2.0l(k)/2.5l Tests W/codes Buick Skylark VI

DIAGNOSIS & TESTING

Note. The following conditions must be met before testing

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

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 208) 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 208

DIAGNOSTIC SYSTEM OPERATION

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

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

"HARD FAILURES"

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

"INTERMITTENT FAILURES"

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

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

BASIC DIAGNOSTIC PROCEDURE

Note. Most 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 DEFINITION table in this article.
3. If no trouble codes were displayed, perform FIELD SERVICE MODE CHECK procedures.
4. If no trouble is indicated by FIELD SERVICE MODE CHECK and/or a driveability problem exists, refer to SYMPTOM DIAGNOSIS and/or SCAN TESTER USAGE in this article. The comments there will send you to the proper component charts or tell you the most likely system/component to check.
5. After any repairs are made, clear any trouble codes and perform FIELD SERVICE MODE check again.

Scheme 209

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. For exact location of ALDL, see appropriate COMPONENT LOCATION figure at the end of this article. Insert jumper wire across terminal "B" (diagnostic "test" terminal), and terminal "A" (ground). (Scheme 209) CAUTION: Inserting spade lug (jumper lead) into terminals of ALDL connector grounds ALDL "test" terminal. Do not ground ALDL connector until after ignition is on (engine not running).
2. "SERVICE ENGINE SOON" light should flash Code 12. Code 12 consists of "FLASH", pause, "FLASH", "FLASH" followed by a longer pause. 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 digital voltmeter. 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 "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 Code 21. The first series of flashes are the first digit of trouble code. The 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 "test" 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 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. Anytime Codes 51, 52, 54 or 55 are displayed with another code, start with "50-series" code first, then proceed to low profile numbered codes.

TROUBLE CODE DETERMINATION (HARD OR INTERMITTENT)

During any diagnostic procedure, it must be determined if codes are "hard" failure codes or "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. See CLEARING TROUBLE CODES near the beginning of this article.
2. Apply parking brake and place transmission in Neutral or Park. Block drive wheels and start engine. "SERVICE ENGINE SOON" light should go out. Run warm engine at specified curb idle for 2 minutes and note "SERVICE ENGINE SOON" light.
3. If "SERVICE ENGINE SOON" light comes on, MANUALLY 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.
6. Charts where a stored trouble code leads you to a particular problem. See ECM TROUBLE CODE DEFINITION and DIAGNOSTIC AIDS in this article.
7. 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 many "trouble code" charts) 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.

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.

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 most of the time, a lean exhaust is indicated. If light is on 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 FWD 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 TOOLS (DIAGNOSTIC)

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 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, dwell meter, test light, ohmmeter, digital voltmeter with 10-megohm input impedance (minimum), vacuum pump, vacuum gauge, fuel injector test lights (TBI and PFI) and 6 jumper wires 6" long (one wire with female connectors at both ends, one wire with male connector at both ends and 4 wires with male and female connectors at opposite ends). A test light, rather than a voltmeter, must be used when indicated by a diagnostic chart.

On carbureted models, a dwell meter can be used to measure the time M/C solenoid is on or off. Dwell reading indicates if M/C solenoid is working, as well as 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. Some models may not be compatible with computerized engine control system.

Before engine reaches operating temperature, dwell should be fixed between 10-50 degrees, indicating "open loop" operation. With engine at operating temperature and idling, dwell meter needle should be varying between 10-50 degrees. This indicates "closed 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 in this article. 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 TESTER - TEST DATA PARAMETERS

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

SCAN TESTER - TEST DATA PARAMETERS

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). This is the same test point for connection of aftermarket "Scan" testers.

DIAGNOSTIC CIRCUIT CHECK

The diagnostic circuit check is an organized approach for identifying fuel injection problems created by an electronic engine control malfunction. The Assembly Line Data Link (ALDL) can provide diagnostic information for display on a "Scan" tester or any tester designed for this purpose. If a stored code is displayed, code definitions will aid in determining if fault is still present (hard failure) or result of an intermittent condition not normally diagnosed using code charts. Data parameters shown on tester can be compared to a similar vehicle or typical values shown in SCAN TESTER - TEST DATA PARAMETERS table in this article.

If "Scan" tester is not operating, check on another vehicle. If okay, the cigar lighter socket should be checked for 12 volts and a good ground. Also check for an open ALDL "test" terminal between ALDL terminal "B" and ECM. With ignition on, diagnostic line (ALDL terminal "B") should have about 5 volts present.

If trouble codes are not displayed in diagnostic circuit check, observe scan data parameters using a scan tester. Sensors that are out of specification will not set trouble codes, but may cause driveability problems. See appropriate specifications in the SCAN TESTER - TEST DATA PARAMETERS table in this article.

Diagnostic Circuit Check - Flow Chart. Scheme 210

CHART A1 - NO "SES" LIGHT

The "SERVICE ENGINE SOON" light should come on when ignition is turned on and engine is not running. If engine runs, a no "SERVICE ENGINE SOON" light condition indicates a defective bulb, blown fuse or open control circuit No. 419. If engine cranks but will not run, no "SERVICE ENGINE SOON" light indicates a blown battery fuse, blown fusible link, ECM ignition fuse blown, battery circuit No. 240 to ECM open, ignition circuit No. 439 to ECM open or poor connection to ECM.

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

1. This check determines if there is a fault in "SES" light circuit or in ECM.
2. If circuits No. 240 and 439 except 2.5L A Body (440 & 439 2.5L A Body) have voltage, the ECM grounds or ECM are faulty.
3. Using a test light connected to 12 volts, probe each of the ECM ground circuits.

Chart A1 Schematic, No "SES" Light (Except 2.5L A Body). Scheme 211

Chart A1 Schematic, No "SES" Light (2.5L A Body). Scheme 212

Flow Chart A1, No "SES" Light (All). Scheme 213

CHART A2 - NO CODE 12, "SES" LIGHT ALWAYS ON

The "SERVICE ENGINE SOON" light should come on when ignition switch is in the "ON" position and engine is not running. With diagnostic terminal grounded, light should flash Code 12, followed by other trouble codes stored in memory. A steady light indicates a grounded circuit No. 419 or an open diagnostic circuit No. 451.

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

1. If "Scan" tester is not operating, check on another vehicle. If okay, check circuit No. 461. The cigarette lighter socket should also be checked for 12 volts and a good ground. If all is okay, ECM or PROM is faulty.
2. If the light goes off when the ECM connector is disconnected, then circuit No. 419 is not shorted to ground. Check connector terminals physically for proper contact.
3. This step checks for an open diagnostic circuit No. 451.
4. At this point "SERVICE ENGINE SOON" light wiring is okay. Problem may be a faulty ECM or PROM. If Code 12 does not flash, the ECM should be replaced using the original PROM. Replace the PROM only after trying a new ECM, as a defective PROM is an unlikely cause of the problem.

