5.0l Feedback Carburetor "a" Charts 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 474) 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 474

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, 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 475

"Scan" Diagnostic Circuit Check Flow Chart. Scheme 476

"Scan" Diagnostic Circuit Check Ckt Diag. Scheme 477

"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 478

"Non-Scan" Diagnostic Circuit Check Flow Chart. Scheme 479

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 480

System Performance Check Flow Chart. Scheme 481

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 482

Chart A1 - Dwell Fixed Under 10 Degrees Flow Chart. Scheme 483

Chart A1 - Dwell Fixed Under 10° Ckt Diag. Scheme 484

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 485

Chart A2 - Dwell Fixed Between 10-45 Degrees Or 50 Degrees Flow Chart. Scheme 486

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 487

Chart A3 - Dwell Fixed Over 45 Degrees Or 50 Degrees Flow Chart. Scheme 488

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 489

Chart A5 - "Service Engine Soon" Light Inoperative Flow Chart. Scheme 490

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 491

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 492

Buick (B & G Bodies), (VIN Y) Component Locations. Scheme 493

F Body, (VIN Y) Component Locations. Scheme 494

Oldsmobile (B & G Bodies), (VIN Y/9) Component Locations. Scheme 495

Pontiac & Chevrolet (B & G Bodies), (VIN H/G) Component Locations. Scheme 496

Pontiac & Chevrolet (B Body), (VIN Y) Component Locations. Scheme 497

ECM Terminal Pin Voltage (1986 5.0L VIN G & H). Scheme 498

ECM Terminal Pin Voltage (1986 5.0L VIN Y & 9). Scheme 499

ECM Terminal Pin Voltage (1987 5.0L VIN G & H). Scheme 500

ECM Terminal Pin Voltage (1987 5.0L VIN Y & 9). Scheme 501

Full Function Wiring Diagram (1986 5.0L, VIN Y & 9). Scheme 502

Full Function Wiring Diagram (1986 5.0L, VIN G & H). Scheme 503

Full Function Wiring Diagram (1987 5.0L, VIN 9, "G" Body). Scheme 504

Full Function Wiring Diagram (1987 5.0L, VIN Y). Scheme 505

Full Function Wiring Diagram (1987 5.0L, VIN G & H). Scheme 506