5.0l/5.7l Pfi Tests W/codes Pontiac Firebird III

DESCRIPTION

The computerized engine control system monitors as many as 19 engine/vehicle functions. (Scheme 163) 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 163

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

OPERATION

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

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

"Hard Failures"

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

"Intermittent Failures"

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

DIAGNOSTIC PROCEDURE

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

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

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

Scheme 164

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 164) 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 164): ALDL Connector Terminal Identification NOTE: In some of the diagnostic and trouble shooting charts the Assembly Line Data Link (ALDL) may also be referred to as the Assembly Line Communication Link (ALCL). These are referring to the same connector. It is also the test point for connection of aftermarket "Scan" testers.
2. "SERVICE ENGINE SOON" light should flash Code "12". Code "12" consists of "FLASH", pause, "FLASH", "FLASH" followed by a longer pause. Trouble Code "12" will be repeated 2 more times. If any other trouble codes are stored in ECM memory, they will be displayed in the same manner.
3. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.

READING TROUBLE CODES

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

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

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

ECM TROUBLE CODE DEFINITIONS

ECM TROUBLE CODE DEFINITION

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

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

TROUBLE CODE DETERMINATION (HARD OR INTERMITTENT)

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

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

CLEARING TROUBLE CODES

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

DIAGNOSTIC MATERIALS

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

Diagnostic Charts

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

1. Charts which test the reliability of the self-diagnostic system.
2. Charts which help fix problems which are "SERVICE ENGINE SOON" light related.
3. Charts which test the computerized fuel control system performance.
4. Charts which help fix a problem when the on-car diagnostics don't work.
5. ENGINE CRANKS BUT WON'T RUN charts. Refer to the appropriate DIAGNOSTIC A-CHARTS in this article.
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 FIELD SERVICE MODE CHECK found a problem.

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

Diagnostic Aids

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

Field Service Mode Check (Fuel Injected Models)

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

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

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

SPECIAL DIAGNOSTIC TOOLS

Note. Special "Scan" testers plugged into the ALDL may be used to read trouble codes and check voltages in the system on the serial data line (terminal "E" on EFI and terminal "M" on EFI with P-4 systems). These testers can save a great deal of time. For additional information see SCAN TESTER USAGE and SCAN TESTER - TEST DATA PARAMETERS table in this article.

The computerized engine control system is most easily diagnosed using a "Scan" tester, however, other tools may aid in diagnosing problems if a "Scan" tester is unavailable. These 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.

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 TESTER - TEST DATA PARAMETERS table. They also furnish information on output device (solenoids and motors) status. Status parameters, however, are only an indication that output signals have been sent to devices by the ECM. It does not indicate if devices have responded properly to that signal. This will need to be verified at output device using a voltmeter or test light.

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

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

Note. Information obtained by scan tester is only as accurate as the tester itself. If erroneous voltage signals are suspected, it will be necessary to verify tester information using a digital voltmeter and wiring schematic. If non- existent codes are 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 DATA

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

SCAN TESTER - TEST DATA PARAMETERS

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

DIAGNOSTIC CIRCUIT CHECK

The Diagnostic Circuit Check is an organized approach for identifying fuel injection problems using the Assembly Line Data Link (ALDL). This communication link can provide diagnostic information for display on any "Scan" tester designed for this purpose.

If the "Scan" tester is not operating, check tester on another vehicle. If okay, the cigar lighter socket should be checked for 12 volts and a good ground. With the ignition on, if the "Scan" tester reads "NO DATA" or "NO ALDL", check the serial data wire for an open or short to ground. Also check for an open diagnostic "test" terminal "B". Refer to appropriate CHART A1 schematic for circuit wiring reference. With the ignition on, the serial data line should vary between 2-5 volts and the diagnostic line should have about 5 volts present. See SCAN TESTER USAGE and SCAN TESTER - TEST DATA PARAMETERS table in this article.

Diagnostic Circuit Check. Scheme 165

CHART A1 - NO "SERVICE ENGINE SOON" ("SES") LIGHT

"SERVICE ENGINE SOON" light should be on steady when ignition is on and engine is not running. Battery voltage is supplied directly to the bulb. The ECM turns the light on by grounding circuit No. 419 at the ECM.

If both the "keep alive" memory and the battery voltage supply are lost or the ignition feed is not present, the "SERVICE ENGINE SOON" light will not come on.

Engine Runs Okay, Check The Following

1. Faulty light bulb.
2. Circuit No. 419 has an open.
3. Fuel gauge fuse is blown. This will result in no stop, oil or alternator warning lights.

Engine Cranks But Will Not Run, Check The Following

1. Continuous battery power, fuse or fusible link is open.
2. ECM fuse has a open.
3. Battery circuit No. 340 to ECM is open.
4. Ignition circuit No. 439 to ECM is open.
5. Poor plug connection at ECM.

Chart A1, No "SES" Light, Schematic. Scheme 166

Chart A1, No "SES" Light, Schematic. Scheme 167

CHART A2 - NO ALDL/WON'T FLASH CODE 12 "SES" LIGHT ON STEADY

"SERVICE ENGINE SOON" light should be on with ignition on and engine not running. Battery voltage is supplied directly to the bulb. With the diagnostic terminal grounded, the light should flash a Code 12, followed by any other trouble codes stored in memory. A steady light indicates a short to ground in light control circuit No. 419, or an open in circuit No. 451.

Note. Test numbers refer to test numbers on diagnostic chart.

1. If there is a problem with the ECM that causes a "Scan" tester not to read serial data, the ECM should not flash a Code 12. If Code 12 is flashing, check for a short in circuit No. 451. If Code 12 does flash, be sure that "Scan" tester is working properly on another vehicle. If "Scan" tester is functioning properly and circuit No. 461 is okay, PROM, MEM-CAL or ECM may be causing the "NO ALDL" symptom.
2. If the light goes off when the ECM connector is disconnected, circuit No. 419 is not shorted to ground.
3. Checks for an open in diagnostic circuit No. 451.
4. At this point the wiring to the "SERVICE ENGINE SOON" light is okay. The problem could be a faulty ECM, PROM or MEM-CAL. If Code 12 does not flash, the ECM should be replaced using the original PROM or MEM-CAL. Replace PROM or MEM-CAL only after trying a new ECM.

Chart A2, No Code 12 "SES" Light On Steady, Schematic (1 of 2). Scheme 168

Chart A2, No Code 12 "SES" Light On Steady, Schematic (2 of 2). Scheme 169

CHART A3 - CRANKS/WON'T RUN

This chart assumes that battery condition and engine cranking speed are okay, and there is adequate fuel in the tank.

Note. Test numbers refer to test numbers on diagnostic chart.

Test Steps For Chart 1 of 2

1. A "SERVICE ENGINE SOON" light, is a basic test to determine if there is 12 volts to the ECM. "NO ALDL" may be due to an ECM problem. CHART A2 will diagnose the ECM. If TPS is greater than 2.5 volts, the engine may be in the "clear flood" mode, which will cause starting problems. The engine will not start without reference pulses. "Scan" tester should read RPM (reference) while cranking.
2. No spark may be caused by one of several components related to the ignition system. CHART C4 will cover all problems related to the causes of a no spark condition.
3. The test light should blink, indicating that the ECM is controlling the injectors. How bright the light blinks is not important.
4. Use Fuel Pressure Gauge (J 34730-1). Wrap a shop towel around the fuel pressure tap to absorb any fuel leakage that may occur when installing the gauge.

Test Steps For Chart 2 of 2

1. Checks for 12 volts to injectors. Due to the injectors being wired in parallel, there should be a light on at both terminals.
2. Checks continuity of circuits No. 467 and 468.
3. All checks made to this point would indicate that the ECM is at fault, however, there is a possibility of circuits No. 467 or 468 being shorted to a voltage source either in the engine harness or in the injector harness.

An EGR valve sticking open can cause a low air/fuel ratio during cranking. Unless engine enters "clear flood" at the first indication of a flooding condition, it can result in a no start. Check for fouled plugs.

a defective cold start circuit or water in fuel line can cause a no start condition in cold weather. See CHART A9. A defective MAF sensor may cause a no start or a stall after start. To determine if the sensor is causing the problem, disconnect it. The ECM will then use a default value for the sensor. If the condition is corrected and the connections are okay, replace the sensor.

Also check that injectors on both sides of engine will cause a test light to blink. If not okay, check injector fuses. If all checks are okay, see TROUBLE SHOOTING in the article CEC TESTING W/O CODES (TROUBLE SHOOTING) in the ENGINE PERFORMANCE section.