Chart A2, No Code 12 Schematic, "SES" Always On (2.5L A Body). Scheme 214

Flow Chart A2, No Code 12 "SES" Light Always On (All). Scheme 215

CHART A3 - ENGINE CRANKS BUT WON'T RUN (2.0L VIN K) (1 OF 2)

1. A "SERVICE ENGINE SOON" light on checks for ignition and battery supply to ECM. If TPS signal is greater than 2.5 volts, the engine may be in the "clear flood" mode, which will cause starting problems. Engine will not start without RPM reference signals. "Scan" tester should display engine RPM while cranking.
2. If RPM was indicated while cranking, the ignition module is receiving a crank signal. No spark at this point indicates the ignition module is not triggering the coil.
3. While cranking engine, there should be no fuel spray with injector disconnected. Replace injector if injector leaks or sprays.
4. Test light should blink, indicating the ECM is controlling the injector. How bright the light blinks is not important.
5. Fuel spray from injector indicates that fuel is available. Check engine for flooding.
6. This test will determine if ignition module is not generating the reference pulse, or if the wiring or ECM are at fault. RPM should be indicated on "Scan" tester when test light is touched and removed from circuit No. 430.

1. Water or foreign material can cause a no start during freezing weather. The engine may start after 5 or 6 minutes in a heated shop. The problem may not re-occur until an overnight park in freezing temperatures.
2. 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.
3. Low fuel pressure can result in a very lean air/fuel ratio. See CHART A7 - FUEL SYSTEM DIAGNOSIS.

Chart A3 Schematic, Cranks But Won't Run (2.0L VIN K). Scheme 216

Flow Chart A3, Cranks But Won't Run (2.0L VIN K) (1 of 2). Scheme 217

CHART A3 - ENGINE CRANKS BUT WON'T RUN (2.0L VIN K) (2 OF 2)

Ignition voltage is supplied to the fuel injector through circuit No. 439. The injector will be pulsed (sprayed) when the ECM grounds and opens circuit No. 467.

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

1. This check determines if injector connector has ignition voltage on one terminal only.
2. A faulty ECM may result in damage to the injector.
3. A test light connected from ground to the ECM harness terminal "W1" should light due to continuity through injector.

Flow Chart A3, Cranks But Won't Run (2.0L VIN K) (2 of 2). Scheme 218

CHART A3 - CRANKS BUT WON'T RUN (2.5L A & N BODY)

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

1. A "SERVICE ENGINE SOON" light on checks for ignition and battery supply to ECM. If TPS signal is greater than 2.5 volts, engine may be in "clear flood" mode, causing engine not to start. "Scan" tester should display engine RPM while cranking.
2. Because the Direct Ignition System (DIS) uses 2 plugs and wires to complete the circuit of each coil, the opposite spark plug wire should be left connected. Using Spark Tester (ST-125), check for spark to spark plugs. No spark indicates a possible ignition module problem.
3. While cranking engine, there should be no fuel spray with injector disconnected. Replace injector if it sprays or if it drips.
4. If test light "blinks" while cranking, ECM control is functioning properly. How bright the light blinks is not important.
5. Reconnect injector. The EFI system is considered okay if fuel sprays from the injector; however, the engine could be severely flooded due to too much fuel.

1. Water or foreign material can cause a no start during freezing weather. The engine may start after 5 or 6 minutes in a heated shop. The problem may not re-occur until an overnight park in freezing temperatures.
2. 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, a "no start" condition may result.
3. Low fuel pressure can result in a very lean air/fuel ratio. See CHART A7 - FUEL SYSTEM DIAGNOSIS.

Chart A3 Schematic, Cranks But Won't Run (2.5L A & N Body). Scheme 219

Flow Chart A3, Cranks/Won't Run (2.5L A & N Body) (1 of 3). Scheme 220

CHART A3 - CRANKS BUT WON'T RUN (2.5L A & N BODY) (CONTINUED)

A magnetic pick-up crank sensor in the side of the block determines engine crankshaft position. Crankshaft rotation past the magnetic pick-up creates a reference signal for the DIS ignition module. The module processes this signal, creating necessary reference pulses needed by the ECM to trigger the coil at the correct time.

If "Scan" tester did not indicate cranking RPM, and there is no spark present at the plugs, the problem lies in the DIS unit or the power and ground supplies to the module.

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

1. Because the Direct Ignition System (DIS) uses 2 spark plugs and wires to complete the circuit of each coil, the opposite spark plug wire should be left connected.
2. This test will determine if the DIS system has a 12 volt supply and a good ground.
3. This test will determine if the ignition module is generating a reference pulse, or if the wiring or ECM are faulty.
4. This test will determine if the ignition module is not triggering the problem coil or if the tested coil is at fault. Secondary coil resistance should be 5000-10,000 ohms.
5. This test checks for continuity of the crank sensor and connections, as well as sensor magnetism.

Note. An intermittent problem with the crank sensor could cause an intermittent "cranks but won't run" condition. If ignition module is replaced due to results of step 5), replace crankshaft sensor as well.

Flow Chart A3, Cranks/Won't Run (2.5L A & N Body) (2 of 3). Scheme 221

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

1. This check determines if injector connector has ignition voltage on one terminal only.
2. A faulty ECM may result in damage to the injector.
3. A test light connected from ground to the ECM harness terminal "W1" should light due to continuity through injector.

Flow Chart A3, Cranks/Won't Run (2.5L A/N Body) (3 of 3). Scheme 222

CHART A5 - FUEL PUMP RELAY (2.0L)

The ECM turns in-tank fuel pump on as long as ignition is on and the engine is cranking or running. The ECM will allow the pump to run as long as it receives reference pulses from distributor. If there are no reference pulses, the ECM will shut off the fuel pump within 2 seconds after ignition switch is in "ON" position.

Should the fuel pump relay, or the 12-volt relay drive from the ECM fail, the fuel pump will run through the oil pressure switch back-up circuit.

An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The extended cranking time is caused by the time necessary for the oil pump to build up enough pressure to close the oil pressure switch and turn on the fuel pump.

Chart A5 Schematic, Fuel Pump Relay (2.0L). Scheme 223

Flow Chart A5, Fuel Pump Relay (2.0L). Scheme 224

CHART A5 - FUEL PUMP RELAY (2.5L A BODY)

The ECM turns in-tank fuel pump on as long as ignition is on and the engine is cranking or running. The ECM will allow the pump to run as long as it receives reference pulses from distributor. If there are no reference pulses, the ECM will shut off the fuel pump within 2 seconds after ignition switch is in "ON" position.

Should the fuel pump relay, or the 12-volt relay drive from the ECM fail, the fuel pump will run through the oil pressure switch back-up circuit.

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

1. This check is to determine if the oil pressure switch is faulty. Since it is the back-up component, it must be checked without the fuel pump relay connected.
2. This check will determine if the oil pressure switch is shorted internally.

An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The extended cranking time is caused by the time necessary for the oil pump to build up enough pressure to close the oil pressure switch and turn on the fuel pump.

