4. If no trouble is indicated by SYSTEM PERFORMANCE CHECK check and/or a driveability problem exists, refer to SYMPTOM DIAGNOSIS and/or SCAN TESTER USAGE in the TROUBLE SHOOTING charts in the CCC 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 SYSTEM PERFORMANCE CHECK.

Scheme 356

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 356) 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 356): 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 DATA TABLES and SCAN TESTER USAGE in the TROUBLE SHOOTING charts in the CCC TESTS W/O CODES article in this section.

If a "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 CCC 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 section. Charts which are used because the SYSTEM PERFORMANCE 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.

SYSTEM PERFORMANCE CHECK (CARBURETED MODELS)

This check verifies that computerized engine control system is functioning correctly. This check should always be made after any repair of computerized engine control system. Performance check chart can be found by proceeding to appropriate SYSTEM PERFORMANCE CHECK chart for that system type.

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 "D" on carbureted, 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 tables 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 tools are: a tachometer, a dwell meter, test light, ohmmeter, digital voltmeter with 10-megohm impedance (minimum), vacuum pump, vacuum gauge, fuel injector test lights (TBI and PFI) 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.

On carbureted models, dwell meter is used to measure the time the M/C solenoid is on or off. Dwell reading indicates if the M/C solenoid is working and fuel mixture strength (rich or lean). The dwell meter is set on the 6-cylinder scale regardless of the number of cylinders in engine.

Dwell meter is connected to Green connector located near carburetor. This connector will not be connected to any circuit EXCEPT when testing with dwell meter. DO NOT allow terminal wire to come in contact with any ground source, including rubber hoses.

Note. If engine operation seems to change when dwell meter is connected to Green wire, remove dwell meter and use another type. A few brands are not compatible with computerized engine control system.

When engine is at operating temperature and idling, dwell meter needle should vary between 10-50 degrees. This indicates closed loop operation. Before engine reaches operating temperature, dwell should be fixed between 10-50 degrees, indicating open loop operation. If after reaching normal operating temperature dwell is fixed between 10-50 degrees, less than 10 degrees or more than 50 degrees, refer to appropriate CHART A - DWELL FIXED diagnostic chart for that system.

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 DATA tables. 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 SCAN DATA tables in this section.

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 in evidence, turn ignition off, remove tester, turn ignition on and 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 and information obtained by it will not be guaranteed accurate.

SCAN TESTER - TEST DATA PARAMETERS

Note. Information in the following table is a typical reading 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.

FULL FUNCTION (CARBURETED)

"SCAN" DIAGNOSTIC CIRCUIT CHECK

The "Scan Diagnostic Circuit Check determines if: 1) the "SERVICE ENGINE SOON" light works, 2) the ECM is operating and can recognize the fault, and 3) if any codes are stored. This is the starting point for any diagnosis. If no codes are indicated, see SYSTEM PERFORMANCE CHECK. If any additional checks are called out from the System Performance Check, see the CCC TESTS W/O CODES article in this section.

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

1. Check operation of the "SERVICE ENGINE SOON" LIGHT. With key on, scanner not connected and engine not running, light should be on steady.
2. Grounding test terminal will flash a Code 12 and any stored trouble codes. The light must go on and off to indicate a code. If the light goes from bright to dim, see CHART A6. This is not considered a code.
3. If the Scanner is not operating, try it on another vehicle. If it works on other vehicle, cigar lighter should be checked for battery voltage and a good ground. If the scanner displays "NO DATA" or "NO ALCL" with the ignition on, see CHART A6.
4. No codes at this point indicate the problem is intermittent and the SYSTEM PERFORMANCE CHECK should be performed. If a code or codes is displayed, see appropriate DIAGNOSTIC CHART.

"Scan" Diagnostic Circuit Check Flow Chart. Scheme 357

"Scan" Diagnostic Circuit Check Flow Chart. Scheme 358

"Scan" Diagnostic Circuit Check Ckt Diag. Scheme 359

"NON-SCAN" DIAGNOSTIC CIRCUIT CHECK

The "Non-Scan" Diagnostic Circuit Check determines if: 1) the "SERVICE ENGINE SOON" light works, 2) the ECM is operating and can recognize a fault, and 3) if any codes are stored. It also checks to see if stored codes indicate an intermittent problem. This is the starting point for any diagnosis. If no codes are indicated, see SYSTEM PERFORMANCE CHECK. If no additional checks are called out from the System Performance Check, see TROUBLE SHOOTING charts in the CCC TESTS W/O CODES article in this section.

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

1. Check operation of the "SERVICE ENGINE SOON" light. Key on and engine not running, light should be on steady.
2. Grounding test terminal will flash a Code 12 and any stored trouble codes. The light must go on and off to indicate a code. If the light goes from bright to dim, see CHART A6. This is not considered a code.
3. This step will determine if any codes, other than Code 12, are still present or were intermittent and are no longer stored. Clear memory. Run vehicle for 2 minutes. See if trouble code(s) reset.
4. If light is on, fault is still present. See applicable trouble code chart.
5. If light is off, fault is either intermittent, or it is a code that cannot be set with vehicle stationary. For codes that cannot be set during the Diagnostic Circuit Check, the applicable trouble code chart will determine if those codes are intermittent.

"Non-Scan" Diagnostic Circuit Check Flow Chart. Scheme 360

"Non-Scan" Diagnostic Circuit Check Flow Chart. Scheme 361

SYSTEM PERFORMANCE CHECK

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

1. 1) This test checks carburetor ability to change air/fuel mixture. Disconnecting M/C solenoid makes carburetor run full rich, reconnecting it with dwell lead grounded makes carburetor run full lean. RPM normally drops 300-1000 RPM as solenoid is reconnected.
2. 1A) If plugging PCV, Purge, or Bowl Vent hose causes RPM to drop more than 300 RPM, that hose leads to the source of the problem.
3. 2) This test checks for proper control of idle circuit.
4. 2A) This indicates a full rich command to the carburetor, caused by: lean engine condition, grounded O2 sensor wire or bad sensor, open wire from ECM terminal "14" to ground, open wire to ECM terminal "22" or open coolant sensor circuits 410 or 452.
5. 2B) This indicates an open loop condition that can be caused by: an open O2 sensor circuit or bad sensor, an open coolant sensor circuit or an open wire from ECM terminal "14" to ground.
6. 2C) This indicates a full lean command which can be caused by: M/C solenoid wires reversed, leaking Bowl Vent valve, excessive fuel in vapor canister, fuel in crankcase, faulty carburetor calibration or carburetor or silicone contaminated O2 sensor.
7. 2D) Indicates "closed loop" operation. On 1987-1988 5.0L (VIN G/H), normal dwell reading is 10-45 degrees but varying (TSB No. 88-445-6E, December,1990). Run engine at 2000 RPM for one minute to ensure O2 sensor is warm.
8. 3) Checks for proper control of main metering system. RPM must be at least 3000 to get into the main metering system operation.
9. 3A) A missing "O" ring between the switching valve solenoid and valve, or a defective valve, may cause air to leak into the exhaust ports at higher RPM only.