To test for this condition, disconnect all injectors. Turn ignition on. Probe circuits No. 467 and 468 on the ECM side of injector harness with a test light connected to ground (test one injector harness on each side of engine). There should no light. If light is on, repair short to voltage. If okay, check resistance of the injectors. Resistance should be 10 ohms or more.

Check injector harness connector. Be sure terminals are not backed out of connector and contacting each other. If all is okay, replace ECM.

Chart A3 (1 of 2), Cranks/Won't Run. Scheme 170

Chart A3 (2 of 2), Cranks/Won't Run. Scheme 171

CHART A7 - FUEL SYSTEM DIAGNOSIS

With the ignition on, the ECM will turn on the in-tank fuel pump. It will remain on as long as the engine is cranking or running, and the ECM is receiving HEI distributor reference pulses.

If there are no reference pulses (cranking or running), the ECM will shut off the fuel pump within 2 seconds. If fuel pump relay fails, a back-up oil pressure switch will energize the fuel pump when oil pressure reaches about 4 psi.

The fuel pump delivers fuel to the fuel rail and injectors, then to the pressure regulator, where system pressure is controlled to about 30-44 psi (2.1-3.0 kg/cm 2 ) when the engine is running. Excess fuel is returned to the fuel tank via the fuel return line.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Use Fuel Pressure Gauge (J 34730-1). Wrap a shop towel around the fuel pressure tap to absorb any fuel leakage that may occur when installing the gauge. With the ignition on and engine off, fuel pressure should about 40-47 psi (2.8-3.2 kg/cm 2 ). This pressure is controlled by spring pressure within the regulator assembly.
2. When the engine is idling, manifold vacuum is high and is applied to the fuel regulator diaphragm. This will overcome regulator spring pressure, open fuel tank return passage and lower fuel pressure. Idle pressure will vary somewhat depending on barometric pressure, however, the pressure idling should be less, indicating proper pressure regulator control.
3. Pressure that continues to fall is caused by one of the following conditions: In-tank fuel pump check valve not holding. Pump coupling hose or pulsator leaking. Fuel pressure regulator valve leaking. Injector(s) sticking open.
4. Check for an injector sticking open by checking for a fouled or saturated spark plug. If a leaking injector can not be determined by a fouled or saturated spark plug, the following procedure should be used: Remove plenum, cold start valve and fuel rail bolts. Reconnect cold start valve. Connect a hose to valve nozzle and insert into a gasoline container. Lift fuel rail out just enough to leave injector nozzles in the ports. Turn key on to pressurize fuel system. DO NOT start engine. Lift each side of rail up and check for injector leaking.
5. See HOW TO USE THIS TROUBLE SHOOTING ARTICLE in the article «CEC TESTING W/O CODES (TROUBLE SHOOTING)»(/pontiac/firebird/iii-1982-1992/remont/testing-diagnostics/#cec-testing-wo-codes-trouble-shooting) in the ENGINE PERFORMANCE section.

1. If fuel pressure is less than 40 psi (2.8 kg/cm 2 ) check the following: System has regulated pressure but pressure is less than 40 psi (2.8 kg/cm 2 ). Amount of fuel to injectors okay, but pressure is too low. Fuel system will run lean and may set Code 44. Also, vehicle is hard starting cold with overall poor performance. Restricted fuel flow that causes pressure drop. Normally a vehicle with a fuel pressure of less than 24 psi (1.6 kg/cm 2 ) at idle will not be driveable, however, if the pressure drop occurs only while driving, the engine will normally surge then stop as pressure begins to drop rapidly.
2. Restricting the fuel return line allows the fuel pump to develop its maximum pressure (dead head pressure). When battery voltage is applied to the pump test terminal or fuel pump relay, pressure should be greater than 60 psi (4.1 kg/cm 2 ).
3. This test determines if the high fuel pressure is due to a restricted fuel return line or a pressure regulator problem.

Chart A7 (1 of 2), Fuel System Diagnosis, Schematic. Scheme 172

Chart A7 (1 of 2), Fuel System Diagnosis, Schematic (1 of 2 PART A). Scheme 173

Chart A7 (2 of 2), Fuel System Diagnosis, Schematic (2 of 2 PART B). Scheme 174

CHART A9 - COLD START VALVE

The cold start valve is used to provide additional fuel during the crank mode to improve cold starts. This circuit is necessary because when engine coolant temperature is low, injector pulse width is not long enough to provide the needed amount of fuel for cold start.

This circuit is completely independent of the ECM. The circuit is activated only in the crank mode. The power is supplied directly from the starter solenoid and is protected by a fuse. The system is controlled by a cold start thermo time switch which provides a ground path for the valve during cranking when engine coolant is less than 95°F (35°C).

The cold start thermo time switch consist of a bi-metallic contact which opens at a specified coolant temperature. This bi-metallic switch is also heated by the circuit winding in the switch. This allows the valve to energize for a maximum of 8 seconds even at coolant temperatures as low as -4°F (-20°C). The time the switch will stay closed (1-8 seconds) varies with coolant temperature. In other words, as the coolant temperature goes up, the cold start valve "on" time goes down.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Disconnecting the distributor 4-way connector will disable the other injectors. The amount of pressure drop depends on the temperature of the engine. This test could also be performed by removing the 2 injector fuses.
2. This test will determine the resistance through the switch to ground.

Chart A9, Cold Start Valve, Schematic. Scheme 175

Chart A9, Cold Start Valve (1 of 3). Scheme 176

Chart A9, Cold Start Valve (2 of 3). Scheme 177

Chart A9, Cold Start Valve (3 of 3). Scheme 178

CHART B1 - RESTRICTED EXHAUST SYSTEM CHECK

Before any components are replaced, exhaust system must be checked for restrictions. Check at AIR pipe or check at O2 sensor procedure may be used to diagnose condition, depending on engine or tool used.

Check At Air Pipe

Remove rubber hose at exhaust manifold AIR pipe check valve and remove check valve. Install fuel pump pressure gauge to hose and nipple via Propane Enrichment Device (J26911) as shown in Fig. (Scheme 179). Nipple should be inserted into exhaust manifold AIR pipe.

Restricted Exhaust System Check at Air Pipe. Scheme 179

Check At O2 Sensor

Remove O2 sensor. Install back pressure tester in place of O2 sensor as shown in illustration. After test is completed, ensure that O2 sensor threads are coated with anti-seize compound before installation.

Restricted Exhaust System Check at O2 Sensor. Scheme 180

Diagnosis

1. Start engine and bring to operating temperature. Allow engine to idle and observe exhaust system back pressure gauge. Reading should not exceed 1.25 psi (.09 kg/cm 2 ).
2. Increase engine speed to 2000 RPM and note gauge. Reading should not exceed 3 psi (.21 kg/cm 2 ).
3. If during steps 1) or 2), specification is exceeded, exhaust system restriction is indicated.
4. Check complete exhaust system for collapsed pipe, heat distress and possible internal muffler failure.
5. If none of the conditions in step 4) exist, check for restricted catalytic converter. Replace if necessary.

CODE 13 - OPEN OXYGEN SENSOR CIRCUIT

The ECM supplies a voltage of about .45 volt between circuits No. 412 and 413. If measured with a 10-megohm digital voltmeter, this may read as low as .32 volt. The oxygen sensor varies the voltage within a range of about one volt if the exhaust is rich, down to about .10 volt if exhaust is lean. The sensor is like an open circuit and produces no voltage when it is less than 600°F (316°C). An open sensor circuit or cold sensor causes "open loop" operation.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 13 will set when the following conditions occur: Engine at normal operating on temperature. At least 2 minutes elapsed since start-up. Oxygen signal voltage steady between .35 and .55 volt. Throttle angle greater than 5 percent (about .3 volt greater than closed throttle voltage). All conditions must be met for about 60 seconds. If conditions for a Code 13 exist, the system will not go "closed loop".
2. This test will determine if the sensor, wiring or ECM is the cause of Code 13.
3. When conducting this test, use only a high impedance (10-megohm) digital volt/ohmmeter. This test checks the continuity of circuits No. 412 and 413. If circuit No. 413 is open the ECM voltage on circuit No. 412 will be greater than .6 volt.

Normal "Scan" tester voltage varies between 100 and 999 mV while in "closed loop". Code 13 will set in one minute if voltage remains between .35-.55 volt, however, system will go "open loop" in about 15 seconds.