Chart A5 Schematic, Fuel Pump Relay (2.5L A Body). Scheme 225

Flow Chart A5, Fuel Pump Relay (2.5L A Body). Scheme 226

CHART A5 - FUEL PUMP RELAY (2.5L N BODY)

The ECM turns in-tank fuel pump on as long as ignition is on and the engine is cranking or running. The ECM will allow the pump to run as long as it receives reference pulses from distributor. If there are no reference pulses, the ECM will shut off the fuel pump within 2 seconds after ignition switch is in "ON" position.

Should the fuel pump relay, or the 12-volt relay drive from the ECM fail, the fuel pump will run through the oil pressure switch back-up circuit.

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

1. This check is to determine if the oil pressure switch is faulty. Since it is the back-up component, it must be checked without the fuel pump relay connected.
2. This check will determine if the oil pressure switch is shorted internally.

An inoperative fuel pump relay can result in long cranking times, particularly if the engine is cold. The extended cranking time is caused by the time necessary for the oil pump to build up enough pressure to close the oil pressure switch and turn on the fuel pump.

Chart A5 Schematic, Fuel Pump Relay (2.5L N Body). Scheme 227

Flow Chart A5, Fuel Pump Relay (2.5L N Body). Scheme 228

CHART A7 - FUEL SYSTEM DIAGNOSIS

Note. Use appropriate CHART A5 wiring schematic for circuit reference.

When the ignition switch is turned to the "ON" position, the ECM will turn on the in-tank electric fuel pump by energizing the fuel pump relay through circuit No. 465. The fuel pump will deliver fuel to the TBI unit, where the system pressure is controlled by the pressure regulator. Fuel pressure will be 9-13 psi (.6-.9 kg/cm 2 ) with the vacuum hose connected to the pressure regulator. Excess fuel is returned to the fuel tank via the fuel return line. A fuel pump test terminal is located on the left side of the engine compartment. When the engine is stopped, the pump can be turned on by applying battery voltage to the test terminal.

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

1. Test determines if the fuel pump will supply fuel pressure when energized through the fuel pump test terminal. If fuse in jumper wire blows, this indicates a short to ground in circuit No. 120. Use appropriate CHART A5 for determining applicable circuits and terminals.
2. Pressure less than 9 psi (.6 kg/cm 2 ) falls into 2 categories: If regulated pressure is less than 9 psi (.6 kg/cm 2 ) and volume to injector is adequate, system will run lean and cause Code 44. Engine will be hard to start when cold and have poor overall performance. A restricted fuel flow is causing pressure drop. Normally, a vehicle with fuel pressure of less than 9 psi (.6 kg/cm 2 ) at idle will not be driveable; however, if pressure drop occurs only when driving, the engine will surge then stop as pressure begins to drop.
3. When battery voltage is applied to the pump test terminal, and fuel line is gradually restricted, maximum fuel pump pressure of approximately 13-18 psi (.9-1.3 kg/cm 2 ) will develop.
4. This test determines if high fuel pump pressure is due to a restricted fuel line or a throttle body pressure regulator problem.

Improper fuel system pressure may also cause vehicle to

1. Crank but not run.
2. Set Code 44 or 45.
3. Cut out (may feel like ignition problem).
4. Hesitate.

Chart A7 Schematic, Fuel Line (A, J Body). Scheme 229

Chart A7 Schematic, Fuel Line (N Body). Scheme 230

Flow Chart A7, Fuel System Diagnosis (All). Scheme 231

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 procedures listed below 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 (J-26911 ) (Scheme 232) Nipple should be inserted into exhaust manifold AIR pipe.

Flow Chart B1, Restriction Check At AIR Pipe. Scheme 232

Check At O2 Sensor

Remove O2 sensor. Install backpressure 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.

Flow Chart B1, Restriction Check At O2 Sensor. Scheme 233

Diagnosis

1. Start engine and bring to operating temperature. Allow engine to idle and observe exhaust system backpressure 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) exists, check for restricted catalytic converter. Replace if necessary.

CODE 13 - OPEN OXYGEN SENSOR CIRCUIT

The O2 sensor is like an open circuit and produces no voltage when it is below about 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 at normal operating temperature if at least 2 minutes have passed since engine start, O2 signal voltage is steady between .35 and .55 volt and throttle position sensor signal is above 7 percent (1200 RPM). All conditions must be met for at least 60 seconds.
2. Determines if the O2 sensor, ECM or wiring is at fault.
3. Use only a high impedance DVOM for this test. This test checks for continuity in circuits No. 412 and 413. If circuit No. 413 is open, voltage on circuit No. 412 will be greater than .6 volt.

Verify a clean, tight ground connection for circuit No. 413. An open circuit at terminal No. 412 or No. 413 will cause a Code 13.

Code 13 Schematic, Open Oxygen Sensor. Scheme 234

Code 13 Flow Chart, Open Oxygen Sensor Circuit. Scheme 235

CODE 14 - COOLANT SENSOR SIGNAL VOLTAGE LOW

Coolant temperature is one of the inputs used to control fuel delivery, engine timing (EST), idle (IAC), and converter clutch (TCC). As the engine warms, the sensor resistance becomes less. At normal operating temperature, voltage signal will be about 1.5-2.0 volts at terminal "B8" of ECM.

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

1. This test checks to see if code was set as a result of a hard failure or intermittent condition. Code 14 will set if signal voltage indicates a coolant temperature above 275°F (135°C) for more than 2 seconds.
2. This test simulates conditions for a Code 15. If the ECM recognizes the open circuit by displaying a low temperature, the ECM and wiring are not at fault.

After the engine is started, the temperature should rise steadily to about 194°F (90°C), then stabilize, when thermostat opens. If the engine is allowed to cool overnight, the coolant and MAT sensors, when measured with a "Scan" tester, should read close to each other. When Code 14 is set, the ECM will turn on the cooling fan. A Code 14 will also set if circuit No. 410 is shorted to ground.

Code 14 Schematic, Coolant Sensor Signal Voltage Low. Scheme 236

Code 14 Flow Chart, Coolant Sensor Signal Voltage Low. Scheme 237

CODE 15 - COOLANT SENSOR SIGNAL VOLTAGE HIGH

As the engine warms, the sensor resistance becomes less and voltage drops. At normal engine operating temperature, voltage will be about 1.5-2.0 volts at terminal "B8" of ECM. If circuit No. 410 opens with ignition off, ECM will see -40°F (-40°C) and deliver fuel for this temperature.

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

1. This test checks to see if code was set as a result of a hard failure or intermittent condition. Code 15 will set if time since engine start is more than 2 minutes and signal voltage indicates a coolant temperature less than -24°F (-31°C) for more than 60 seconds.
2. This test simulates conditions for a Code 14. If the ECM recognizes the grounded circuit, and displays a high temperature, the ECM and wiring are not at fault.
3. Determines if there is a problem with ECM or wiring. If circuit No. 452 is open, there may also be a Code 21 stored.

After the engine is started, the temperature should rise steadily to about 194°F (90°C), then stabilize, when thermostat opens. If the engine is allowed to cool overnight, the coolant and MAT sensors, when measured with a "Scan" tester, should read close to each other. When Code 15 is set, the ECM will turn on the cooling fan. Code 15 will also set if circuits No. 410 and 452 are open.