System Performance Check Flow Chart. Scheme 362

System Performance Check Flow Chart. Scheme 363

CHART A1 - DWELL FIXED UNDER 10°

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

1. 1) This test determines if problem is CCC or engine related. On 1987-1988 5.0L (VIN G/H) B & F Body, dwell should start increasing as soon as engine is choked and increase until it is greater than 45 degrees (TSB No. 88-445-6E, December,1990). If dwell responds, problem is lean engine.
2. 1A) This test checks for cause of lean condition that resulted in full rich command.
3. 2) This test checks for ECM response to input to O2 sensor circuit. The voltmeter is used to put a voltage on the O2 sensor circuit to simulate a rich condition. Dwell should increase (lean command) if ECM and harness are good.
4. 3) This test checks for normal coolant sensor circuit condition. Voltage on a normalized hot engine should be under 2.5 volts.
5. 4) This step checks for an open in the ground circuit to ECM terminal "14" and grounded O2 sensor circuit. Terminal "2" voltage should be under 1.0 volt at idle. A high voltage could be caused by an open in circuit at terminal "22". Normally this will cause Codes 21 and 34 (and 35 if equipped with idle speed control) but won't set them on some engines.

Chart A1 - Dwell Fixed Under 10° Flow Chart. Scheme 364

Chart A1 - Dwell Fixed Under 10 Degrees Flow Chart. Scheme 365

Chart A1 - Dwell Fixed Under 10° Ckt Diag. Scheme 366

CHART A2 - DWELL FIXED BETWEEN 10-45° OR 50°

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

Note. On 1987-88 5.0L (VIN G/H), B & F Body dwell is 10-45 degrees (TSB No. 88-445-6E December, 1990).

1. 1) Run engine at part throttle for 1 minute to warm O2 sensor. Grounding O2 sensor input checks ECM response to a "lean" signal. Normal response is dwell decreasing to full rich command.
2. 1A) On some ECMs, an open circuit to terminal "14" can cause open loop.
3. 1B) Checks output of O2 sensor with full rich command from ECM caused by grounding O2 sensor input circuit. Normal response is voltage at O2 sensor over .8 volt.
4. 2) This step grounds O2 sensor circuit at ECM to check for open in wiring to ECM terminals "9". Normal response to "lean" signal is dwell decrease.
5. 3) This step checks for voltage to the coolant sensor. Normal reading on a warm engine is less than 2.5 volts. An open circuit would cause a reading of approximately 5 volts.

Chart A2 - Dwell Fixed Between 10-45° Or 50° Flow Chart. Scheme 367

Chart A2 - Dwell Fixed Between 10-45 Degrees Or 50 Degrees Flow Chart. Scheme 368

CHART A3 - DWELL FIXED OVER 45 OR 50°

Note. On 1987-88 5.0L (VIN G/H), B & F Body dwell is 10-45 degrees (TSB No. 88-445-6E December, 1990).

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

1. This test determines whether problem is related to engine or electronics. Normal response is dwell decrease. This indicates that O2 sensor, harness and ECM are okay. Problem is a rich condition. If engine is very rich, a large air leak may be required to lean mixture. When mixture is lean enough, engine will begin to run rough.
2. This test checks ECM response to a "lean" O2 signal. Normal response to this test is low dwell. No dwell change indicates a defective ECM. This test also eliminates possibility of an open sensor wire. An open wire will cause open loop operation and may set Code 13.
3. This test checks for excessive voltage in O2 line. If under .55 volt, wire and ECM are okay. Fault is in O2 sensor. If over .55 volt, wire is shorted to battery voltage or ECM is faulty.
4. If plugging PCV or bowl vent vacuum hose causes dwell to decrease, that hose leads to the source of the problem.

Chart A3 - Dwell Fixed Over 45° Or 50° Flow Chart. Scheme 369

Chart A3 - Dwell Fixed Over 45 Degrees Or 50 Degrees Flow Chart. Scheme 370

CHART A5 - "SERVICE ENGINE SOON" LIGHT INOPERATIVE

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

1. 1) This checks for blown gauge fuse or open in "SERVICE ENGINE SOON" light circuit (including I.P. connector), printed circuit and "SERVICE ENGINE SOON" light. Normal response is light on.
2. 2) This test checks for shorted ECM. A grounded ECM terminal "G" will turn the "SERVICE ENGINE SOON" light off. If disconnecting the ECM turns light on, ECM is shorted. Normal response is light on.
3. 3) This test checks for grounded wire from terminal "C" of light driver to terminal "G" of ECM, an open circuit to terminal "B" of light driver, a bad ground or faulty light driver. Normal reading is about 9 to 11 volts. Over 11 volts indicates a bad ground or a faulty light driver.
4. 4) This test checks for open in wire to terminal "B". Normal reading is approximately battery voltage.
5. 4A) This test checks for an open wire to terminal "E" from the "SERVICE ENGINE SOON" light. With terminal "E" grounded, normal response is light on.
6. 5) This test checks for a grounded wire from driver terminal "C" to ECM terminal "G". Normal response is light on.

Chart A5 - "Service Engine Soon" Light Inoperative Flow Chart. Scheme 371

Chart A5 - "Service Engine Soon" Light Inoperative Flow Chart. Scheme 372

CHART A6 - WON'T FLASH CODE 12

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

1. This step checks for short to battery voltage in wire to terminal "C" or faulty light driver. Normal reading is 9 to 11 volts.
2. This step checks to see if problem is related to the ECM or light driver. Grounding terminal "C" should turn light off.
3. Grounding terminal "G" at ECM and finding light on indicates an open in wire to terminal "C" of light driver. Grounding terminal "G" should turn light off.
4. This step checks for open in wire from ECM to test terminal in ALCL connector. The light should flash Code 12 when terminal "5" is grounded.
5. This checks for proper voltage supply to ECM. Both should read over 9 volts. Term. "C" is ignition and terminal "R" is constant battery voltage for long term memory.
6. This test checks for bad ground in ECM. Terminals "A" and "U" are connected together inside ECM.
7. This step distinguishes between a faulty ECM and PROM. Normal response is for Code 51 to flash even though PROM is not installed in ECM. If no Code 51, ECM is faulty.

Chart A6 - Won't Flash Code 12 Flow Chart. Scheme 373

Note. Check fuses that supply power to ECM. Repeat Diagnostic Circuit Check after any repair.

Chart A6 - Won't Flash Code 12 Flow Chart. Scheme 374

CODE 12 - NO DISTRIBUTOR REFERENCE PULSES

Code 12 indicates the ECM is on and sees no reference pulse from the distributor. This is a normal code with the ignition on and the engine not running. Code 12 is not stored and will only flash when the fault is present. With engine running, Code 12 could mean an open or ground in distributor reference circuit. Code 41 will appear with Code 12 if engine is running with no distributor reference signal. If problem clears, Code 41 will store.

