2.8l/4.3l Tbi Tests W/codes GMC Cab & Chassis S15

BASIC DIAGNOSTIC PROCEDURE

Diagnosis of the CCC system should be performed in the following order

1. Make sure that all engine systems not related to the CCC system are operating properly. Do not proceed with testing unless all other problems have been repaired.
2. Go to the Diagnostic Circuit Check chart and follow all instructions given there to verify proper operation of ECM self-diagnostics and to obtain any stored trouble codes.
3. If trouble codes were displayed, determine whether the codes are "intermittent" or "hard". Go to numbered Code Charts for further diagnosis of stored trouble codes.
4. If no trouble codes were displayed, proceed to Field Service Mode on TBI models (part of Diagnostic Circuit Check).
5. If no trouble is indicated by any of these charts, use the TROUBLE SHOOTING material in this article. The comments there will send you to the proper component charts or tell you what to fix.
6. After any repairs have been made, always perform System Performance Check. Clear any trouble codes.

Note. Each of the steps listed here are described later in this section. If you are unsure of the proper way to test, read through the following material.

Scheme 138

1. Turn ignition switch on but do not start engine. "SERVICE ENGINE SOON" lamp should glow. Locate Assembly Line Communication Link (ALCL) connector attached to ECM wiring harness under instrument panel near the steering column (under driver's seat on "G" model vans). Insert spade lug terminal across the "TEST" terminal and "GROUND" terminal. (Scheme 138) CAUTION: Inserting spade lug in terminals of ALCL connector grounds "TEST" terminal lead. Do not ground ALCL connector until after ignition is on or engine is started. (Scheme 138): ALCL Connector Terminal Locations
2. "SERVICE ENGINE SOON" light should flash code "12" ("FLASH", pause, "FLASH", "FLASH"). Code "12" will be repeated 3 times. If any trouble codes are stored in the ECM memory, they will be displayed in the same manner.
3. Trouble codes will be displayed from lowest to highest numbered codes (3 times each) and be repeated as long as the "TEST" terminal of the ALCL connector is grounded.
4. To exit diagnostic mode, turn ignition switch off and remove spade lug terminal from ALCL connector.

CLEARING TROUBLE CODES

Trouble codes are cleared by removing battery voltage from the ECM for at least 10 seconds. To do so, be sure ignition switch is "OFF" and remove battery positive terminal, or disconnect ECM harness from positive battery pigtail, or remove ECM fuse from fuse block.

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. When recalled, these codes will be displayed by flashes of the "SERVICE ENGINE SOON" light. Trouble codes are displayed starting with the lowest numbered code. Only codes that represent a definite malfunction will be shown.

Trouble codes are read by counting flashes of the "SERVICE ENGINE SOON" light, or by reading output of a diagnostic tool connected to the ALCL connector. These special tools are faster and more accurate, but are not mandatory.

If a special diagnostic tool is not available, read the flashes of the "SERVICE ENGINE SOON" lamp. For example, "FLASH", "FLASH", pause, "FLASH", longer pause, identifies "21". The first flashes are the first digit of the trouble code, the second set of flashes represent the second digit.

TROUBLE CODE IDENTIFICATION - TBI MODELS

ECM TROUBLE CODE IDENTIFICATION

TROUBLE CODE DETERMINATION (HARD OR INTERMITTENT)

During any diagnostic procedure, you must decide between "hard" codes and "intermittent" codes. Diagnostic charts will not help analyze intermittent failures. Proper use of the Diagnostic Circuit Check chart will determine whether a stored code is "hard" or "intermittent".

An "intermittent" code is one which does not reset itself and is not present while diagnosing vehicle. Intermittent codes are frequently caused by loose connections. A "hard" code will repeat itself during circuit check and reset itself while diagnosing vehicle.

Diagnostic Charts

The Diagnostic Charts are used to find and repair problems which On-Vehicle Diagnostics have found. These charts include

1. Charts which fix a problem when the On-Vehicle Diagnostics don't work.
2. Charts where a stored trouble code leads you to a particular problem.
3. Charts which are used because the Field Service Mode (TBI models) found a problem.
4. "Engine Cranks But Won't Run" charts.

Diagnostic Circuit Check

1. If complaint is "SERVICE ENGINE SOON" light related, this check will lead to the most likely problem area (if a malfunction exists). Enter diagnostic mode and record stored trouble codes.
2. Begin diagnosis with the lowest numbered code shown and go to the numbered trouble code chart. If code "51" is displayed, see PROM removal and installation in this article.

Diagnostic Symptom Check

1. If complaint is NOT "SERVICE ENGINE SOON" light related, this check will lead to most likely problem area. However, first make checks that would normally be made for the complaint on a vehicle without CCC system.
2. Follow instructions in diagnostic chart and repair malfunction. After repair, perform Field Service Mode check (TBI models).

Field Service Mode

1. This test confirms proper operation of fuel system and verifies Closed Loop operation. It is part of the Diagnostic Circuit Check for TBI models. Clear codes and perform this test after any repair is completed.
2. When performing this check, always engage parking brake and block drive wheels.
3. On some engines, the 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 for several minutes, accelerating from idle to part throttle several times.

DIAGNOSTIC TOOLS

The CCC system does not require special tools for diagnosis. A tachometer, dwell meter, test light, ohmmeter, digital voltmeter with 10 megohms impedance (minimum), vacuum pump, vacuum gauge and 6 jumper wires 6" long (1 wire with female connectors at both ends, 1 wire with male connectors at both ends, 4 wires with male and female connectors at opposite ends) are the only tools necessary for diagnosis.

A test light, rather than a voltmeter, must be used when indicated by a diagnostic chart.

Some brands of dwell meter are not compatible with General Motors CCC systems. If engine operation seems to change as meter is connected, remove dwell meter and use another type.