Code 15 Flow Chart, Coolant Sensor Signal Voltage High. Scheme 238

CODE 21 - TPS SIGNAL VOLTAGE HIGH

The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to throttle valve angle. Signal voltage will vary from less than 1.25 volts at idle to 4.5 volts at wide open throttle.

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

1. Confirms Code 21 and checks to see if fault is the result of a hard failure or an intermittent condition. Code 21 sets if TPS voltage is greater than 2.5 volts for 2 seconds, engine speed is less than 1800 RPM or if MAP sensor signal is equal to a no load condition.
2. Simulates conditions for Code 22. If the ECM recognizes the low voltage signal and sets Code 22, the ECM and circuits No. 416 and 417 are not at fault.
3. Isolates a faulty sensor, ECM, or an open circuit No. 452. If circuit No. 452 is open, there may also be a Code 15 stored.

A "Scan" tester displays throttle position in volts. Closed throttle voltage should be less than 1.25 volts. TPS voltage should increase at a steady rate, up to 4.5 volts, as throttle angle is increased. Code 21 will also result if circuit No. 452 is open or circuit No. 417 is shorted to voltage.

Code 21 Schematic, TPS Signal Voltage High. Scheme 239

Code 21 Flow Chart, TPS Signal Voltage High. Scheme 240

CODE 22 - TPS SIGNAL VOLTAGE LOW

The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to throttle valve. Signal voltage will vary from less than 1.25 volts at idle to 4.5 volts at wide open throttle.

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

1. Confirms Code 22 and tests if the fault is the result of a hard failure or an intermittent condition. Code 22 will set if engine is running, TPS voltage is less than .2 volt for 2 seconds.
2. This test simulates Code 21. If the ECM recognizes a high voltage signal and sets Code 21, the ECM and wiring are not at fault.
3. This test checks for voltage to the ECM. If the ECM recognizes the voltage as over 4 volts, the ECM and circuit No. 417 are not at fault.
4. If circuit No. 416 is open or shorted to ground, there may also be a Code 34 stored.

A "Scan" tester displays throttle position in volts. Closed throttle voltage should be less than 1.25 volts. TPS voltage should increase at a steady rate, up to 4.5 volts, as throttle angle is increased. Code 22 will also set if circuits No. 416 or 417 are open or grounded.

Code 22 Flow Chart, TPS Signal Voltage Low. Scheme 241

CODE 23 - MAT SENSOR TEMPERATURE LOW

The ECM supplies and monitors a voltage signal (4-6 volts) to the sensor through circuit No. 472. When temperatures are low, sensor resistance is high and the ECM will see a high monitored voltage signal. As temperatures increase, sensor resistance decreases and the voltage sensed by the ECM drops.

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

1. This step checks to see if Code 23 is the result of a hard failure or an intermittent condition. Code 23 will set if the engine has been running for more than 58 seconds and the signal voltage indicates a MAT temperature less than -22°F (-30°C).
2. Simulates conditions for a Code 25. If the "Scan" tester displays a high temperature, the ECM and wiring are not at fault.
3. This step checks for continuity of circuits No. 472 and 469. If circuit No. 469 is open there may also be a Code 33 stored.

If the engine is allowed to cool overnight, the coolant and MAT sensors, when measured with a "Scan" tester, should read close to each other.

When Code 15 is set, the ECM will turn on the cooling fan. A Code 23 will also result if circuits No. 472 or 469 become open.

Code 23 Schematic, MAT Sensor Temp Low. Scheme 242

Code 23 Flow Chart, MAT Sensor Temp Low. Scheme 243

CODE 24, VEHICLE SPEED SENSOR (VSS) A, J & N BODIES (WITH PM GENERATOR)

The speed sensor, which is a Permanent Magnet (PM) generator, provides the ECM with vehicle speed information. The PM generator, mounted in the transmission, produces a pulsing voltage signal whenever the vehicle speed is over 3 MPH. The voltage level and the number of pulses increase with vehicle speed. The ECM converts the pulsing voltage to MPH, which is used by the ECM in calculations to determine vehicle adjustments.

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

1. A Code 24 will set when MPH reads zero, transmission is not in Park or Neutral, engine speed is between 1800-4400 RPM, TPS is at zero percent (closed throttle), and a high manifold vacuum is sensed by the MAP sensor. All of these conditions must be met for 3-5 seconds. The PM generator only produces a voltage signal if drive wheels are turning greater than 3 MPH.
2. Before replacing the ECM, PROM should be checked for correct application.

A faulty or misadjusted Park/Neutral switch may set a false Code 24. Use "Scan" tester and check for proper signal in Drive, while wiggling shifter.

Code 24 Schematic, VSS W/PM Generator (2.5L N Body). Scheme 244

Code 24 Schematic, VSS W/PM Generator (2.0L). Scheme 245

Code 24 Flow Chart, VSS W/PM Generator (All). Scheme 246

CODE 25 - MAT SENSOR TEMPERATURE HIGH

The ECM applies and monitors a voltage signal (4-6 volts) to circuit No. 472. When manifold air is cold, the sensor resistance is high and the ECM will see a high signal voltage. As air warms, resistance decreases and voltage sensed by the ECM drops. Sensor resistance can be measured at ECM terminals "B5" and "W14".

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

1. Checks to determine if the code is a hard failure or an intermittent condition. Code 25 will set if a MAT temperature greater than 275°F (135°C) is sensed for more than 2 seconds.

If engine is allowed to cool overnight, the coolant and MAT sensors, when measured with a "Scan" tester, should read close to each other. A Code 25 also results if circuit No. 472 is shorted to ground.

Code 25 Flow Chart, MAT Sensor Temp High. Scheme 247

CODE 32 - EGR SYSTEM ERROR 2.0L (VIN K) & 2.5L A BODY

A properly operating EGR will directly affect the air/fuel mixture requirements of the engine. Since the exhaust gas introduced into the airflow is an inert gas (contains little or no oxygen), less fuel is required to maintain a correct air/fuel mixture. If the EGR system were to become inoperative, the inert exhaust gas would be replaced with oxygen and the air/fuel mixture would become leaner. This would be reflected in an increased block learn value.

The engine control system operates within 2 block learn cells, a closed throttle cell, and an open throttle cell. The block learn value of these 2 cells should be close to the same. The open throttle cell is affected by the operation of a functional EGR system. If the EGR system malfunctions, the open throttle cell block learn would increase, but the block learn for the closed throttle cell would remain unchanged. When the change becomes too great, ECM sets Code 32.

The Code 32 chart is a functional check of the EGR system. If the EGR system works properly, but a Code 32 has been set, check for other causes for an increased open throttle block learn value.

1. Check for a blocked/restricted EGR passage.
2. Check the MAP sensor function. A MAP sensor may shift enough in calibration to affect the air/fuel mixture without setting a MAP related code.

Code 32 Schematic, EGR System (2.0L VIN K & 2.5L A Body). Scheme 248

Code 32 Flow Chart, EGR System (2.0L VIN K & 2.5L A Body). Scheme 249

CODE 33 - MAP SENSOR SIGNAL VOLTAGE HIGH

The Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If the MAP sensor fails, ECM substitutes a fixed MAP value and uses TPS to control fuel delivery.