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

1. This test checks for a poor connection at EST connector as being the cause of no reference pulse. Check for corrosion, connector terminals not fully seated or terminal not properly attached to wire. Terminal must be removed from the connector and carefully inspected.
2. Voltage should normally be more than .5 volt, indicating that the signal is being generated by the module and fault is a bad connection at the ECM, or faulty ECM. To check ECM connection, terminal must be removed from connector.
3. If circuit from terminal "10" to module is not open or grounded, source of no signal is the module.

Code 12 - No Distributor Reference Pulses Flow Chart. Scheme 375

Code 12 - No Distributor Reference Pulses Flow Chart. Scheme 376

CODE 13 - OPEN OXYGEN SENSOR CIRCUIT

Code 13 indicates an open in the O2 sensor circuit with the following conditions

1. O2 sensor voltage is within a specified range.
2. Above a specified TPS value.
3. More than specified time after engine has warmed up.

The ECM supplies about .45 volt between terminals "9" and "14". Voltage may read as low as .32 volt when measured with a 10 megohm digital voltmeter. The O2 sensor varies the voltage within a range of about 1 volt (rich exhaust) to about .1 volt (lean exhaust).

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

1. This test checks to see if problem still exists. Normal dwell reading will vary, indicating fault is not present. Fixed dwell indicates fault.
2. By grounding the O2 sensor circuit to the ECM, a "low voltage (lean) signal" is sent to the ECM. This should result in a "full rich (low dwell) command" from the ECM.
3. This test checks O2 sensor. With the rich command, O2 sensor should read a high voltage, over .8 volt. If O2 sensor functions, fault is in sensor connections.

Code 13 - Open Oxygen Sensor Circuit Flow Chart. Scheme 377

Code 13 - Open Oxygen Sensor Circuit Flow Chart. Scheme 378

CODE 14 - COOLANT SENSOR SHORTED

Code 14 indicates the ECM has seen low resistance of the coolant sensor circuit as high engine temperature, or low voltage at ECM terminal "3", for a time longer than specified.

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

1. This test determines whether fault is in sensor or circuit. Normal circuit voltage is about 5 volts or Scanner should read very low (-10°C). A low voltage or high coolant temperature would indicate a faulty circuit or ECM.
2. This test checks for grounded circuit between ECM and coolant sensor. Test light to battery positive will be off in an ungrounded circuit. Coolant sensor is not connected during the test.

Code 14 - Coolant Sensor Shorted Flow Chart. Scheme 379

Code 14 - Coolant Sensor Shorted Flow Chart. Scheme 380

CODE 15 - COOLANT SENSOR OPEN

Code 15 indicates the ECM has seen the resistance of the Coolant Sensor circuit too high. This could be due to high resistance (cold engine temperature) or high voltage at ECM terminal "3" for a time longer than specified. This may cause detonation on a warm engine due to excessive spark advance, or poor driveability due to inaccurate fuel control.

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

1. If problem still exists, "SERVICE ENGINE SOON" light will come on and Code 15 will be set.
2. This test checks if fault is coolant sensor or lack of voltage to sensor. Normal reading is 5 volts across coolant sensor connector or a high temperature display (110°C) using the Scanner.
3. This test determines whether low voltage at sensor connector is due to open coolant sensor circuits, or in another part of the 5-volt reference circuit. Normal voltage is about 5 volts between ECM terminals "3" to "7" or a Scanner display of high temperature (110°C).
4. Normally, if voltage reading is more than 4 volts or Scanner displays high voltage (110°C), fault would be intermittent. If voltage is more than 6 volts, circuit 410 could be shorted to another voltage source.
5. This test checks coolant sensor resistance. If resistance is within chart specifications, coolant sensor is okay. Check for corrosion at connector or low coolant level.

Code 15 - Coolant Sensor Open Flow Chart. Scheme 381

Code 15 - Coolant Sensor Open Flow Chart. Scheme 382

CODE 21 - TPS CIRCUIT HIGH

Code 21 indicates the ECM has seen a high TPS voltage for more than 10 seconds, below a specified RPM (normally curb idle) or below a specified engine load. Due to the pull-up resistor between terminals "21" and "2" within the ECM, an open in the TPS circuit will place about 5 volts (high TPS signal) at terminal "2" of ECM.

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

1. This test checks the circuits from the TPS connector back to the ECM. Installing a jumper lead between harness terminals "B" and "C" should lower voltage at ECM terminal "2" below 2.5 volts.
2. A high voltage of 2.5 volts or more at this point would indicate circuit 417 is open or shorted to circuit 416, or circuit 452 is open. If both circuits check okay, problem is faulty ECM.
3. A low voltage reading of less than 2.5 volts indicates the TPS or its connections are faulty.

Code 21 - TPS Circuit High Flow Chart. Scheme 383

Code 21 - TPS Circuit High Flow Chart. Scheme 384

CODE 22 - TPS CIRCUIT LOW (5.0L VIN Y)

Code 22 indicates the ECM has seen low TPS voltage longer than 20 seconds at terminal "2" of the ECM.

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

1. This test checks to see which code will be set with the TPS disconnected. The Scanner checks the TPS voltage and then rechecks the voltage with the TPS disconnected.
2. If Code 22 is reset, or the Scanner still displays less than 200 millivolts, fault is in TPS circuit. If circuit is okay, normal voltage reading will be about 5 volts.
3. If Code 21 is set or Scanner displays 200 millivolts or more, the TPS is faulty.

Code 22 - TPS Circuit Low (5.0L Vin Y) Flow Chart. Scheme 385

Code 22 - TPS Circuit Low (5.0L Vin Y) Flow Chart. Scheme 386

CODE 23 - M/C SOLENOID CIRCUIT LOW

Code 23 indicates that the ECM has sensed a low steady voltage at ECM terminal "18". Normal voltage at terminal "18" is rising and falling as the solenoid is turned on and off. This could be caused by a ground on the ECM side of the M/C solenoid or an open in the M/C solenoid circuit. A grounded circuit will cause a full lean condition and very poor driveability. An open circuit will cause a full rich condition and poor economy, odor, smokey exhaust or poor driveability.

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

1. This test checks for a complete circuit from the battery to the M/C solenoid dwell lead. Normal reading should be battery voltage. Battery voltage means there might be an open circuit between dwell connector and ground. No voltage could be either an open between the connector and battery or a ground on the ECM side of the M/C solenoid.
2. This test checks for battery voltage on the pink ignition source wire. Test light should light between the ignition source and ground.
3. This test checks for an open in the solenoid to ECM circuit. Normal circuit will read about battery voltage at Term. "18" of the ECM.
4. This test determines whether fault is in the M/C solenoid, a ground in the circuit to the ECM or the ECM. A light will indicate a ground in circuit to term. "18" or a faulty ECM. NOTE: A test light must be used in this step. A voltmeter may give an inaccurate indication.
5. This test checks for ground in wire to ECM term. "18". If wire is grounded, light will stay "ON".