Code 13, O2 Sensor Ckt, Schematic. Scheme 181

Code 13, O2 Sensor Ckt, Schematic. Scheme 182

CODE 14 - COOLANT TEMPERATURE SENSOR (CTS) SIGNAL VOLTAGE LOW

The Coolant Temperature Sensor (CTS) uses a thermistor to control the signal voltage to the ECM. The ECM applies and monitors a voltage on circuit No. 410 to the sensor. When the engine is cold, the sensor resistance is high, therefore, the ECM will see high monitored voltage. As the engine warms, the sensor resistance becomes less and the monitored voltage drops. At normal operating temperature, the voltage will measure about 1.5-2.0 volts on circuit No. 410. Coolant temperature is one of the inputs used to control; fuel delivery, spark timing, idle speed, converter clutch, canister purge, air management (man. trans. only), EGR and cooling fan. Code 14 will set if monitored voltage indicates a coolant temperature greater than 266°F (130°C) for about 3 seconds.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 14 will set if signal voltage indicates high coolant temperature for 3 seconds.
2. This test will determine if circuit No. 410 is shorted to ground which will cause the conditions for Code 14.

Check harness routing for a potential short to ground in circuit No. 410. The "Scan" tester displays engine temperature in degrees centigrade. After engine is started, the temperature should rise steadily to about 90°C, then stabilize when thermostat opens.

Code 14, CTS Voltage Low, Schematic. Scheme 183

Code 14, CTS Voltage Low, Schematic. Scheme 184

CODE 15 - CTS SIGNAL VOLTAGE HIGH

The CTS uses a thermistor to control the signal voltage to the ECM. The ECM applies and monitors voltage on circuit No. 410 to the sensor. When the engine is cold, sensor resistance is high, therefore, the ECM will see a high monitored voltage. As the engine warms, the sensor resistance becomes less and the monitored voltage drops. At normal operating temperature, the voltage will measure about 1.5-2.0 volts on circuit No. 410. Coolant temperature is one of the inputs used to control: fuel delivery, spark timing, idle speed, converter clutch, canister purge, air management (manual transmission), EGR and cooling fan.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 15 will set if signal voltage indicates coolant temperature is less than -47°F (-44°C) for 3 seconds.
2. This test simulates a Code 14. If the ECM recognizes the low signal voltage and the "Scan" tester reads 130°C or more, ECM and wiring are okay.
3. This test will determine if circuit No. 410 is open. There should be 5 volts present at sensor connector if measured with a DVOM.

The "Scan" tester reads engine temperature in degrees centigrade. After engine is started, the temperature should rise steadily to about 90°C, then stabilize when thermostat opens.

A faulty connection, or an open in circuits No. 410 or 452 will result in a Code 15. If Code 22 or 23 is also set, check circuit No. 452 for faulty wiring or connections. Check terminals at sensor for a good contact.

Code 15, CTS Voltage High, Schematic. Scheme 185

CODE 21 - TPS SIGNAL VOLTAGE HIGH

The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to throttle angle. Signal voltage will vary from about .5 volt at idle to about 4.5 volts at wide open throttle. Code 21 will set if the following conditions occur

1. TPS voltage is greater than 2.5 volts for at least 2-5 seconds.
2. Airflow less than 12 gm/sec.
3. Engine speed less than 1200 RPM NOTE: Test numbers refer to test numbers on diagnostic chart.

1. Confirms Code 21 and that fault is present.
2. With TPS sensor disconnected, the TPS voltage should go low if the ECM and wiring are okay.
3. Probing circuit No. 452 with a test light checks the 5-volt return circuit. A faulty 5-volt return will cause a Code 21.

The "Scan" tester reads throttle position in volts. Reading should be less than .7 volt with throttle closed and ignition on or at idle. Voltage should increase at a steady rate as throttle is moved toward WOT. An open in circuit No. 452 will result in a Code 21. Some "Scan" testers measure throttle angle as a percent. A fully closed throttle should read zero percent and a wide open throttle should read 100 percent.

Code 21, TPS Voltage High, Schematic. Scheme 186

Code 21, TPS Voltage High, Schematic. Scheme 187

CODE 22 - THROTTLE POSITION SENSOR SIGNAL VOLTAGE LOW

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

Note. Test numbers refer to test numbers on diagnostic chart.

1. Confirms Code 22 and that fault is present.
2. Simulates Code 21. If the ECM recognizes the high signal voltage and sets Code 21, the ECM and wiring are okay.
3. With throttle closed, the TPS voltage reading should be at idle specification. See DIAGNOSTIC AIDS below.
4. This simulates a high signal voltage to check for an open in circuit No. 417.

Using a "Scan" tester, reading should be less than .7 volt with throttle closed and ignition on or at idle. Monitored voltage should increase at a steady rate as throttle is moved toward WOT. An open or short to ground in circuits No. 416 or 417 will result in a Code 22. Some "Scan" testers measure throttle angle as a percent. A fully closed throttle should read zero percent and a wide open throttle should read 100 percent.

Code 22 Flow Chart - TPS Voltage Low. Scheme 188

CODE 23 - MAT SENSOR SIGNAL VOLTAGE HIGH

The Manifold Air Temperature (MAT) sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies and monitors voltage on circuit No. 472 to the sensor. When the air is cold, sensor resistance is high and the ECM will see a high monitored voltage. If the air is warm, the sensor resistance is low and ECM will see a low monitored voltage.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 23 will set if signal voltage indicates manifold air temperature is low, time since engine start is more than 8 minutes and no vehicle speed signal is present (vehicle not moving). Due to the conditions necessary to set a Code 23, the "SERVICE ENGINE SOON" light will only stay on when all 3 conditions are met.
2. A Code 23 will set due to an open sensor, wire or connection. This test will determine if the wiring and ECM are okay. The MAT sensor is difficult to reach on some models so this test can be performed at the MAT sensor harness connector.
3. This will determine if an open is present in the monitored signal circuit No. 472 or the 5-volt return circuit No. 452.

The "Scan" tester reads temperature of the air entering the engine (should read close to ambient air temperature when engine is cold) and rises as underhood temperature increases. Carefully check harness and connections for possible open in sensor circuits. Use the TEMPERATURE-TO-RESISTANCE VALUES table to help determine if sensor calibration has shifted.

Code 23, MAT Sensor Voltage High, Schematic. Scheme 189

Code 23, MAT Sensor Voltage High (1 of 2). Scheme 190

Code 23, MAT Sensor Voltage High (2 of 2). Scheme 191

CODE 24 - VEHICLE SPEED SENSOR

The ECM applies and monitors 12 volts on circuit No. 437. Circuit No. 437 is connected to the vehicle speed sensor buffer in the instrument cluster. The speed sensor buffer alternately grounds circuit No. 437 when drive wheels are turning. This pulsing action takes place about 2000 times per mile and the ECM will calculate vehicle speed based on the time between pulses. "F" Bodies use a Permanent Magnet (PM) generator mounted in the transmission to send signals to the VSS buffer in the instrument panel.

The "Scan" tester reading should closely match the speedometer reading with drive wheels turning.

Note. Test numbers refer to test numbers on diagnostic chart.

1. To avoid possible damage to CV joints on FWD vehicles, always support control arms whenever drive wheels are turning off of ground. Code 24 will set if vehicle is stopped when the following conditions occur: Reference signal indicates engine speed is between 1400 and 4400 RPM. Throttle angle is less than 2 percent (closed throttle). Low load condition (low airflow or high vacuum). Transmission not in Park or Neutral. All conditions met for 4-5 seconds. These conditions are met during a road load deceleration. Disregard Code 24 that sets when drive wheels are not turning.
2. A voltage of less than one volt at the IP connector indicates that circuit No. 437 is shorted to ground. If after disconnecting circuit No. 437 at the vehicle speed sensor, the voltage reads greater than 10 volts, the vehicle speed sensor is faulty. If voltage remains less than 10 volts, then it is possible that circuit No. 437 wire is grounded. If circuit No. 437 is not grounded, there is a faulty connection at the ECM, or a faulty ECM.

A "Scan" tester should indicate a vehicle speed whenever the drive wheels are turning greater than 3 MPH. A faulty or misadjusted Park/Neutral switch can result in a false Code 24. Use "Scan" tester and check for proper signal while wiggling shifter in Drive.