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

1. This test confirms Code 33, and determines if the fault is the result of a hard failure or and intermittent condition. Code 33 will set when voltage signal reading is too high for a time greater than 48 seconds and if the TPS voltage indicates throttle is closed (TPS less than 4 percent).
2. This step simulates conditions for a Code 34. If the ECM recognizes and sets Code 34, low MAP signal, the ECM and circuits No. 416 and 432 are not at fault. If circuit No. 469 is open, there may also be a stored Code 23.

With the ignition switch in the "ON" position and the engine stopped, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Comparison of the BARO readings from a known good vehicle using the same sensor is a good way to check the accuracy of the suspected sensor. Readings should be the same within +/- .4 volt. Code 33 will also result if circuit No. 469 is open, or circuit No. 432 is shorted to voltage or to circuit No. 416.

Code 33 Schematic, MAP Sensor Signal Volt High. Scheme 250

Code 33 Flow Chart, MAP Sensor Signal Volt High. Scheme 251

CODE 34 - MAP SENSOR SIGNAL VOLTAGE LOW

Manifold Absolute Pressure (MAP) sensor responds to changes in manifold pressure (vacuum). If the MAP sensor fails, the ECM substitutes a fixed MAP value and uses TPS to control fuel delivery.

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

1. This test confirms Code 34, and determines if code is the result of a hard failure or an intermittent condition. Code 34 will set when ignition is on and MAP signal voltage is low.
2. Jumpering harness terminal "B" to "C" will determine if sensor is at fault or if there is a problem with the ECM or wiring.
3. "Scan" tester may not display 12 volts. The important thing is the ECM recognizes and sets Code 33, high MAP voltage signal, indicating the ECM and circuit No. 432 are not at fault.

With the ignition switch in the "ON" position and the engine stopped, manifold pressure is equal to atmospheric pressure and the signal voltage will be high. Comparison of the BARO readings from a known good vehicle using the same sensor is a good way to check the accuracy of the suspected sensor. Readings should be the same within +/- .4 volt. A Code 34 will also result if circuits No. 416 and 432 are open or shorted to ground. If circuit No. 416 is open or shorted to ground, there may also be a stored Code 22.

Code 34 Flow Chart, MAP Sensor Signal Volt Low. Scheme 252

CODE 35 - IDLE SPEED ERROR (2.5L A, N BODY)

Code 35 will set when closed throttle engine speed is 150 RPM greater or less than correct idle speed for 20 seconds.

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

1. Continue with test even if engine will not idle properly. If idle is too low, "Scan" tester will display 80 or more counts or steps. 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 and down. Disconnect IAC. If condition is unchanged, IAC is not at fault.
2. When engine was stopped, the IAC valve retracted (more air) to a fixed position to provide increased airflow during next engine start. During next engine start, a "Scan" should display 100 or more counts.
3. Disconnect IAC valve prior to this test. The test light will confirm the ECM signals by a steady or flashing light on all circuits.
4. Test verifies faulty IAC, short to voltage (steady light) or poor IAC connection.

A slow, unstable idle may be caused by a system problem that cannot be overcome by IAC. "Scan" counts will be greater than 60 if too low, and zero counts, if too high. If idle is too high, stop engine. With ignition on, ground ALDL "test" terminal. Wait 45 seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is greater than 800 RPM, look for possible vacuum leaks.

System Too Lean

If air/fuel ratio is too lean, the idle speed may be either too high or too low. Engine speed may vary up and down and disconnecting the IAC may not help. "Scan" and/or digital voltmeter (10 megohm) will read an oxygen sensor output less than 300 mv (.3 volt). Check for low fuel pressure or water in the fuel. A contaminated O2 sensor (caused by silicone) will produce lean air/fuel mixtures with an oxygen sensor output fixed greater than 800 mv (.8 volt). This may also set Code 45.

System Too Rich

If air/fuel ratio is too rich, idle speed will be too low and "Scan" tester counts will usually be greater than 80. The system may be obviously rich, with Black smoke from the tailpipe. "Scan" tester and/or voltmeter will read an oxygen sensor voltage signal fixed greater than 800 mv (.8 volt). Look for high fuel pressure or injectors leaking or sticking. Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging the bore.

Throttle Body

Remove IAC. Inspect bore for evidence of IAC valve dragging.

IAC Valve Connections

Inspect carefully for loose or corroded connections.

PCV Valve

An incorrect PCV valve may cause incorrect idle speed.

Code 35 Schematic, Idle Speed (2.5L A, N Body). Scheme 253

Code 35 Flow Chart, Idle Speed Error (2.5L A, N Body). Scheme 254

CODE 42 - ELECTRONIC SPARK TIMING (EST) 2.0L (VIN K)

Code 42 indicates the ECM has seen an open or short to ground in HEI EST or by-pass circuits.

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

1. This test confirms Code 42 and determines if fault is a hard failure or intermittent condition.
2. This test checks for a normal EST ground path through the ignition module. If circuit No. 423 is shorted to ground, reading will be less than 500 ohms.
3. As test light voltage touches circuit No. 424, the module should switch. This will cause the ohmmeter to "over-range" if meter is in the 1000-2000 ohm position. A higher ohm range will indicate over 5000 ohms. This test assures that the module "switched".
4. If module did not switch, this step will test for a short in circuit No. 423, an open in circuit No. 424, and a faulty ignition module connection or module.
5. This step confirms that Code 42 is a faulty ECM and not an intermittent problem in circuits No. 423 and 424.

The "Scan" tester does not have the ability to help diagnose a Code 42 problem. See INTERMITTENTS in the CCC TESTS W/O CODES article in this section.

Code 42 Flow Chart, EST (2.0L VIN K). Scheme 255

CODE 42 - ELECTRONIC SPARK TIMING (EST) (2.5L A, N BODY)

Code 42 indicates the ECM has seen an open or short to ground in EST or by-pass circuits (DIS ignitions).

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

1. This test confirms Code 42 and determines if fault is hard or intermittent.
2. This test checks for a normal EST ground path through the ignition module. If circuit No. 423 is shorted to ground, reading will be less than 500 ohms.
3. As test light voltage touches circuit No. 424, the module should switch. This will cause the ohmmeter to "over-range" if meter is in the 1000-2000 ohm position. A higher ohm range will indicate over 5000 ohms. This test assures that the module "switched".
4. If module did not switch, this step will test for a short in circuit No. 423, an open in circuit No. 424, and a faulty ignition module connection or module.
5. This step confirms that Code 42 is a faulty ECM and not an intermittent problem in circuits No. 423 and 424.

The "Scan" tester does not have the ability to help diagnose a Code 42 problem. See INTERMITTENTS in the CCC TESTS W/O CODES article in this section.

Code 42 Flow Chart, EST (2.5L). Scheme 256

CODE 44 - LEAN EXHAUST INDICATION

Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature 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 44 sets when O2 sensor signal at ECM is less than .2 volt for 60 seconds or more and system is operating in "closed loop".