Code 23: M/C Solenoid Circuit Low Flow Chart. Scheme 387

Note. Check connections at M/C solenoid. If OK, clear memory and recheck for code(s). If no Code 23, circuit is OK.

Code 23: M/C Solenoid Circuit Low Flow Chart. Scheme 388

CODE 24 - VEHICLE SPEED SENSOR (VSS)

The ECM applies and monitors 12 volts on circuit 437. Circuit 437 connects to vehicle speed sensor which alternately grounds circuit 437 when drive wheels are turning. This pulsing action takes place about 2000 times per mile and ECM calculates 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 drive wheels turning, pulsing action varies voltage. This variation is greater at low speeds to an average of 4-6 volts at about 20 mph (32 km/h).
2. This test checks for a grounded circuit. A voltage of less than 1 volt indicates circuit 437 is shorted to ground. Disconnect circuit 437 at VSS. VSS is faulty if voltage now reads more than 10 volts. If voltage remains less than 10 volts, circuit 437 is grounded. If wire isn't grounded, check for faulty ECM connector or ECM.
3. A Scanner display of "0" MPH would normally indicate a faulty circuit 437, VSS or ECM. If Scanner displays a MPH reading, Code 24 may have been caused by a faulty Park/Neutral switch circuit.
4. A steady 8-12 volts at ECM connector indicates circuit 437 is open or a faulty VSS.
5. Normal voltage is 1-6 volts and varying. This may indicate an intermittent problem if Code 24 is shown.

Code 24 - Vehicle Speed Sensor (VSS) Flow Chart. Scheme 389

Note. To prevent misdiagnosis, the technician should review electrical section or electrical troubleshooting manual and identify the type of vehicle speed sensor used prior to using this chart. Disregard Code 24, if set when drive wheels are turning.

Code 24 - Vehicle Speed Sensor (VSS) Flow Chart. Scheme 390

CHART 24B - PARK/NEUTRAL CIRCUIT (P/N)

The P/N switch is closed when gear selector is in Park or Neutral. One side of switch is connected to ECM which supplies a buffered 12 volts. The other side is grounded. The P/N switch is an input to the ECM. When voltage at ECM terminal "H" is high (12 volts), ECM allows activation, at proper time, of other controls such as TCC, EST, VSS and others.

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

1. This test checks for good P/N circuit. When the switch is closed in Park and Neutral, voltage across P/N switch terminals should be low, usually less than 1 volt. When the switch is open in Drive and Reverse, voltage should be about battery voltage.
2. This step separates a faulty switch or switch adjustment, from a faulty electrical circuit or ECM. Normal voltage across the terminals of the connector, when removed from P/N switch, should be about battery voltage.

Chart 24B - Park/Neutral Circuit (P/N) Flow Chart. Scheme 391

Chart 24B - Park/Neutral Circuit (P/N) Flow Chart. Scheme 392

CODE 32 - BARO SENSOR

Note. The BARO sensor reacts like a MAP sensor in that it measures highest voltage when barometric pressure is highest.

Code 32 says that the ECM has seen a BARO pressure (measured in volts) too low at terminal "1" of the ECM.

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

1. This test checks voltage output from sensor at the ECM. Voltage normally falls within the middle range on table.
2. Installing a jumper lead between Baro sensor harness terminals "B" and "C" should cause a voltage reading of more than 2.5 volts at the ECM. This indicates circuits 416 and 432 are okay.
3. Possible causes of low voltage in step 2) are checked by measuring voltage between Baro harness terminal "A" and "C". A voltage reading of 4-6 volts limit possible causes to be an open circuit 433, a poor connection at ECM terminal "1" or a faulty ECM.
4. This test verifies the presence of reference voltage at Baro harness terminal "C".

Code 32 - Baro Sensor Flow Chart. Scheme 393

Code 32 - Baro Sensor Flow Chart. Scheme 394

Code 34 indicates the ECM has seen the following

1. Pressure outside a specified voltage range (seen by ECM as voltage at terminal "20").
2. Engine RPM less than a given value.
3. Engine at operating temperature.
4. All the above for a time greater than specified.

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

1. This tests output of sensor at idle to determine if sensor is within specification. Normal sensor will read .59-.64 volts with key in the "ON" position and engine not running.
2. Signal voltage should read over 2 volts with engine idling with a minimum of 15 in. Hg vacuum.
3. If voltage reads between 4-6 volts, VAC sensor is faulty.
4. Low voltage indicates a faulty circuit 416.

Code 34 - Diff. Press. (VAC) Sensor Flow Chart - 1986 Models. Scheme 395

Code 34 - Diff. Press. (VAC) Sensor Flow Chart - 1986 Models. Scheme 396

Code 34 indicates the ECM has seen the following

1. Pressure outside a specified voltage range (seen by ECM as voltage at terminal "20").
2. Engine RPM less than a given value.
3. Engine at operating temperature.
4. All the above for a time greater than specified.

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

1. This tests output of sensor at idle to determine if sensor is within specification. Normal sensor will read .59-.64 volts with key in the "ON" position and engine not running.
2. Signal voltage should read over 2 volts with engine idling with a minimum of 15 in. Hg vacuum.
3. If voltage reads below 2 volts and adequate vacuum was available, the VAC sensor is faulty.
4. If voltage remains over 0.5 volts with the sensor harness disconnected the fault is in the harness or ECM. Low voltage indicates a faulty vacuum sensor.
5. If voltage remains low when harness terminals B and C are jumpered together the faulty is in the harness or ECM. High voltage indicates a faulty vacuum sensor.

Code 34 - Diff. Press. (VAC) Sensor Flow Chart - 1987 Models. Scheme 397

CODE 41 - NO DISTRIBUTOR REFERENCE SIGNAL

Code 41 indicates there are no distributor reference pulses to the ECM at a specified engine vacuum. This code could set with the ignition on, engine "Not Running" if the MAP or vacuum sensor indicates "Engine Running" voltage with the ignition on. With a constant open or ground in the reference signal circuit, Code 12 would be set along with Code 41. Use Chart 12, if 12 and 41 are set. Code 41 alone indicates problem is intermittent. When distributor reference line signal is lost, engine runs full rich and with retarded (base) spark timing. The result is poor performance, poor fuel economy, and possibly rotten egg odor from the exhaust.

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

1. This test checks to see if MAP or vacuum sensor voltage changes with loss of vacuum supply. A good sensor will change voltage at terminals "A" to "B" by 1 volt or more.
2. This test checks for cause of an intermittent open or ground in the distributor circuit. Fault could also be a MAP or vacuum sensor that is intermittently stuck, at the same voltage output as an engine "running" condition, with the key only in "ON" position. This condition will produce no reference signal. Terminals must be removed from connector to properly check them. The distributor pick-up coil should also be checked.
3. Since voltage change was less than 1 volt, problem is in MAP/VAC system. The ECM has seen engine "running" vacuum equivalent with no distributor reference signal, with key in "ON" position and engine "not running".