Code 24, Vehicle Speed Sensor, Schematic. Scheme 192

Code 24, Vehicle Speed Sensor, Schematic. Scheme 193

CODE 25 - MANIFOLD AIR TEMPERATURE SENSOR SIGNAL VOLTAGE LOW

The Manifold Air Temperature (MAT) sensor uses a thermistor to control the signal voltage to the ECM. The ECM applies and monitors voltage on circuit No. 472 to the sensor. When manifold air is cold, the sensor resistance is high and the ECM will see a high monitored voltage. As the air warms, the sensor resistance becomes less, and monitored voltage drops. Code 25 will set if monitored voltage indicates manifold air temperature greater than 275°F (134°C) for 3 seconds and time since engine start is 8 minutes or longer. Due to the conditions necessary to set a Code 25, the "SERVICE ENGINE SOON" light will remain on only while the signal is low and vehicle speed is present.

The "Scan" tester reads temperature of the air entering the engine. Parameter should read close to ambient air temperature when engine is cold, and rise as underhood temperature increases). A faulty connection, or an open in the MAT signal or ground circuit will result in a Code 23. Use the TEMPERATURE-TO-RESISTANCE VALUES table and the appropriate Flow Chart/Schematic to help determine if the sensor calibration has shifted.

TEMPERATURE-TO-RESISTANCE

Code 25 Flow Chart - MAT Sensor Voltage Low. Scheme 194

CODE 32 - EGR SYSTEM FAILURE

EGR vacuum is regulated by an ECM-controlled solenoid. The ECM turns the EGR on and off (duty cycle) by grounding and ungrounding circuit No. 435. The duty cycle is calculated by the ECM based on coolant temperature, airflow and engine RPM. There should be no EGR when vehicle is in Park or Neutral, TPS input is less than a specified value or TPS is indicating Wide Open Throttle (WOT). With the ignition on and engine stopped, the EGR solenoid is de-energized. By grounding the diagnostic "test" terminal, the solenoid should energize.

Code 32 means that the EGR solenoid switch was closed during start-up or that the switch was not detected closed under the following conditions

1. Coolant temperature is greater than 176°F (80°C).
2. EGR duty cycle command is greater than 48 percent.
3. TPS less than Wide Open Throttle (WOT), but not at idle.
4. Codes 21, 22, 33 and 34 not set.
5. All conditions above must be met for about 4 minutes.

If the switch is detected closed during start-up, or if the switch is detected open when above conditions are met, the "SERVICE ENGINE SOON" light remains on unless the switch changes state.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test will determine if the ECM set the code due to circuit No. 935 being grounded on start up. If the "Scan" tester does not indicate the switch is closed but the "SERVICE ENGINE SOON" light is on after start up, then this circuit should be checked carefully for an intermittent ground condition.
2. If the "Scan" tester indicates the switch is no longer closed after disconnecting it, be sure the switch is not closed due to heat. Ensure EGR valve is open prior to test.
3. This test will check for a possible open in circuit No. 935. The ECM supplies and monitors 9-12 volts on circuit No. 935. "Scan" tester should indicate switch being closed when circuit No. 935 is grounded.
4. By grounding the diagnostic "test" terminal, the EGR solenoid should close and allow vacuum to be applied. Vacuum should hold.
5. This test will determine if the electrical control part of the system is at fault, or if the connector or solenoid are at fault.
6. By plugging the EGR valve side and ungrounding the diagnostic "test" terminal, the solenoid valve should open and allow vacuum to bleed off through the vent.
7. With the engine not running and vacuum applied to the valve, the valve should move to the fully open position.
8. Due to engine using a negative backpressure valve, the valve should close when the engine is started.

Code 32 Schematic - EGR Sys Failure. Scheme 195

Code 32 - EGR Sys Failure (1 of 3). Scheme 196

Code 32 - EGR Sys Failure (2 of 3). Scheme 197

Code 32 - EGR Sys Failure (3 of 3). Scheme 198

CODE 33 - MAF SENSOR VOLTAGE HIGH

The Mass Airflow (MAF) sensor measures the amount of air entering the engine. The ECM uses this information to determine the operating condition of the engine and to control fuel delivery. The oil pressure switch or ECM, through control of the fuel pump relay, will provide 12 volts for the MAF power relay. This 12-volt signal is used by the Bosch MAF sensor to maintain a "hot wire" sensor circuit within the sensor body. The flow of air across this "hot wire" causes the wire to cool off. The sensor compensates by increasing current flow through the "hot wire" to maintain a calibrated temperature.

The ECM applies and monitors a current limiting 5 volts on circuit No. 998. As the MAF sensor varies current on the "hot wire" circuit, internal sensor circuitry changes the monitored 5-volt signal. Sensor circuitry will drop the voltage proportionately so that with low airflow, the ECM sees voltage as low as .4 volt and with high airflow, the ECM will see near the full 5-volt supply.

Due to the "hot wire" being exposed to air which always contains some contaminants, deposits may form on the "hot wire". To keep the system functioning properly, whenever the ignition is shut off, the sensor wire is heated to a temperature of 1000°F (538°C) by the MAF burn-off relay. With the relay energized, the ECM then monitors the MAF signal line to determine if the burn-off took place. If it did not occur, Code 36 will set in memory and the "SERVICE ENGINE SOON" light will come on the next time the vehicle is started.

Note. Test numbers refer to test numbers on diagnostic chart.

Code 33 indicates ECM has seen flow in excess of 45 grams per second (greater than about 2.2 volts) for one second when engine is first started, or TPS is less than 1/4 throttle and engine speed is less than 2000 RPM. Due to the 5-volt pull-up resistor in ECM circuit, if circuit No. 998 becomes open, the ECM will see a high monitored voltage signal and set Code 33.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Determines if the conditions to set Code 33 exist.
2. With the ALDL terminal "G" jumpered to 12 volts, there should be 12 volts at the sensor. If no voltage is present, make sure that the fuel pump is running. If not, repair fuel pump circuit.
3. If a burn-off signal is present at the MAF sensor with the engine running, a Code 33 will set. Be sure no voltage is present on circuit No. 994 for the first 2 seconds after the ignition is turned on or the first 25 seconds that the fuel pump is running.
4. The ECM supplies a voltage of 4-6 volts to the MAF sensor on circuit No. 998. This test checks for that voltage.

By jumpering the fuel pump test terminal (terminal "G" of ALDL) to 12 volts, the MAF sensor will stay powered up and the monitored signal line should see a low voltage (less than 250 mV or low gram per second on the "Scan" tester). By wiggling the related wiring, the intermittent may be detected. Also, an erratic signal with the engine running may indicate faulty wiring or components.

Code 33, MAP Sensor, Schematic. Scheme 199

Code 33, MAP Sensor (1 of 4). Scheme 200

Code 33, MAP Sensor (2 of 4). Scheme 201

Code 33, MAP Sensor (3 of 4). Scheme 202

Code 33, MAP Sensor (4 of 4). Scheme 203

CODE 34 - MAF SENSOR VOLTAGE LOW

The Mass Airflow (MAF) sensor measures the amount of air entering the engine. The ECM uses this information to determine the operating condition of the engine and to control fuel delivery. The oil pressure switch or ECM, through control of the fuel pump relay, will provide 12 volts for the MAF power relay. This 12-volt signal is used by the Bosch MAF sensor to maintain a "hot wire" sensor circuit within the sensor body. The flow of air across this "hot wire" causes the wire to cool off. The sensor compensates by increasing current flow through the "hot wire" to maintain a calibrated temperature.

The ECM applies and monitors a current limiting 5 volts on circuit No. 998. As the MAF sensor varies current on the "hot wire" circuit, internal sensor circuitry changes the monitored 5-volt signal. Sensor circuitry will drop the voltage proportionately, so that with low airflow, the ECM sees voltage as low as .4 volt and with high airflow, the ECM will see near the full 5-volt supply.

Due to the "hot wire" being exposed to air which always contains some contaminants, deposits may form on the "hot wire". To keep the system functioning properly, whenever the ignition is shut off, the sensor wire is heated to a temperature of about 1000°F (538°C) by the MAF burn-off relay. With the relay energized, the ECM monitors the MAF signal line to determine if the burn-off took place. If it did not occur, Code 36 will set in memory and the "SERVICE ENGINE SOON" light will come on the next time the vehicle is started.

Note. Test numbers refer to test numbers on diagnostic chart.

1. A Code 34 may be caused by an engine that exhibits a low, rough, unstable or incorrect idle problem. If this condition exists, disconnect MAF sensor. If condition improves with sensor disconnected, replace sensor.
2. Tests to determine if the conditions still exist to set Code 34. With the MAF sensor disconnected, the ECM should see a high signal voltage and set Code 33. If Code 34 resets, wiring or ECM is faulty.

Be sure air ducts are clean and tight. Code 34 could result from a dirty or misadjusted throttle body.