Using the "Scan" tester, observe the Block Learn Memory (BLM) value at different RPMs. If conditions for a Code 44 exist, the block learn value will be around 150.

O2 Sensor Wire

O2 sensor wire may be mispositioned and laying against the exhaust manifold. Check for ground between sensor and wire connector.

Fuel Contamination

Water, even small amounts, near the in-tank fuel pump inlet can be delivered to the injector. The water may cause a lean exhaust and set Code 44.

Fuel Pressure

System will be lean if fuel pressure is low. It may be necessary to monitor fuel pressure while driving vehicle. Refer to CHART A7 - FUEL SYSTEM DIAGNOSIS in this article.

Exhaust Leaks

If the exhaust system has large leaks, exhaust system negative pressure pulses can cause outside air to be drawn into the system and past the O2 sensor. Vacuum or crankcase leaks can also cause a lean condition.

If Code 44 is intermittent, see INTERMITTENTS in the CCC TESTS W/O CODES article in this section.

Code 44 Flow Chart, Lean Exhaust Indication. Scheme 257

CODE 45 - RICH EXHAUST INDICATION

Sensor acts like an open sensor circuit and produces no voltage when exhaust temperature is below 600°F (316°C). An open sensor circuit or cold sensor causes "open loop" operation.

Code 45 indicates a rich exhaust and diagnosis should begin with these items: fuel pressure, leaking injector, HEI shielding, canister purge saturation, coolant sensor, MAP sensor, O2 sensor contamination and TPS intermittent output.

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

1. Test checks to see if O2 sensor is registering a rich condition. Code 45 is set when vehicle is at operating temperature (in "closed loop"), throttle angle is between 3 and 45 percent, O2 sensor signal at ECM is greater than .7 volt for 30 seconds and time since engine start is one minute or more.

Code 45, rich exhaust, is most likely caused by one of the following

Fuel Pressure High

If fuel pressure is too high, air/fuel ratios will be rich. See appropriate CHART A7 - FUEL SYSTEM DIAGNOSIS . The ECM can compensate for slight increases but if air/fuel ratio becomes too rich a Code 45 will be set.

Ignition Ground

If an open occurs at circuit No. 453, HEI induced electrical "noise" may result, causing simulated reference pulses to be picked up by the ECM on the reference line of the 4-wire EST harness. The additional pulses result in a higher than actual engine speed signal. The ECM will increase injector pulse width ("on" time) to match the increased RPM signal. "Scan" tester will show higher than actual RPM, which can help in diagnosing this problem.

Fuel Canister

Fuel saturation of the charcoal canister will cause a rich air/fuel ratio. If full of fuel, check canister control and hoses.

MAP Sensor

An output that causes the ECM to sense a higher than normal manifold pressure (low vacuum) can cause the system to go rich. Disconnecting the MAP sensor will allow the ECM to substitute a fixed value for the MAP sensor. If the condition disappears, substitute a different MAP sensor and continue testing.

TPS

An intermittent TPS output will cause the system to operate rich due to a false indication of engine acceleration.

O2 Sensor Contamination

O2 sensor contamination, caused by silicone in certain fuels or use of improper RTV sealant, may cause a White powdery coating to cover the exterior of the O2 sensor. The false high signal voltage (low oxygen content sensed) produced is interpreted by the ECM as a rich mixture, causing the ECM to set Code 45.

EGR Problem

EGR valve sticking open at idle is usually accompanied by a rough idle and/or stalling. If Code 45 is intermittent, see INTERMITTENTS in the CCC TESTS W/O CODES article in this section.

Code 45 Flow Chart, Rich Exhaust Indication. Scheme 258

CODE 51 - FAULTY PROM

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

CODE 53 - SYSTEM OVERVOLTAGE

This code indicates a basic charging system problem. Code 53 will set when voltage at ECM terminal is greater than 16.9 volts for 50 seconds.

CHART C1A - PARK/NEUTRAL SWITCH

The Park/Neutral switch contacts are part of neutral start switch and are closed to ground in Park or Neutral, and open in Drive ranges. The ECM supplies ignition voltage through a current limiting resistor to circuit No. 434, and senses a closed switch when voltage on circuit No. 434 drops to less than one volt. The ECM uses this signal as one of the inputs to control IAC and VSS diagnostics.

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

1. Checks for closed switch to ground in Park position. Different makes of "Scan" tester will read P/N differently. Refer to manufacturer's operating manual for type of display used.
2. Checks for an open switch in Drive range.
3. Be sure "Scan" tester indicates Drive. Wiggle shifter to test for an intermittent or misadjusted switch.

Chart C1A Schematic, Park/Neutral Switch. Scheme 259

Flow Chart C1A, Park/Neutral Switch. Scheme 260

CHART C1D - MAP OUTPUT CHECK

The MAP sensor measures manifold pressure (vacuum) and sends that signal to ECM. The ECM uses this information for fuel and spark control.

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

1. Check MAP sensor output voltage to ECM. This voltage, without engine running, represents a barometer reading to ECM.
2. Applying 10 in. Hg (34 kPa) vacuum to MAP sensor should cause voltage to be 1.2-2.3 volts less than voltage at step 1). Upon applying vacuum to sensor, change in voltage should be instantaneous. A slow voltage change indicates a faulty sensor.
3. Check vacuum hose to sensor for leaks or restriction. Be sure no other vacuum devices are connected to MAP sensor signal hose.

Flow Chart C1D, MAP Output Check. Scheme 261

CHART C1E - POWER STEERING PRESSURE SWITCH (PSPS)

The Power Steering Pressure Switch (PSPS) is normally open to ground. Circuit No. 495 should be near battery voltage. Turning the steering wheel increases power steering oil pressure, and its load on an idling engine. The pressure switch will close before load can cause an idle problem. Closing switch causes circuit No. 495 to read less than one volt. The ECM will increase idle air rate and retard timing. A pressure switch that won't close, or an open circuit No. 495 or 450 may cause engine to stop when power steering loads are high. A switch that will not open, or a circuit No. 450 or 495 shorted to ground, will cause timing to retard at idle and may affect idle quality.

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

1. Different makes of "Scan" tester will read P/S differently. Refer to manufacturer's operating manual for type of display used.
2. Determines if circuit No. 495 is shorted to ground.
3. This test simulates a closed switch.

Chart C1E Schematic, P/S Pressure Switch. Scheme 262

Flow Chart C1E, P/S Pressure Switch. Scheme 263

CHART C2C - IDLE AIR CONTROL CHECK

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

1. Continue with test, even if engine won't idle. If idle is too low, "Scan" tester will display 80 or more counts. If idle is high, it will display zero counts. Sometimes, an erratic or unstable idle may occur. Engine speed may vary 200 RPM or more up and down. Disconnect IAC. If condition is unchanged, the IAC is not at fault.
2. When the engine was stopped, the IAC plunger retracted (more air) to a fixed "park" position for increased airflow and idle speed during the next engine start. A "Scan" tester will display 100 or more counts. When performing this test, immediately note RPM on start-up. On a warm engine, RPM will decrease rapidly.
3. Be sure to disconnect the IAC valve prior to this test. Test light will confirm the ECM signals with 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.