Code 41 - No Distributor Reference Signal Flow Chart. Scheme 398

Code 41 - No Distributor Reference Signal Flow Chart. Scheme 399

CODE 42 - ELECTRONIC SPARK TIMING (EST)

Code 42 says that the ECM has seen

1. Open or grounded By-Pass Circuit (terminal "11").
2. Open or grounded EST Circuit (terminal "12").

A grounded EST circuit may not set a code unless cranked 10 seconds or longer with circuit grounded.

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

1. This test checks operation of EST. Grounding test terminal causes timing to go to a fixed value which is normally different from that obtained with EST operating. Therefore, the timing should change. Usually the change can be heard in engine RPM. If timing varies with increase in engine RPM (module advance), a problem is indicated.
2. This step eliminates the ECM and ECM connections from module input. By jumpering terminals "A" and "B", the distributor reference signal is fed directly into the EST line of the module. By putting voltage through the test light on harness terminal "C", the module is switched to the EST mode and vehicle should run. If engine stops, there is no EST signal reaching the module due to open or poor connections, or the module is faulty.
3. By removing the jumper, you are opening the EST signal, and the engine should stop.
4. Since the engine ran when the module was jumpered, the problem is not in the distributor (if correct HEI module is installed). The wrong HEI module can set a Code 42.

Code 42 - Electronic Spark Timing (EST) Flow Chart. Scheme 400

Code 42 - Electronic Spark Timing (EST) Flow Chart. Scheme 401

CODE 43 - ELECT SPARK CONTROL (ESC) VOLTAGE LOW TO ECM

Code 43 indicates that the Electronic Spark Control (ESC) retard signal has been seen by the ECM for too long a period of time. When voltage at ECM terminal "L" is low, spark is retarded. Normal voltage in non-retard mode is about 7.5 volts or more.

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

1. Normal voltage would be over 7.5 volts. If 7.5 volts is present at terminal "L", the reason for a Code 43 is a poor connection to ECM or faulty ECM.
2. Over 6 volts indicates an overly sensitive knock sensor or controller, or noise in the engine that triggers knock sensor.
3. This test checks for grounded ECM.
4. This test checks for an open in wire from ESC to ECM terminal "L". If over 6 volts was obtained at terminal "C" of the ESC, fault is an open circuit 457.
5. This test checks for 12-volt ignition source to ESC terminal "B". Reading at terminal "B" should be battery voltage.
6. This tests if spark retard is due to engine knock or faulty knock sensor. If spark timing advances when knock sensor is disconnected, fault is in engine noise or sensor. Normally no increase would be noted.
7. This tests if spark retard is due to noise on knock sensor wiring to ESC or whether faulty ESC controller is the problem. By removing terminal "E" from the connector, determination of faulty component can be made. If spark advances, check for improper routing of knock sensor wiring.

Code 43 - Electronic Spark Control (ESC) Voltage Low to ECM Flow Chart. Scheme 402

Code 43 - Electronic Spark Control (ESC) Voltage Low to ECM Flow Chart. Scheme 403

CODE 44 - LEAN EXHAUST INDICATION

Code 44 indicates the ECM has seen O2 sensor voltage lower than specified, in closed loop, above a specified TPS value and for a time longer than specified.

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

1. A fixed dwell of under 10° indicates problem is still present. A fixed dwell under 10° at idle, with dwell varying at 3000 RPM, usually indicates an intake leak. Check this area prior to replacing O2 sensor.
2. This test checks if ECM is able to respond to a rich condition caused by choking the engine. If it does, problem is a lean engine condition, NOT ELECTRICAL.
3. This step puts a rich O2 signal (about 1 volt) into terminal "9" of the ECM. Dwell should increase (lean command).
4. If dwell increases to over 50° with heavy choking, fault is an air leak, since the ECM was able to respond. If air is going to the exhaust ports, disconnect solenoid(s) for the air control valve. If air still goes to the ports, it is a faulty valve.

Code 44 - Lean Exhaust Indication Flow Chart. Scheme 404

Code 44 - Lean Exhaust Indication Flow Chart. Scheme 405

CODE 45 - RICH EXHAUST INDICATION

Code 45 indicates the ECM has seen

1. High oxygen sensor voltage.
2. More than specified time (about 2 minutes).
3. Above a specified TPS value.
4. Closed loop.

A high voltage can be caused by a rich exhaust or an O2 sensor contaminated with silicone.

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

1. On 1987-1988 5.0L (VIN G/H), B & F Body, dwell less than 45 degrees indicates that engine should be checked for cause of intermittent rich condition; purge or bowl vent valves leaking, fuel in crankcase, fuel in evaporative canister, or sticking mixture control solenoid or metering rods.
2. This step tests ECM response to a lean engine condition. A drop in dwell indicates that ECM and O2 sensor are not faulty.
3. This step tests ECM response to a lean O2 signal (low voltage). If there is no dwell change with a grounded lead to O2 sensor terminal "9", fault is in ECM. Open O2 sensor wire would have set Code 13.
4. This step checks voltage from ECM at O2 sensor harness. Normal voltage at this point is ECM bias voltage for no O2 signal, approximately .45 volt. If voltage is high, wire to ECM could be shorted to battery voltage, or ECM is faulty.

Code 45 - Rich Exhaust Indication Flow Chart. Scheme 406

Code 45 - Rich Exhaust Indication Flow Chart. Scheme 407

CODE 51 - PROM

Code 51 sets if any of the following occur

1. Faulty PROM unit.
2. PROM unit improperly installed (may not set a code if installed backward).
3. Some PROM pins not making contact (i.e. bent).

Ensure PROM pins are not bent and are inserted properly into ECM. Ensure PROM is installed in proper direction as shown in chart.

Code 51 - PROM Replacement. Scheme 408

CODE 53 - EGR CONTROL ERROR (5.0L VIN G & H)

With the EGR valve closed, manifold vacuum will be greater than it was during normal EGR operation. This change will be relayed to the ECM by the VAC sensor. If the change is not within the calibrated window, a Code 53 will be set. The EGR solenoid is always energized whenever vehicle is in Park/Neutral, throttle position greater than specified and coolant less than specified.

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

1. This test checks to see if the EGR passages are restricted or if valve is stuck open.
2. With 4-wire EST connector disconnected, the ECM thinks the engine is not running. Grounding test terminal with EST disconnected, causes ECM to pulse EGR solenoid on and off for testing. This creates a controlled vacuum supply to EGR valve at 2000 RPM.
3. By disconnecting the EGR solenoid electrical connector, ECM control is by-passed and the normally open EGR solenoid will pass available vacuum. At 2000 RPM, EGR valve should move if EGR control system is functioning properly.
4. If vacuum is below 7 in. Hg at 2000 RPM with EGR solenoid electrical connector disconnected, a leak or restriction between the EGR diaphragm and vacuum source is indicated.
5. This test checks EGR solenoid electrical control circuit. The test light should flicker, if harness and connections are okay.