Code 34 Flow Chart - MAF Sensor. Scheme 204

CODE 36 - MAF BURN-OFF FAILURE

The Mass Airflow (MAF) sensor measures the amount of air which passes through it. The ECM uses this information to determine the operating condition of the engine to control fuel delivery. Due to contaminates in the atmosphere, a residue may build up on the MAF sensor "hot wire". To maintain an accurate reading from the sensor, a burn-off cycle will occur when the ignition is turned off after the engine has been running a specified amount of time and engine is at operating temperature. The burn-off function takes place when the ECM grounds circuit No. 900 which energizes the MAF sensor burn-off relay. With the MAF sensor burn-off relay energized, voltage will be supplied to the MAF sensor terminal "D". Voltage will also be supplied through the normally closed set of contacts in the MAF power relay which will supply 12 volts to terminal "E" of the MAF sensor. During the burn-off cycle, the ECM monitors the 5-volt MAF signal line (circuit No. 998). The increase in temperature, up to 1000°F (538°C), on the MAF "hot wire" will be reflected as a voltage change on circuit No. 998. If this change is not reflected at the ECM, a Code 36 will be set in memory and the "SERVICE ENGINE SOON" light will come on the next time the vehicle is started.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test will determine if the burn-off function is operative or if the code was set due to an intermittent condition.
2. Check for continuous 12-volt supply to burn-off relay.
3. Grounding circuit No. 900 should energize the burn-off relay and close the contacts.
4. With the burn-off relay energized, there should be 12 volts supplied to the MAF sensor on terminals "D" and "E" (circuits No. 993 and 994).

Code 36 could have been set due to a poor connection at any of the relays or the MAF sensor. Be sure that these connections and terminals are okay. A faulty MAF sensor should not be considered as the cause if Code 36 is set.

Code 36, MAF Burn-Off Failure, Flow Chart. Scheme 205

CODE 41 - CYL SELECT ERROR

Note. Test numbers refer to test numbers on diagnostic chart.

1. The ECM used for this engine can also be used for other engines. The difference is in the MEM-CAL. If a Code 41 sets, the incorrect MEM-CAL has been installed or MEM-CAL is faulty and must be replaced.

Check MEM-CAL to be sure locking tabs are secure. Also check the pins on both the MEM-CAL and ECM to verify they are making proper contact. Check the MEM-CAL part number for proper application. If the correct MEM-CAL is installed and is defective, it is possible the ECM will also need to be replaced.

Code 41, Cylinder Select Error, Schematic. Scheme 206

Code 41, Cylinder Select Error, Schematic. Scheme 207

CODE 42 - EST (W/HEI)

When the system is running on the ignition module (no voltage on the by-pass line), the ignition module grounds the EST signal. The ECM expects to see no voltage on the EST line during this condition. If it sees a voltage, it sets Code 42 and won't go into the EST mode.

When the RPM for EST operation is reached (400 RPM), by-pass voltage is applied. At this time, the EST should no longer be grounded in the ignition module, so the EST voltage should be varying. If the by-pass line is open or grounded, the ignition module will not switch to EST mode, so the EST voltage will be low and Code 42 will be set. If the EST line is grounded, the ignition module will switch to EST, but because the line is grounded, there will be no EST signal and Code 42 will be set.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 42 means the ECM has seen an open or short to ground in the EST or by-pass circuits. This test confirms Code 42 and that the fault causing the code is present.
2. Checks for a normal EST ground path through the ignition module. If EST circuit No. 423 is shorted to ground, it will also read less than 500 ohms. This will be checked later.
3. As the test light voltage touches circuit No. 424, the module should switch, causing the ohmmeter to overrange if the meter is in the 1000-2000 ohms position. Selecting the 10,000-20,000 ohms position will indicate a reading greater than 5000 ohms. The important thing is that the module switched.
4. The module did not switch and this test checks for EST circuit No. 423 for being shorted to ground, by-pass circuit No. 424 open, faulty ignition module connections or module.
5. Confirms that Code 42 is a faulty ECM and not an intermittent in circuits No. 423 or 424.

The "Scan" tester does not have the ability to help diagnose a Code 42 problem. A PROM/Mem-Cal not fully seated in the ECM can result in a Code 42.

Code 42, Electronic Spark Timing, Schematic. Scheme 208

Code 42, Electronic Spark Timing (1 of 2). Scheme 209

Code 42, Electronic Spark Timing (2 of 2). Scheme 210

CODE 43 - ELECTRONIC SPARK CONTROL

Electronic Spark Control (ESC) is accomplished using a knock sensor and a control module which sends a voltage signal to the ECM. As the knock sensor detects engine knock, the voltage from the ESC module to the ECM drops, signaling the ECM to retard timing. The ECM will retard the timing when knock is detected and engine speed is greater than 900 RPM.

Code 43 means the ECM has seen low voltage on circuit No. 485 (terminal "B7") for longer than 5 seconds with the engine running, or the system has failed the functional check. This system performs a functional check once per start-up to check the ESC system. To perform this test, the ECM will advance the spark when coolant temperature is greater than 194°F (95°C) and a high load condition exists (near WOT). If knock occurs, the functional test has passed. If the ECM detected knock before coolant temperature reached 194°F (95°C), the system is operating properly and the functional check will not be run. If the functional check fails, the "SERVICE ENGINE SOON" light will remain on until ignition is turned off, or until a knock signal is detected.

Note. Test numbers refer to test numbers on diagnostic chart.

1. If the conditions for a Code 43 exist, the "Scan" tester will always display "Yes". There should not be a knock at idle unless an internal engine problem or a system problem exists.
2. This test will determine if the system is functioning properly at this time. Usually, a knock signal can be generated by tapping on the right exhaust manifold. If no knock signal is generated, try tapping on block closer to the area of the sensor.
3. Because Code 43 sets when the signal voltage on circuit No. 485 remains low, this test should cause the signal on circuit No. 485 to go high. The 12-volt signal should be seen by the ECM as "no knock" if the ECM and wiring are okay.
4. This test will determine if the knock signal is being detected on circuit No. 496, or if the ESC module is at fault.
5. If circuit No. 496 is routed to close to secondary ignition wires, the ESC module may see the induced interference as a knock signal.
6. This checks the ground circuit to the module. An open ground will cause the voltage on circuit No. 485 to be about 12 volts, which would cause the Code 43 functional tests to fail.
7. Touching circuit No. 496 with a test light to 12 volts should generate a knock signal. This will determine if the ESC module is operating correctly.

Code 43 can be caused by a faulty connection at the knock sensor, ESC module or at the ECM. Also, check circuit No. 485 for possible open or short to ground.

Code 43 Flow Chart - ESC. Scheme 211

Code 43 Flow Chart - ESC (1 of 2). Scheme 212

Code 43 Flow Chart - ESC (2 of 2). Scheme 213

CODE 44 - LEAN EXHAUST INDICATION

The ECM supplies a voltage of about .45 volt between circuits No. 412 and 413. If measured with a 10-megohm digital voltmeter, this may read as low as .32 volt. The oxygen sensor varies the voltage within a range of about one volt if the exhaust is rich, down to about .10 volt if exhaust is lean. The sensor is like an open circuit and produces no voltage when it is less than about 600°F (360°C). An open sensor circuit or cold sensor causes "open loop" operation.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 44 is set when the oxygen sensor signal voltage on circuit No. 412 remains less than .2 volt for at least 60 seconds and the system is operating in "closed loop".

Using the "Scan" tester, observe the block learn values under different RPM and airflow conditions. If the conditions for Code 44 exists, the block learn values will be around 150.

1. Oxygen sensor pigtail may be mispositioned and contacting the exhaust manifold.
2. Check for intermittent ground in wire between connector and sensor.
3. A MAF sensor output that causes the ECM to sense a less than normal airflow will cause the system to go lean. If this occurs, disconnect the MAF sensor. If the lean condition is gone, replace the MAF sensor. MAF Tester (J 36101) may be used to test for an out-of-calibration sensor.
4. Water, even in small amounts, near the in-tank fuel pump inlet can be delivered to the injectors. The water causes a false lean exhaust condition and can set a Code 44.
5. System will be lean if pressure is too low. It may be necessary to monitor fuel pressure while driving the vehicle at various speeds. Check fuel volume from pump (one pint in 30 seconds) as well as pressure. Also check rubber fuel lines for internal collapse.
6. If there is an exhaust leak, outside air may be pulled into the exhaust and past the sensor. Vacuum or crankcase leaks can cause A lean condition.
7. If the above tests are okay, replace oxygen sensor.