A slow unstable idle may be caused by a system problem that cannot be overcome by the IAC. A "Scan" tester can be used to monitor desired RPM, actual RPM and IAC counts helps isolate a system problem. For example, a vacuum leak may be indicated if desired engine speed is 900 RPM, IAC counts are zero and actual engine speed is 1500 RPM.

Idle speed may be too high or too low. Engine speed may vary up and down. Disconnecting IAC does not help. This may set Code 44. A "Scan" tester and/or voltmeter will read an oxygen sensor output less than .3 volt. Check for low regulated fuel pressure or water in fuel. A lean exhaust, with an oxygen sensor output fixed greater than .8 volt, will be a contaminated sensor. This may also set Code 45.

Idle speed too low. If "Scan" tester counts are more than 80, system is rich and may exhibit Black smoke from the exhaust. The "Scan" tester and/or voltmeter will read an oxygen sensor signal fixed greater than .8 volt. Check for high fuel pressure and/or injector leaking or sticking.

Remove IAC and inspect bore for foreign material or evidence of IAC valve dragging in the bore.

CHART C4C - IGNITION SYSTEM CHECK 2.0L (VIN K)

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

1. Two wires are checked to ensure that there is not an open spark plug wire. Checks for proper output from ignition system. Spark tester requires a minimum of 25,000 volts to fire. This check can be used in case of an ignition misfire because the system may provide enough voltage to run engine but not enough to fire spark plug under heavy load. A) If spark occurs with EST connector disconnected, pick-up coil output is too low for EST operation.
2. A spark indicates that problem is a faulty distributor cap or rotor.
3. Normally, there should be battery voltage at "C" and "+" terminals. Low voltage indicates an open or high resistance circuit from the distributor to the coil or ignition switch. If "C" terminal voltage was low, but "+" terminal voltage is 10 volts or more, circuit from "C" terminal to the ignition coil or the ignition coil primary winding is open.
4. Checks for a shorted module or grounded circuit from ignition coil to module. The distributor module should be turned off so normal voltage should be about 12 volts. If module is turned on, the voltage would be low but more than one volt. This could cause ignition coil to fail from excessive heat. With an open ignition coil primary winding, a small mount of voltage will leak through module from the battery terminal to tach terminal.
5. Checks for an open module or circuit to it. The 1.5-8 volts applied to module "P" terminal should turn module on and voltage should drop to about 7-9 volts. This test can be performed using a DC battery with a 1.5-8 volt rating.
6. This should turn off module and cause a spark. If no spark occurs, the fault is most likely in ignition coil because most module problems would have been found before this point in procedure. A module test could determine which is at fault.

Flow Chart C4C, Ignition System Check (2.0L VIN K). Scheme 264

CHART C4D1 - "DIS" MISFIRE AT IDLE (2.5L A, N BODY)

The Direct Ignition System (DIS) uses a waste spark system for spark distribution. The ignition module triggers the ignition coils and fires the spark plugs in pairs (1-4 and 2-3). While one cylinder is at TDC compression, the opposite cylinder is at TDC exhaust. The exhaust cylinder requires less voltage to fire the spark plug, leaving the remainder of the high voltage to fire the compression cylinder spark plug.

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

1. If misfire condition exists only under load, CHART C4D-2 must be used. RPM should drop equally on all plug leads.
2. A Spark Plug Tester (ST-125), requiring a minimum of 25,000 volts to fire, must be used because it is essential to verify adequate available secondary voltage at the spark plug.
3. If the spark jumps the test gap after the opposite plug wire is grounded, it indicates excessive resistance in the plug which was by-passed. Also check for carbon deposits in spark plug boots.
4. Excessive wire resistance or faulty connections could cause the coil to be damaged. If carbon tracking exists, replace coil.
5. If no spark conditions follows the suspected coil, replace coil. Otherwise, the ignition module is the cause of no spark. This test could also have been performed by substituting a known good coil for the suspected coil.

Flow Chart C4D1, "DIS" Misfire At Idle (2.5L). Scheme 265

CHART C4D2 - "DIS" MISFIRE UNDER LOAD (2.5L A, N BODY)

The Direct Ignition System (DIS) uses a waste spark system for spark distribution. The ignition module triggers the ignition coils and fires the spark plugs in pairs (1-4 and 2-3). While one cylinder is at TDC compression, the opposite cylinder is at TDC exhaust. The exhaust cylinder requires less voltage to fire the spark plug, leaving the remainder of the high voltage to fire the compression cylinder spark plug.

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

1. If misfire condition exists only at idle, CHART C4D-1 must be used. A Spark Plug Tester (ST-125 ), requiring a minimum of 25,000 volts to fire, must be used because it is essential to verify adequate available secondary voltage at the spark plug. Spark should jump the test gap on all 4 leads. This simulates a "load" condition.
2. If the spark jumps the test gap after the opposite plug wire is grounded, it indicates excessive resistance in the plug which was by-passed. Also check for carbon deposits in spark plug boots.
3. Excessive wire resistance or faulty connections could cause the coil to be damaged. If carbon tracking exists, replace coil.
4. If no spark condition follows the suspected coil, replace coil. Otherwise, the ignition module is the cause of no spark. This test could also have been performed by substituting a known good coil for the suspected coil.

Flow Chart C4D2, "DIS" Misfire Under Load (2.5L). Scheme 266

Flow Chart C7, EGR Valve Check (2.5L A, N Body). Scheme 267

CHART C8 - TORQUE CONVERTER CLUTCH

The purpose of automatic transmission/transaxle Torque Converter Clutch (TCC) feature is to eliminate power loss of torque converter stage when vehicle is in cruise condition. Fused battery ignition is supplied to TCC solenoid through brake switch, and transmission 3rd gear apply switch. The ECM will engage TCC by grounding circuit No. 422 to energize solenoid. TCC will engage when vehicle speed is greater than 30 MPH, engine at normal operating temperatures greater than 158°F (70°C), throttle position sensor output not changing (indicating a steady road speed), transmission 3rd gear switch closed, and brake switch closed.

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

1. Light off confirms that the transmission 3rd gear apply switch is open.
2. At 30 MPH transmission/transaxle 3rd gear switch should close. Test light will come on and confirm battery supply and closed brake switch.
3. Grounding ALDL "test" terminal with engine off should energize the TCC solenoid. This test checks capability of ECM to control solenoid.

An engine coolant thermostat that is stuck open or opens at too low a temperature may result in an inoperative TCC.

Chart C8 Schematic, Torque Converter Clutch. Scheme 268

Flow Chart C8, Torque Converter Clutch. Scheme 269

CHART C8B - M/T SHIFT LIGHT (2.0L VIN K, 2.5L N BODY)

The shift light indicates best transmission shift point for maximum fuel economy. The light is controlled by ECM. The ECM uses measured RPM and vehicle speed to calculate what gear vehicle is in. When indicator light is desired, ECM turns on light by grounding circuit No. 456. The ECM uses TPS, RPM, VSS and coolant temperature inputs for control of the shift light.