Code 53 - EGR Control Error (5.0L VIN G & H) Flow Chart. Scheme 409

Code 53 - EGR Control Error (5.0L VIN G & H) Flow Chart (1 Of 2). Scheme 410

Code 53 - EGR Control Error (5.0L VIN G & H) Flow Chart (2 Of 2). Scheme 411

Code 53 - EGR Control Error (5.0L VIN G & H) Schematic. Scheme 412

CODE 54 - M/C SOLENOID CIRCUIT HIGH

Code 54 will be set if there is constant high voltage at ECM terminal "18". A short circuit to 12 volts will cause M/C solenoid to remain in full rich position, resulting in potential ECM damage, excessive fuel consumption and excessive exhaust odor.

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

1. This test checks the M/C solenoid resistance to determine if fault is in the solenoid, ECM harness or ECM. Normal reading for a solenoid is 20-32 ohms.
2. This test checks if reason for high voltage to terminal "18" is a faulty ECM or a short to 12 volts on that wire. If test light to ground lights at M/C solenoid test lead with both ends of harness disconnected, there is a short to 12 volts in wire.

Code 54 - M/C Solenoid Circuit High Flow Chart. Scheme 413

Code 54 - M/C Solenoid Circuit High Flow Chart. Scheme 414

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 (CARBURETED)

Scheme 415

CHART C1D - MAP & BARO CHECK

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

1. Checks sensor voltage, which is highest when manifold pressure is highest. Output is normally in the middle range for specific altitudes. Low voltage increases spark advance. High voltage decreases spark advance. MAP and BARO sensors are essentially the same device. One measures pressure in manifold and the other measures atmospheric pressure.
2. Checks if rate of change of output verses change in pressure is correct. Normal change should be in the middle of the range listed. Voltage should change as soon as vacuum is changed.

Chart C1D - Map & Baro Check Flow Chart. Scheme 416

Chart C1D - Map & Baro Check Flow Chart. Scheme 417

CHART C1E - DIFFERENTIAL PRESS (VAC) SENSOR

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

1. This tests output of sensor. Normal reading with engine stopped and ignition on is less than 1 volt and at idle should be more than 3 volts. High voltage increases spark advance, while low voltage reduces spark advance.
2. This tests rate of change of output with change of vacuum. Normal reading should be in the middle of the range. Voltage should change as soon as vacuum changes.

Chart C1E - Differential Pressure (VAC) Sensor Flow Chart. Scheme 418

Chart C1E - Differential Pressure (VAC) Sensor Flow Chart. Scheme 419

CHART C2B - IDLE STOP SOLENOID (5.0L)

The idle stop solenoid is used to increase throttle opening during certain deceleration modes and when A/C is on. The solenoid is controlled by a relay which has 2 sets of contacts. The ECM controls relay by completing ground circuit to energize relay. When relay is energized, throttle opening is increased only if A/C is on. When relay is de-energized, a fixed throttle opening is maintained during deceleration regardless of A/C position.

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

1. This tests for normal solenoid operation. With ignition on and engine not running, solenoid plunger should extend.
2. This tests for normal solenoid retraction when test terminal is grounded.

Chart C2B - Idle Stop Solenoid Flow Chart (5.0L). Scheme 420

Chart C2B - Idle Stop Solenoid Flow Chart (5.0L, 1 Of 2). Scheme 421

Chart C2B - Idle Stop Solenoid Flow Chart (5.0L, 1 Of 2). Scheme 422

CHART C2C - IDLE LOAD COMPENSATOR (ILC) CHECK (5.0L VIN Y)

The ILC is used to control throttle angle during long periods of deceleration and certain wide open throttle positions.

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

1. This tests for normal ILC operation at idle. ILC plunger should be retracted.
2. By depressing TPS plunger, the ILC should extend.
3. This tests for energizing signal to ILC solenoid. With TPS depressed, light should normally go on if circuit is okay.
4. This test checks if plunger remains extended due to an electrical failure of a grounded circuit, including ECM or faulty ILC vacuum solenoid. If ILC and vacuum are okay, ILC plunger will retract.
5. This tests anti-dieseling solenoid for vacuum to ILC from vacuum tank during ignition shutdown. Plunger should retract fully.
6. This tests anti-dieseling solenoid for blockage of vacuum from DVDV to ILC. With engine running, solenoid should be energized and allowing vacuum from DVDV to ILC.

Chart C2C - ILC Flow Chart (5.0L VIN Y) (1 Of 2). Scheme 423

Chart C2C - ILC Flow Chart (5.0L VIN Y) (1 Of 2, Part 1). Scheme 424

Chart C2C - ILC Flow Chart (5.0L VIN Y) (1 Of 2, Part 2). Scheme 425

Chart C2C - ILC Flow Chart (5.0L VIN Y) (2 Of 2). Scheme 426

Chart C2C - ILC Flow Chart (5.0L VIN Y) (2 Of 2, Part 1). Scheme 427

Chart C2C - ILC Flow Chart (5.0L VIN Y) (2 Of 2, Part 2). Scheme 428

Chart C2C - ILC Ckt Diag. (5.0L VIN Y). Scheme 429

CHART C2D - REAR VACUUM BREAK (RVB) (5.0L VIN Y)

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

1. Grounding diagnostic test terminal and disconnecting coolant sensor with engine at normal operating temperature causes ECM to energize RVB solenoid. This shuts off vacuum from RVB plunger and allows it to extend.
2. This step distinguishes between an electrical short to ground from a vacuum problem. By de-energizing the solenoid, the RVB should normally retract.
3. This test checks if retracted RVB from step 1) was caused by no electrical signal to energize solenoid or a faulty solenoid. Solenoid should be energized with ignition on and engine not running. Test light should be on in this step if electrical circuit is okay.

Chart C2D - Rear Vacuum Break (RVB) Flow Chart (5.0L VIN Y). Scheme 430

Chart C2D - Rear Vacuum Break (RVB) Flow Chart (5.0L VIN Y). Scheme 431

CHART C2F - TPS ENRICHMENT CHECK

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

1. This test checks that the TPS will cause an ECM rich command when fully depressed manually. On some cars, this will cause a low dwell (full rich). Code 21 should normally be set.
2. This step distinguishes between a faulty TPS, ECM or ECM harness. Code 21 should normally be set if engine RPM is not set above specifications.
3. This tests for 5-volt reference signal at TPS. It should be about 5 volts. If loss of voltage reference were in ECM, it should set codes 21, 32 and 34, since it is the same 5 volt reference to MAP, BARO or VAC sensors. Therefore it must be an open in wiring.
4. This tests for a grounded circuit. Normal circuit should read about 5 volts. Checking for grounded TPS output to terminal "2" of ECM, or short in wiring to terminals "2" and "22" from TPS, indicates whether fault is in wiring or ECM. A voltmeter with a 10 megaohm resistance must be used for an accurate reading.