Code 44, Lean Exhaust Indication, Schematic. Scheme 214

CODE 45 - RICH EXHAUST INDICATION

The ECM supplies a voltage of about .45 volt between circuits No 412 and 413. The oxygen sensor varies the voltage within a range of about one volt if the exhaust is rich, down to about .10 volt if exhaust is lean. The oxygen sensor is like an open circuit and produces no voltage when it is less than about 600°F (360°C). An open sensor circuit or cold sensor causes "open loop" operation.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Code 45 is set when the oxygen sensor signal voltage on circuit No. 412 remains greater than .7 volt for 30 seconds, in "closed loop", engine time after start is one minute or more and throttle angle is greater than 2 percent.

Using the "Scan" tester, observe the block learn values at different RPM and airflow conditions. If the conditions for Code 45 exists, the block learn values will be around 115.

1. Fuel system will go rich if pressure is too high. The ECM can compensate for some increase, however, if it gets too high, a Code 45 may be set.
2. Check for fuel contaminated oil.
3. An open ground circuit No. 453 (ignition module ground to ECM) may result in induced electrical "noise". The ECM looks at this "noise" as reference pulses (RPM). The additional pulses result in a higher than actual engine speed signal. The ECM then delivers too much fuel, causing system to go rich. If this problem occurs, "Scan" tester will show a greater than actual engine speed, which can help in diagnosing this problem.
4. Check vapor canister for fuel saturation. If canister is full of fuel, check canister liquid/vapor control and hoses.
5. An output that causes the ECM to sense a higher than normal airflow can cause the system to go rich. Disconnecting the MAF sensor will allow the ECM to set a fixed value for the sensor. Substitute a different MAF sensor, if the rich condition is gone when the sensor is disconnected, or test MAF sensor using MAF Tester (J 36101).
6. Check for leaking fuel pressure regulator diaphragm by checking vacuum line to regulator for fuel.
7. An intermittent TPS output will cause the system to go rich due to a false indication of the engine accelerating.

Code 45 Flow Chart - Rich Exhaust Indication. Scheme 215

CODE 51 - FAULTY MEM-CAL

Check that all pins are fully inserted in socket and that MEM-CAL is properly latched. If ok, replace, MEM-CAL, clear memory, and recheck. If code 51 reappears, replace ECM.

CODE 53 - SYSTEM OVER VOLTAGE

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

CODE 54 - FUEL PUMP VOLTAGE LOW

Fuel pump circuit No. 120 is monitored by the ECM and is used to compensate fuel delivery based on system voltage. This signal is also used to store a trouble code if the fuel pump relay is defective or if fuel pump voltage is lost while the engine is running. There should be about 12 volts on circuit No. 120 for 2 seconds after the ignition is turned on, or any time references pulses are being received by the ECM.

Code 54 sets if the voltage on circuit No. 120 is less than 2 volts for 1.5 seconds since the last reference pulse is cycled off, however, if the voltage is detected less than 2 volts with the engine running, the light will only remain on while the condition exists.

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

Code 54 Flow Chart - Fuel Pump Voltage Low. Scheme 216

Code 54 Flow Chart - Fuel Pump Voltage Low (1 of 2). Scheme 217

Code 54 Flow Chart - Fuel Pump Voltage Low (2 of 2). Scheme 218

CODE 55 - ECM ERROR

Ensure ECM grounds are okay. If okay, replace ECM. Clear codes and confirm "closed loop" operation and no "SERVICE ENGINE SOON" light.

CHART C1A - PARK/NEUTRAL SWITCH

The Park/Neutral switch contacts are a part of the neutral start switch. Contacts are closed to ground in Park or Neutral. The ECM applies and monitors a 12-volt signal on circuit No. 434 and senses a closed switch when voltage drops to less than one volt. If circuit No. 434 indicates Park/Neutral (grounded) while in Drive, the EGR system would be inoperative, resulting in possible detonation. If circuit No. 434 indicates Drive (open), a dip in idle may exist when gear selector is moved to Drive.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Checks for a switch closed to ground in Park.
2. Checks for an open switch in Drive.
3. To test for an intermittent or misadjusted switch in Drive, be sure "Scan" tester indicates Drive, even while wiggling shifter.

C1A - Park/Neutral Switch. Scheme 219

C1A - Park/Neutral Switch. Scheme 220

CHART C2A - INJECTOR BALANCE TEST

The injector balance test is used to pulse the injector for a precise amount of time, spraying a measured amount of fuel in the intake manifold. As each injector is pulsed, a drop in fuel rail pressure occurs. This pressure drop can be recorded and compared to other injectors. An injector that has a pressure drop of 1.5 psi (.11 kg/cm 2 ) or more, greater than or less than other injectors, should be considered faulty.

Note. Allow engine to cool down to avoid irregular readings due to "Hot Soak" fuel boiling. In order to prevent flooding, the INJECTOR BALANCE TEST should not be repeated more than once, without starting and running engine.

1. With ignition off, connect Fuel Pressure Gauge (J 34730-1) to pressure tap. Unplug harness connector at all injectors. Connect Injector Tester (J 34730-3) to one of the injectors. On turbocharged engines, use adapter harness supplied with injector tester to pulse injectors that are not accessible.
2. Follow manufacturer's instructions for use of the adapter harness. Ignition should be turned off for at least 10 seconds to complete ECM shutdown cycle.
3. Turn ignition on. Fuel pump should run at least 2 seconds after ignition is turned on. Bleed air from gauge and hose to ensure accurate gauge reading. Repeat this procedure until all air is bled from system. Turn ignition off for at least 10 seconds.
4. Turn ignition on again to bring fuel pressure to maximum. Record initial pressure reading. Energize tester one time and note pressure drop at lowest point.
5. Disregard any slight pressure drop after low point is reached. Subtracting second pressure reading from initial reading indicates amount of injector pressure drop.
6. Repeat test step 4) on each injector and compare amount of pressure drop. Recheck injectors that do not read within pressure drop range. Replace injector(s) that fail second check.
7. If injectors are all okay, plug in harness connectors and review SYMPTOMS in TROUBLE SHOOTING section.

Chart C2A, Injector Balance Test. Scheme 221

CHART C2C - IDLE AIR CONTROL

The ECM will control engine idle speed by moving the Idle Air Control (IAC) valve to control airflow around the throttle plate. It does this by sending voltage pulses to the proper motor winding for each internal IAC motor. This will cause the motor shaft and valve to move in or out of the motor a given distance for each pulse received. The IAC position is measured in counts. Zero counts is a fully extended valve (no airflow), 255 counts is a fully retracted valve (maximum airflow).

To increase idle speed, the ECM will send a signal to retract the IAC valve and allow more air to flow through the idle air passage and by-pass the throttle plate. This will increase the IAC counts. To decrease idle speed the ECM will send a signal to extend the IAC valve and reduce airflow through the idle air passage around the throttle plate. This will reduce the IAC counts.

Each time the engine is started and then the ignition is turned off, the ECM will reset the IAC valve. This is done by sending enough extend pulses to seat the valve (to zero counts). The fully seated valve is the ECM reference zero. A given number of counts are then calculated by the ECM. This is how the ECM knows what the motor position is for a given idle speed.

Note. Test numbers refer to test numbers on diagnostic chart.

1. Continue with test even if engine will not idle. If idle is too low, "Scan" tester will display 80 or more counts. If idle is high it will display zero counts. Occasionally, an erratic or unstable idle may occur. Engine speed may vary 200 RPM or more up or down. If this condition exists, disconnect IAC motor. If the condition is unchanged, the IAC is not at fault. There is a system problem. Proceed to DIAGNOSTIC AIDS.
2. When the engine was stopped, the IAC valve retracted to a fixed "Park" position for increased airflow and idle speed during the next engine start. A "Scan" tester will display 40 or more counts.
3. Be sure to disconnect the IAC valve prior to this test. The test light will confirm the ECM signals by a steady or flashing light on all circuits.
4. There is a remote possibility that one of the circuits is shorted to voltage which would have been indicated by a steady light. Disconnect ECM and turn the ignition on. Probe terminals to check for this condition.