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

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

Chart C8B Schematic, M/T Shift Light (2.0L VIN K, 2.5L N Body). Scheme 270

Flow Chart C8B, M/T Shift Light (2.0L VIN K, 2.5L N Body). Scheme 271

CHART C10 - A/C CLUTCH CONTROL (2.0L VIN K)

When A/C switch is in the "ON" position, ignition voltage is supplied to the compressor low pressure switch. If there is sufficient freon pressure, switch will close and complete the circuit to close the high pressure cut-off switch and circuits No. 603 and 604. The voltage on circuit No. 604 to the ECM is shown on the "Scan" tester as A/C request "on" (voltage present) or "off" (no voltage). When a request for A/C is seen by the ECM, ECM will respond by grounding circuit No. 459 of the A/C clutch control relay. The relay will then close and current will flow from circuit No. 604 to circuit No. 59 and engage the A/C compressor clutch. When voltage is seen by the ECM on circuit No. 603, the cooling fan will be turned on.

Both pressure switches are located on the high side of the A/C system, at the rear of the compressor. The low pressure switch (Blue insert) will be closed at 40-47 psi (2.8-3.3 kg/cm 2 ) and allow A/C clutch operation. At less than 37 psi (2.6 kg/cm 2 ), the low pressure switch will be open and A/C clutch will not operate. At about 430 psi (30.2 kg/cm 2 ), the high pressure switch (Red insert) will open and disengage the A/C clutch, preventing damage to the system.

Chart C10 Schematic, A/C Clutch Control (2.0L VIN K). Scheme 272

Flow Chart C10, A/C Clutch Control (2.0L VIN K). Scheme 273

CHART C10 - A/C CLUTCH CONTROL (2.5L A BODY)

When A/C switch is in the "ON" position, ignition voltage is supplied to the compressor low pressure switch. If there is sufficient freon pressure, switch will close and complete the circuit to close the high pressure cut-off switch and circuits No. 67 and 459. The voltage on circuit No. 459 to the ECM is shown on the "Scan" tester as A/C request "on" (voltage present) or "off" (no voltage). When a request for A/C is seen by the ECM, ECM will respond by grounding circuit No. 458 of the A/C clutch control relay. The relay will then close and current will flow from circuit No. 459 to circuit No. 59 and engage the A/C compressor clutch. When voltage is seen by the ECM on circuit No. 67, the cooling fan will be turned on.

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

1. ECM will energize the A/C relay only when the engine is running. This test determines if relay or circuit No. 458 is faulty.
2. The low pressure and high pressure switches must be closed so that the A/C request signal (12 volts) will be present at the ECM.
3. A short to ground in any part of the A/C request or A/C clutch control circuits could be the cause of the blown fuse.
4. With the engine idling and the A/C on, the ECM should be grounding circuit No. 458, causing the test light to be on.
5. Determines if the signal is reaching the low pressure switch on circuit No. 66 from the A/C control panel. The signal should be present only when the A/C mode or defrost mode has been selected.

If the complaint was insufficient cooling, the problem may be caused by an inoperative cooling fan or A/C pressure fan switch. The engine cooling fan should turn on when A/C pressure exceeds a value to open the switch, causing the ECM to energize the cooling fan relay. See CHART C12 - ENGINE COOLING FAN for cooling fan diagnosis.

Chart C10 Schematic, A/C Clutch Control (2.5L A Body). Scheme 274

Flow Chart C10, A/C Clutch Control (2.5L A Body). Scheme 275

CHART C10 - A/C CLUTCH CONTROL (2.5L N BODY)

When A/C switch is in the "ON" position, ignition voltage is supplied to the compressor low pressure switch. If there is sufficient freon pressure, switch will close and complete the circuit to close the high pressure cut-off switch and circuits No. 67 and 604. The voltage on circuit No. 604 to the ECM is shown on the "Scan" tester as A/C request "on" (voltage present) or "off" (no voltage). When a request for A/C is seen by the ECM, ECM will respond by grounding circuit No. 459 of the A/C clutch control relay. The relay will then close and current will flow from circuit No. 604 to circuit No. 59 and engage the A/C compressor clutch. When voltage is seen by the ECM on circuit No. 67, the cooling fan will be turned on.

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

1. ECM will energize the A/C relay only when the engine is running. This test will determine if the relay or circuit No. 459 is faulty.
2. The low pressure and high pressure switches must be closed so that the A/C request signal (12 volts) will be present at the ECM.
3. A short to ground in any part of the A/C request or A/C clutch control circuits could be the cause of the blown fuse.
4. With the engine idling and the A/C on, the ECM should be grounding circuit No. 459, causing the test light to be on.
5. Determines if the signal is reaching the low pressure switch on circuit No. 66 from the A/C control panel. The signal should be present only when the A/C mode or defrost mode has been selected.

If the complaint was insufficient cooling, the problem may be caused by an inoperative cooling fan or A/C pressure fan switch. The engine cooling fan should turn on when A/C pressure exceeds a value to open the switch, causing the ECM to energize the cooling fan relay. See CHART C12 - ENGINE COOLING FAN for cooling fan diagnosis.

Chart C10 Schematic, A/C Clutch Control (2.5L N Body). Scheme 276

Flow Chart C10, A/C Clutch Control (2.5L N Body). Scheme 277

CHART C12 - ENGINE COOLING FAN

Battery voltage is supplied to fan relay terminal "A" and from ignition to terminal "D". Grounding relay terminal "F" will close relay and supply battery voltage to fan motor. Above 30 MPH, the ECM will remove ground from circuit No. 335. If coolant temperature and A/C pressure switches are open, fan will stop. Fan is energized when

1. A/C is "on" and vehicle speed is less than 30 MPH.
2. Coolant temperature is greater than 230°F (108°C).
3. Code 14 or 15 is set.

If an overheating condition is suspected, verify if this is due to actual boilover. If gauge or light indicates an overheat condition, and no boilover is in evidence, inspect the gauge/light circuit for malfunction.

If the vehicle is overheating and the gauge or light indicates the same, but the cooling fan is not coming on, check the coolant sensor temperature using a "scan" tester. The sensor may have shifted calibration.

Chart C12 Schematic, Engine Cooling Fan. Scheme 278

Flow Chart C12, Engine Cooling Fan. Scheme 279

Scheme 280

Scheme 281

Scheme 282

TERMINAL ID & PIN VOLTAGE CHARTS

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

The following conditions must be met before testing

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

Scheme 283

Scheme 284

Scheme 285

Scheme 286

Wiring Diagram (J Body 2.0L VIN K). Scheme 287

Wiring Diagram (Celebrity, Century, Ciera & 6000 2.5L VIN R). Scheme 288

Wiring Diagram (Calais, Grand Am & Skylark 2.5L VIN U). Scheme 289

See also:
• SCAN TESTER USAGE
• SCAN TESTER - TEST
• CHART A7 - FUEL SYSTEM DIAGNOSIS