Chart C2F - TPS Enrichment Flow Chart. Scheme 432

Chart C2F - TPS Enrichment Flow Chart. Scheme 433

CHART C4A - IGN SYSTEM CHECK WITH INTEGRAL COIL

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

1. 1) This tests for proper output from the ignition system. Using Tester (ST-125), check for spark at plugs. Tester requires a minimum of 25,000 volts to fire. This test can be used in case of an ignition miss because the system may provide enough voltage to run the engine, but not enough to fire the spark plug under load.
2. 1A) If spark occurs with EST connector disconnected, pick-up coil output is too low for EST operation.
3. 2) Normal reading during cranking is about 9-10 volts.
4. 3) This tests for a shorted module or grounded circuit from the ignition coil to the module. The distributor module should be turned off so that normal voltage should be about 12 volts. If module is turned on, 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 amount of voltage will leak through the module from "Bat." terminal to "Tach." terminal.
5. 4) This checks voltage output with pick-up coil triggering module. A spark indicates ignition system has enough output. An intermittent no start or poor performance can result if polarity of ignition coil and pick-up coil is not correct. The color of the pick-up coil connector has to be Yellow if 1 of the ignition coil leads is Yellow. If the ignition coil has a white lead, any pick-up coil connector color EXCEPT Yellow is okay.
6. 5) This tests for an open module or module circuit. Applying 12 volts to module "P" terminal should turn module on and voltage should drop to about 7-9 volts.
7. 6) This should turn off the module and cause a spark. If no spark occurs, fault is most likely in the ignition coil, not the module. A module tester could determine which is at fault.

Chart C4A - Ignition System Check W/Integral Coil Flow Chart. Scheme 434

Chart C4A - Ignition System Check W/Integral Coil Flow Chart (1 Of 2). Scheme 435

Chart C4A - Ignition System Check W/Integral Coil Flow Chart (2 Of 2). Scheme 436

Chart C4A - Ignition System Check W/Integral Coil Ckt Diag. Scheme 437

CHART C4K - EST PERFORMANCE CHECK

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

1. Grounding test terminal causes system to go to a fixed spark advance, which should be different from that obtained with EST operating.
2. Check timing change with vehicle in Drive. Some engines do not have EST operating in Park/Neutral.
3. This test checks if fault is in MAP/VAC system.

Chart C4K - EST Performance Check Flow Chart. Scheme 438

Chart C4K - EST Performance Check Flow Chart. Scheme 439

CHART C5 - ELECTRONIC SPARK CONTROL

If timing is retarded at idle, it may be caused by ESC operating. ESC should not operate unless a knock is present.

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

1. This is an ESC functional test. Simulating engine knock by tapping on the engine block should cause a drop in RPM (decrease in timing). If RPM doesn't drop, either timing is not retarding or is retarded all the time.
2. This should cause timing to fully retard by dropping voltage at ECM terminal "L". Retarding timing should cause RPM to drop.
3. Normally voltage should be .08 volts or more for a good knock sensor circuit.
4. "SERVICE ENGINE SOON" light should be on and a Code 43 set because ESC system would be retarded too long. If no light comes on, ECM is not retarding the spark because of voltage in circuit to terminal "L" or ECM is faulty.
5. This tests if knock sensor is reason for retard signal. If engine knock is not present, and timing increases when knock sensor is disconnected, faulty knock sensor is the cause.
6. This tests if retard signal is due to "noise" on signal wire or a faulty controller. If timing increases when wire is disconnected from controller, fault is due to knock sensor signal wire running too close to an ignition or charging system wire. Reroute wire to correct.

Chart C5 - Electronic Spark Control Flow Chart. Scheme 440

Note. This chart is used for diagnosis of engine knock, poor performance or poor fuel economy (No Code 43). Only after all causes have been checked, i.e., timing, EGR, MAP engine temperature etc.

Chart C5 - Electronic Spark Control Flow Chart. Scheme 441

CHART C6A - AIR MANAGEMENT CHECK (ED/ES)

Air management is controlled by air diverter and air switching valves, each with an ECM controlled vacuum solenoid. When solenoid is grounded by ECM, manifold vacuum will activate valve and allow air pump air to be directed as follows

1. Air diverter solenoid not grounded by ECM - Air pump air is diverted.
2. Air diverter solenoid grounded by ECM - Air pump air to air switching valve.
3. Air switching solenoid not grounded by ECM - Air pump air to converter.
4. Air switching solenoid grounded by ECM - Air pump air to exhaust ports. Loss of vacuum would cause air to be diverted to air cleaner.

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

1. Air is directed to ports during open loop and engine starting. Engine always starts in open loop, even on a warm engine.
2. Disconnect M/C solenoid, this should set a Code 23. When any code is set, the ECM opens the ground circuit to the air diverter valve. This checks the ECM response to a fault. A ground in the diverter valve circuit to the ECM would prevent diverter valve action.
3. This test checks for a grounded circuit to the ECM. Test light off is normal and indicates the circuit is not grounded.
4. This test checks for an open in solenoid control circuits. Grounding test terminal should ground both solenoid circuits. Normally test light should be on, which indicates problem is not in ECM or wiring. Problem is in solenoid valve or connections.
5. This test checks for a grounded switching valve circuit. Test light off indicates circuit is okay and fault is in valve.

Chart C6A - Air Management Check (ED/ES). Scheme 442

Chart C6A - Air Management Check (ED/ES). Scheme 443

CHART C7A - PULSE WIDTH MODULATED (PWM) EGR

The EGR solenoid is always energized (EGR off) when any 1 of the following conditions are met

1. Park/Neutral switch closed (selector in Park or Neutral).
2. Throttle position greater than specified.
3. Coolant temperature less than specified.

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

1. This tests if EGR passages are restricted or if valve is stuck open.
2. With 4-wire EST connector disconnected, the ECM thinks engine is not running. Grounding test terminal under this condition causes ECM to pulse EGR solenoid on and off for testing. This results in a controlled vacuum supply to EGR valve diaphragm at 2000 RPM.
3. By disconnecting the EGR solenoid electrical connector, ECM control of the valve is overridden, and the normally open EGR solenoid will pass available vacuum. At 2000 RPM, the EGR valve should move if the EGR control system is functioning properly.
4. If vacuum is below 7 in. Hg at 2000 RPM with EGR solenoid electrical connector disconnected, a leak or restriction between EGR diaphragm and vacuum source is indicated.
5. This tests solenoid electrical control circuit. Test light should flicker dimly if the ECM harness and connections are okay.

Chart C7A - Pulse Width Modulated (PWM) EGR (5.0L VIN G & H). Scheme 444

Chart C7A - Pulse Width Modulated (PWM) EGR (5.0L VIN G & H, 1 Of 2). Scheme 445

Chart C7A - Pulse Width Modulated (PWM) EGR (5.0L VIN G & H, 2 Of 2). Scheme 446

CHART C7C - EGR (5.0L VIN Y)

EGR valve is controlled by a normally open solenoid (allows vacuum to pass when de-energized). When the ECM energizes the solenoid by completing the ground circuit, EGR is turned off. The ECM controls the EGR based on inputs from the coolant temperature sensor, throttle position sensor, engine RPM, TCC position and barometric pressure.