Engine idle speed can be adversely affected by the following

1. If ECM thinks the vehicle is always in Neutral, idle will not be controlled to the specified Drive RPM.
2. Leaking injectors will cause fuel imbalance and poor idle quality due to different air/fuel ratios in each cylinder. See appropriate CHART A7.
3. Vacuum or crankcase leaks can affect idle.
4. When the throttle shaft or TPS is binding or sticking in an open throttle position, the ECM does not know the vehicle has stopped and does not control idle.
5. Check air management system for intermittent air to ports while in "closed loop".
6. In addition to electrical control of EGR, be sure to examine the EGR valve for proper seating.
7. Faulty battery cables can result in voltage variations. The ECM will try to compensate by adjusting engine RPM. This results in erratic idle speeds.
8. The ECM will compensate for A/C compressor clutch loads. Loss of the A/C "on" signal would be most apparent in Neutral.
9. Contaminated fuel can adversely affect idle.
10. Perform injector balance test, see CHART C2A. If all checks okay, refer to HOW TO USE THIS TROUBLE SHOOTING ARTICLE in the «CEC TESTING W/O CODES (TROUBLE SHOOTING)»(/pontiac/firebird/iii-1982-1992/remont/testing-diagnostics/#cec-testing-wo-codes-trouble-shooting) article in the ENGINE PERFORMANCE section.

C2C, Idle Air Control, Schematic. Scheme 222

C2C, Idle Air Control, Schematic. Scheme 223

CHART C3 - CANISTER PURGE CHECK

Canister purge is controlled by a solenoid that allows manifold vacuum to purge the canister when de-energized. The ECM supplies a ground on circuit No. 428 to energize the solenoid.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks to see if the solenoid is open or closed. The solenoid is normally energized in this step. It should be closed.
2. This test checks for open or grounded solenoid circuit.
3. Completes functional check by grounding ALDL "test" terminal. This should normally energize the solenoid and allow the vacuum to drop.

Chart C3, Canister Purge Check, Schematic. Scheme 224

Chart C3, Canister Purge Check (1 of 2). Scheme 225

Chart C3, Canister Purge Check (2 of 2). Scheme 226

CHART C4 - IGNITION SYSTEM CHECK (W/HEI)

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test checks 2 wires, to ensure that an open is not present in a spark plug wire. 1A) If spark occurs with EST connector disconnected, pick-up coil output is too low for EST operation.
2. A spark indicates the problem must be the distributor cap or rotor.
3. There should be battery voltage at terminal "C" and at the "+" terminal. Low voltage would indicate an open or a high resistance circuit from the distributor to the coil or ignition switch. If terminal "C" voltage was low, but "+" terminal voltage is 10 volts or more, check circuit from terminal "C" to ignition coil for open or check ignition coil primary winding for open.
4. This test checks for a shorted module or grounded circuit from the ignition coil to the module. The distributor module should be turned off, so normal voltage should be about 12 volts. If the module is turned on, the voltage would be low, but should be greater than one 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 the battery to the tachometer terminal.
5. Applying 1.5-8 volts to module terminal "P" should turn the module on and the tachometer terminal voltage should drop to about 7-9 volts. This test will determine whether the module or coil is faulty or if the pick-up coil is not generating the proper signal to turn the module on. This test can be performed by using a DC battery with a rating of 1.5-8 volts. The use of the test light is mainly to allow the terminal "P" to be probed more easily. Some digital multi-meters can also be used to trigger the module by selecting the ohms scale, usually in the "diode" position. In this position, the meter may have a voltage across the terminals which can be used to trigger the module. The voltage in the "ohm" position can be checked by using a second meter or by checking the manufacturer's specification of the tester being used.
6. This should turn off the module and cause a spark. If no spark occurs, the fault is most likely in the ignition coil because most module problems would have been found before this point in the procedure. Use a module tester to determine which is at fault.

Chart C4, Ignition System (W/ HEI), Schematic. Scheme 227

Chart C4, Ignition System (W/ HEI) (1 of 4). Scheme 228

Chart C4, Ignition System (W/ HEI) (2 of 4). Scheme 229

Chart C4, Ignition System (W/ HEI) (3 of 4). Scheme 230

Chart C4, Ignition System (W/ HEI) (4 of 4). Scheme 231

CHART C5 - ELECTRONIC SPARK CONTROL

The Electronic Spark Control (ESC) system on these vehicles consists of a knock sensor and an ESC controller which sends a voltage signal to the ECM. As engine knock is detected by the knock sensor, a signal is sent to the ESC controller. The controller responds to this knock signal by cutting off current to ECM on circuit No. 485. The ECM will then retard timing if engine speed is greater than 850 RPM.

Note. Test numbers refer to test numbers on diagnostic chart.

1. If a Code 43 is not set, but a knock signal is indicated while running at 1500 RPM, listen for an internal engine noise. Under a no load condition, there should be no detonation. If knock is indicated, an internal engine problem may exist.
2. Usually, a knock signal can be generated by tapping on the right exhaust manifold. This test can also be performed at idle. Test No. 1 was run at 1500 RPM to determine if a constant knock signal was present, which would affect engine performance.
3. This tests whether the knock signal is due to the sensor, a basic engine problem, or the ESC module.
4. If the module ground circuit is faulty, the ESC module will not function correctly. The test light should light, indicating the ground circuits are okay.
5. Contacting circuit No. 496 with a test light to 12 volts should generate a knock signal to determine if the knock sensor is faulty, or if the ESC module can't recognize a knock signal.

Most "Scan" testers have 2 different parameter positions to diagnose the ESC system. The knock signal can be monitored to see if the knock sensor is detecting a knock condition and if the ESC module is functioning. The knock signal should display "YES", whenever detonation is present. The knock retard position on the "Scan" tester displays the amount of spark retard the ECM is commanding. The ECM can retard the timing up to 20 degrees. Also, see additional reasons for detonation/spark knock in DETONATION/SPARK KNOCK in the CEC TESTING W/O CODES (TROUBLE SHOOTING) article in the ENGINE PERFORMANCE section.

Chart C5, Electronic Spark Control (1 of 2). Scheme 232

Chart C5, Electronic Spark Control (2 of 2). Scheme 233

CHART C6B - AIR MANAGEMENT CHECK

Air management is controlled by a pressure operated port valve and a converter valve, each with an ECM controlled solenoid. When the solenoid is grounded by the ECM, air pressure will activate the valve and allow pump air to be directed as follows

1. Cold Mode Port solenoid is grounded by the ECM. Pump air then goes to exhaust ports.
2. Warm Mode Port solenoid is de-energized. Converter solenoid is grounded by the ECM. Pump air then goes to converter.
3. Divert Mode Neither solenoid is grounded by the ECM. Pump air is diverted to atmosphere. NOTE: Test numbers refer to test numbers on diagnostic chart.

1. This test is a system functional check. Air is directed to exhaust ports during "open loop". All engine starts are in "open loop", even on a warm engine. Since the air-to-ports time may be very short on a warmed-up vehicle, prepare to observe port air prior to engine start up. This can be done by squeezing air hose.
2. This should normally set a Code 22. When any code is set, the ECM opens the ground to the converter solenoid and allows air to divert to atmosphere. This checks for ECM response to a fault. A ground in the control valve circuit to the ECM would prevent divert action.
3. This checks for a grounded circuit to the ECM. Test light off is normal and would indicate that the circuit is not grounded.
4. Checks for an open in the solenoid control circuits. Grounding the "test" terminal should ground both solenoid circuits. Normally, the test light should be on, which indicates the problem is not in the ECM or wiring but at the solenoid connections or valve itself.
5. Checks for a grounded solenoid circuit. Test light off would indicate the circuit is okay and fault is in the valve.

Chart C6B, Air Management Check, Schematic. Scheme 234

Chart C6B, Air Management Check (1 of 3). Scheme 235

Chart C6B, Air Management Check (2 of 3). Scheme 236

Chart C6B, Air Management Check (3 of 3). Scheme 237

CHART C7 - EGR CHECK

The EGR valve is controlled by a normally open Pulse Width Modulated (PWM) solenoid. The ECM turns the solenoid off to allow vacuum to pass to the EGR and turns the solenoid on to prohibit EGR operation. When EGR is commanded, the solenoid is turned on and off many times a second (duty cycle).

The duty cycle is calculated by the ECM based on information from the coolant, MAT, TPS and MAF sensors. Also, engine RPM and the Park/Neutral switch inputs affect EGR. There should be no EGR when in Park or Neutral and TPS is below a calibrated value or TPS is indicating Wide Open Throttle (WOT). With the ignition on and engine stopped, the EGR solenoid is de-energized. The solenoid, however, should be energized if the diagnostic "test" terminal is grounded with the ignition on and engine not running.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This will test the solenoid to determine if it is capable of closing off the manifold vacuum from the EGR valve. The vacuum may bleed off slowly but this should not be considered a fault.
2. As soon as backpressure is available at the EGR valve, the bleed portion in the valve should open and cause the valve to go to its seated position.
3. The EGR valve will be inoperative if the Park/Neutral switch is misadjusted or defective. Use "Scan" tester and check Park/Neutral switch. See appropriate CHART C1A.