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

1. This tests for restricted valve or passage in manifold. Engine should run roughly or stall as valve is opened manually.
2. The EGR valve should begin to open as engine speed approaches 2000 RPM.
3. This should result in EGR solenoid being energized, shutting off vacuum to EGR valve. This indicates EGR system is functioning properly.
4. Vacuum below 7 In. Hg at 2000 RPM is insufficient for proper EGR operation. Lower vacuum readings require repair.
5. Disconnecting 4-wire EST connector with diagnostic terminal grounded causes the ECM to ground circuit from ECM terminal "T" to EGR solenoid terminal "B". The light should be on (EGR vacuum off).
6. This isolates cause of low vacuum through EGR solenoid as being faulty circuit, ECM or EGR solenoid.

Chart C7C - Exhaust Gas Recirculation (EGR) (5.0L VIN Y). Scheme 447

Chart C7C - Exhaust Gas Recirculation (EGR) (5.0L VIN Y). Scheme 448

CHART C8A1 - TRANSMISSION/TRANSAXLE CONVERTER CLUTCH (TCC)

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

1. This tests voltage from ignition switch through brake switch, 3rd gear apply switch (if used) and TCC solenoid. Test light should light by 35 MPH. Test light may light momentarily between 3rd gear apply switch closing and ECM grounding TCC circuit from ECM terminal "P".
2. This tests if ECM completes ground to energize TCC solenoid. Light should go off.
3. This tests for open in circuits to terminals "N" and "17". The ECM supplies 12 volts to these terminals through a resistor. Normally both circuits have low voltage readings, since they involve normally closed circuits with vehicle stopped. An open circuit would give a reading of about 12 volts.
4. Switch(es) open when transmission/transaxle upshifts. This checks that transmission circuit functions normally by voltage going high (about battery voltage) as switch opens.
5. This increases throttle opening to increase TPS output. If TPS output is too low, the clutch will not apply. On some models, coasting doesn't require enough throttle opening to allow the transmission to shift.
6. This tests for low TPS input voltage at ECM. At wide open throttle, voltage should be about 5 volts. Too low a TPS voltage output could prevent transmission from shifting.
7. This tests for VSS signal at ECM. VSS signal is necessary to engage TCC.

Chart C8A1 - TCC Flow Chart (1 Of 2). Scheme 449

Chart C8A1 - TCC Flow Chart (1 Of 2, Part 1). Scheme 450

Chart C8A1 - TCC Flow Chart (1 of 2, Part 2). Scheme 451

CHART C8A-2 - TRANSMISSION/TRANSAXLE CONVERTER CLUTCH (TCC)

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

1. This tests for ground in circuit to ECM terminal "P". Normally light should be off.
2. This tests for ignition voltage to terminal "A" of the transmission connector. Light should normally be on.
3. This tests for complete circuit from transaxle to TCC test terminal. Normally light should go on if harness is good.

Chart C8A2 - TCC Flow Chart & Ckt Diag. (2 Of 2). Scheme 452

Chart C8A2 - TCC Flow Chart & Ckt Diag. (2 Of 2). Scheme 453

CHART C8B - MANUAL TRANSMISSION SHIFT LIGHT (5.0L VIN G & H)

The ECM uses input from the coolant temperature sensor, TPS, VSS and engine RPM to determine when to turn on the shift light. The ECM uses indicated RPM to compare with calibrated RPM value. With this information and vehicle speed, the ECM can calculate which gear ratio the drive train is in.

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

1. This test should not turn shift light on. If the shift light is on, there is a short to ground in circuit 456 or a faulty ECM.
2. This test should turn shift light on.
3. This tests for an open in shift light circuit or a faulty ECM.

Chart C8B - M/T Shift Light Flow Chart (5.0L VIN G & H). Scheme 454

Chart C8B - M/T Shift Light Flow Chart (5.0L VIN G & H). Scheme 455

CHART C9C - VACUUM ACTUATED EFE SYSTEM CHECK (5.0L)

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

1. Engine coolant temperature should be below 105°F (40°C).
2. EFE valve should be open above 105°F(40°C).
3. There should be at least 10 In. Hg vacuum available to EFE actuator diaphragm.
4. Valve may be seized. It may be freed up using Heat Valve Lubricant (1052627). If valve does not free up, replace valve.

Chart C9C - Vacuum Actuated EFE System Flow Chart (5.0L). Scheme 456

Chart C9C - Vacuum Actuated EFE System Flow Chart (5.0L). Scheme 457

CHART C10C - A/C CUT-OUT RELAY (5.0L VIN Y)

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

1. Checks for normal A/C clutch operation at idle. If clutch is not engaged, ensure other A/C components (belt tension, proper freon charge, A/C fuse not blown) are okay before checking relay.
2. This tests for normal A/C clutch cut-out during WOT operation. If compressor disengages for at least 10 seconds, system is okay. Clear long term memory.
3. By connecting a jumper wire across relay coil connector terminals, it can be determined if the compressor remained engaged from a faulty relay (test light off) or from a lack of signal to energize coil to open the relay points (test light on).
4. By connecting a jumper wire across the relay contact points connector terminals, it can be determined if lack of compressor operation is caused by open relay contact points. If compressor engages, fault is in relay or incorrect ECM circuit operation.
5. This test checks if open is in clutch coil (test light on) or in clutch coil circuits (test light off). If test light is off, there is either an open in 12 volt power feed to clutch coil or to ground from clutch coil.

Chart C10C - A/C Cut-Out Relay Flow Chart (5.0L VIN Y). Scheme 458

Chart C10C - A/C Cut-Out Relay Flow Chart (5.0L VIN Y). Scheme 459

Buick (B & G Bodies), (VIN Y) Component Locations. Scheme 460

F Body, (VIN Y) Component Locations. Scheme 461

Oldsmobile (B & G Bodies), (VIN Y/9) Component Locations. Scheme 462

Pontiac & Chevrolet (B & G Bodies), (VIN H/G) Component Locations. Scheme 463

Pontiac & Chevrolet (B Body), (VIN Y) Component Locations. Scheme 464

ECM Terminal Pin Voltage (1986 5.0L VIN G & H). Scheme 465

ECM Terminal Pin Voltage (1986 5.0L VIN Y & 9). Scheme 466

ECM Terminal Pin Voltage (1987 5.0L VIN G & H). Scheme 467

ECM Terminal Pin Voltage (1987 5.0L VIN Y & 9). Scheme 468

Full Function Wiring Diagram (1986 5.0L, VIN Y & 9). Scheme 469

Full Function Wiring Diagram (1986 5.0L, VIN G & H). Scheme 470

Full Function Wiring Diagram (1987 5.0L, VIN 9, "G" Body). Scheme 471

Full Function Wiring Diagram (1987 5.0L, VIN Y). Scheme 472

Full Function Wiring Diagram (1987 5.0L, VIN G & H). Scheme 473