Chart C7, EGR Check, Schematic. Scheme 238

Chart C7, EGR Check (1 of 2). Scheme 239

Chart C7, EGR Check (2 of 2). Scheme 240

CHART C8A - TCC, (1 OF 2)

The purpose of the Torque Converter Clutch (TCC) feature is to eliminate the power loss of the torque converter stage when the vehicle is in a cruise condition. This allows the convenience of the automatic transmission and the fuel economy of a manual transmission. The heart of the system is a solenoid located inside the automatic transmission which is controlled by the ECM.

When the solenoid coil is activated, the TCC is applied through a mechanical coupling from the engine to transmission. When the transmission solenoid is de-energize, the TCC is released which allows the torque converter to operate in the conventional manner (fluidic coupling between engine and transmission). The ECM turns on the TCC when coolant temperature is greater than 149°F (65°C), TPS is not changing and vehicle speed is greater than a specified value.

Note. Test numbers refer to test numbers on diagnostic chart.

1. When a test light is connected from ALDL terminal "F" to ground, a test light on indicates battery voltage is okay and the TCC solenoid is disengaged.
2. When the diagnostic "test" terminal is grounded, the ECM should energize the TCC solenoid and the test light should go out.

A "Scan" tester only indicates when the ECM has turned on the TCC driver (grounded circuit No. 422). This does not confirm that the TCC has engaged. To determine if TCC is functioning properly, observe engine RPM. RPM should decrease when the "Scan" tester indicates the TCC driver has turned on. The switches will not prevent TCC from functioning, but will affect TCC lock and unlock points. If the 4th gear switch circuit is always open the TCC may engage as soon as sufficient oil pressure is reached.

Chart C8A (1 of 2), TCC, Schematic. Scheme 241

Check Made In This Chart Will Not Prevent The TCC From Working, But Will Affect Engagement Or Disengagement Points

Chart C8A (1 of 2), TCC, Schematic. Scheme 242

CHART C8A - TCC, (2 OF 2)

A 4th gear switch (mounted in the transmission) opens when the transmission shifts into 4th gear. This switch is used by the ECM to modify TCC lock and unlock points (when in a 4-3 downshift).

Note. Test numbers refer to test numbers on diagnostic chart.

1. Unless the switch or circuit No. 446 is open, the "Scan" tester should display "NO", indicating the transmission is not in 4th gear. The 4th gear switch should only be open while in 4th gear.
2. This test determines if the ECM and wiring are okay. Grounding circuit No. 446 should cause the "Scan" tester to display "NO", indicating the transmission is not in 4th gear.
3. Checks the operation of the 4th gear switch. When the transmission shifts into 4th gear, the switch should open and the "Scan" tester should display "YES".
4. Disconnecting the TCC connector simulates an open switch to determine if circuit No. 446 is shorted to ground or the problem is in the transmission.

A road test may be necessary to verify the problem. If the "Scan" tester indicates TCC is turning on and off erratically, check the status of the 4th gear switch to be sure it is not changing under a steady throttle position. If the switch is changing, check connections and wire routing carefully. If the 4th gear switch is always open the TCC may engage as soon as sufficient oil pressure is reached.

Using A "Scan" Tool, Check The Following And Correct If Necessary

*

Coolant Temperature Should Be Above 65°C

TPS - Be Sure TPS Signal Is Not Erratic

VSS - Be Sure "Scan" Displays VSS With Drive Wheels Turning

Codes - If Code 24 Is Present, See Code Chart 24

Chart C8A - TCC (2 Of 2), (1 Of 2 Part A). Scheme 243

Chart C8A - TCC (2 Of 2), (2 Of 2 Part B). Scheme 244

CHART C8B - SHIFT INDICATOR

The shift light indicates the best transmission shift point for maximum fuel economy. The light is controlled by the ECM and is turned on by grounding circuit No. 422/456. To control the shift light, the ECM uses information from the CTS, TPS, VSS and ignition module reference (RPM). The ECM uses the measured RPM and the vehicle speed to calculate what gear the vehicle is in. This calculation determines when the shift light should be turned on.

Note. Test numbers refer to test numbers on diagnostic chart.

1. This test should not turn on the shift light. If the light is on, there is a short to ground in circuit No. 422/456 wiring or a fault in the ECM.
2. When the diagnostic "test" terminal is grounded, the ECM should ground circuit No. 422/456 and the shift light should come on.
3. This checks the shift light circuit up to the ECM connector. If the shift light illuminates, the ECM connector is faulty or the ECM does not have the ability to ground the circuit.

C8B, Shift Light Indicator, Schematic. Scheme 245

C8B, Shift Light Indicator, Schematic. Scheme 246

CHART C12 - COOLING FAN CKT, (1 OF 2)

The cooling fan is totally controlled by the ECM based on inputs from the coolant sensor and fan control switch. The fan should run, if coolant temperature is greater than 226°F (108°C). Battery voltage is supplied to the fan relay on terminal "E" and ignition voltage to terminal "D".

Grounding circuit No. 335 (relay terminal "F") will energize the relay and supply battery voltage to the fan motor. Once the fan relay is energized by the ECM, it will remain on for a minimum of 30 seconds. The ECM will remove the ground to circuit No. 335 if vehicle speed is greater than 40 MPH or the engine is overheating.

The fan control switch is mounted in the A/C high pressure line and will open when head pressure exceeds 233 psi (16 kg/cm 2 ) and this input causes the ECM to ground circuit No. 335. If Code 14 or 15 sets or the ECM is operating in the fuel back-up mode, the ECM will turn on the cooling fan.

If there is an overheating problem, it must be determined if the complaint was due to an actual boil over or if the hot light or temperature gauge indicates overheating.

If the gauge or light indicates overheating but no boil over is detected, the gauge circuit should be checked. The gauge accuracy can also be checked by comparing the coolant sensor reading using a "Scan" tester and comparing its reading with the gauge reading.

If the engine is actually overheating and the gauge indicates overheating, but the cooling fan is not coming on, the coolant sensor has probably shifted out of calibration and should be replaced. If the engine is overheating and the cooling fan is on, the cooling system should be checked.

Chart C12, Cooling Fan Circuit, Schematic (1 of 2). Scheme 247

Chart C12 (1 of 2), Cooling Fan Circuit, Schematic (1 of 3 PART A). Scheme 248

Chart C12 (1 of 2), Cooling Fan Circuit, Schematic (2 of 3 PART B). Scheme 249

Chart C12 (1 of 2), Cooling Fan Circuit, Schematic (3 of 3 PART C). Scheme 250

CHART C12 - COOLING FAN CKT, (2 OF 2)

The cooling fan is totally controlled by the ECM based on inputs from the coolant sensor and fan control switch. The fan should run if coolant temperature is greater than 226°F (108°C). Battery voltage is supplied to the fan relay on terminal "E" and ignition voltage to terminal "D".

Grounding circuit No. 335 (relay terminal "F") will energize the relay and supply battery voltage to the fan motor. Once the fan relay is energized by the ECM, it will remain on for a minimum of 15 seconds. The ECM will remove the ground to circuit No. 335 if vehicle speed is greater than 40 MPH, unless the engine is overheating.

The fan control switch is mounted in the A/C high pressure line and will open when head pressure exceeds 233 psi (16 kg/cm 2 ). This input causes the ECM to ground circuit No. 335. If Codes 14 or 15 set, or the ECM is operating in the fuel back-up mode, the ECM will turn on the cooling fan.

If there is a problem of overheating, it must be determined if the problem was due to actual boil over or if the hot light or temperature gauge indicated overheating.

If the gauge or light indicates overheating but no boil over is detected, the gauge circuit should be checked. the gauge accuracy can also be checked by comparing the coolant sensor reading using the "Scan" tester and comparing its reading with the gauge reading.

If the engine is actually overheating and the gauge indicates overheating, but the cooling fan is not coming on, the coolant sensor has probably shifted out of calibration and should be replaced. If the engine is overheating and the cooling fan is on, the cooling system should be checked.

Chart C12, Cooling Fan Circuit, Schematic (2 of 2). Scheme 251

Chart C12 (2 of 2), Cooling Fan Circuit, Schematic. Scheme 252

Component Locations (Camaro). Scheme 253

Component Locations (Firebird). Scheme 254

ECM Terminal Identification. Scheme 255

Wiring Diagram. Scheme 256

See also:
• CEC TESTING W/O CODES (TROUBLE SHOOTING)