Note. Special "SCAN" testers can be used to read trouble codes and check voltages in the system. These tools can save a great deal of time, but are not required. Refer to tester manual for operating procedures. Also see "SCAN" charts in this article.

CHART A1 - NO "SERVICE ENGINE SOON" LIGHT

There should always be a steady "SERVICE ENGINE SOON" light when ignition is on with engine not running (battery voltage supplied directly to light). The ECM will control the light and turn it on by providing a ground path through circuit No. 419 to the ECM.

If engine runs okay, check for: faulty light bulb; circuit No. 419 open; gauge fuse blown (resulting in no stop lights, oil or generator lights, seat belt reminder, etc).

If engine cranks but will not run, check: continuous battery (fuse or fusible link open); ECM ignition fuse open; battery circuit No. 340 to ECM open; ignition circuit No. 439 to ECM open; poor connector to ECM.

Solenoids and relays are turned on or off by the ECM. Solenoid and relay coil resistance must measure more than 20 ohms. Less resistance will cause early failure of the ECM "driver". Before replacing the ECM, be sure to check the coil resistance of all solenoids and relays controlled by the ECM. See ECM wiring diagram for solenoids, relays and coil terminal identification.

Chart A1, No "Service Engine Soon" Light. Scheme 139

CHART A2 - NO CODE 12 "SERV ENG SOON" LIGHT ON STEADY

There should always be a steady "SERVICE ENGINE SOON" light when ignition is on with engine not running (battery voltage supplied directly to light). The ECM will control the light and turn it on by providing a ground path through circuit No. 419 to the ECM.

With diagnostic terminal grounded, light should flash a Code 12, followed by any trouble code(s) stored in memory. A steady light suggests a short to ground in light control circuit No. 419, or an open in diagnostic circuit No. 451.

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

1. If light goes off when ECM connector is disconnected, circuit No. 419 is not shorted to ground. Check connector terminals physically for proper contact.
2. This step checks for an open diagnostic circuit No. 451.
3. "SERVICE ENGINE SOON" light wiring is okay. Problem is a faulty ECM or PROM. ECM is okay if a Code 51 is stored when PROM is removed. Replace PROM.
4. Before replacing ECM, be sure to check coil resistance of all solenoids and relays controlled by the ECM. See ECM wiring diagram for solenoids and relays and coil terminal identification.

Chart A2, No Code 12 "Service Engine Soon" Light On Steady. Scheme 140

CHART A3 - ENGINE CRANKS, BUT WILL NOT RUN

Engine cranks but won't run, or engine may start, but immediately stops running. Battery condition and engine cranking speed are okay. There is enough fuel in tank.

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

Scheme 141

Scheme 142

1. "SERVICE ENGINE SOON" light on is basic check for ignition and battery supply to ECM.
2. Fuel spray from injector indicates that fuel is available. Engine could be flooded due to too much fuel.
3. While cranking, there should be no fuel spray with injector disconnected. Replace injector if it sprays fuel or drips.
4. Check voltage at spark plugs with ST-125 (or equivalent). No spark indicates basic HEI problem. If spark is okay, check for: TPS sticking or binding, open coolant sensor circuit with ignition off (see Code 15), EGR sticking open, open crank signal (may cause a no start during very cold weather), and low fuel pressure or volume (see Chart A5).
5. EFI system okay, reconnect injector.
6. No fuel spray from injector indicates a faulty fuel system or no ECM control of injector. If test light blinks while cranking (even if dim) ECM control is okay. The test light should be an 1847 bulb (6.2 volts).
7. No blinking light indicates no ECM control of injector. With voltmeter selector switch in AC volts position and voltage scale switch in 2 volts range, voltage should be greater than.7 volt AC. If voltage is less than.7 volt AC, there is an open or short to ground in HEI reference circuit No. 430. If circuit is okay, there is a basic HEI problem. (Scheme 141): Chart A3 Schematic, Engine Cranks, But Will Not Run (Scheme 142): Chart A3, Engine Cranks, But Will Not Run (1 of 2) NOTE: The following step numbers refer to the numbers in the accompanying flow chart(s).

1. No fuel spray from one injector indicates a faulty fuel injector or no ECM control of injector. If test light blinked while cranking, ECM control should be considered okay. Be sure test light makes good contact between connector terminals during test. The light may be a little dim when blinking. This is due to current draw of the test light. How bright it blinks is not important. However, the test light bulb should be an 1847 or equivalent.
2. Circuit Nos. 481 and 482 supply ignition voltage to injectors. Probe each connector terminal with a test light to ground. There should be a light on at one terminal. If test light confirms ignition voltage at connector, then ECM injector control circuit No. 467 or 468 may be open. Reconnect injector using a test light connected to ground, and check for light at applicable ECM connector terminal D14 or D16. A light at this point indicates that injector drive circuit involved is okay. If an ECM repeat failure has occurred, the injector is shorted. Replace injector and ECM.

Chart A4, Engine Cranks, But Will Not Run (2 of 2). Scheme 143

CHART A5 - FUEL SYSTEM DIAGNOSIS

When ignition switch is turned to "ON" position, ECM will turn in-tank fuel pump on. Pump will remain on as long as engine is cranking or running, and ECM is receiving HEI distributor reference pulses. If there are no reference pulses, ECM will shut off fuel pump within 2 seconds after key is turned on. Fuel pressure at TBI unit is controlled at 9-13 lbs. Excess fuel is returned to fuel tank.

Fuel pump test terminal is located in ALCL connector terminal "G". When engine is stopped, pump can be turned on by applying battery voltage to test terminal.

Improper fuel system pressure will result in one or all of the following symptoms: Cranks But Won't Run, Code 44, Code 45, cuts out (may feel like ignition problem), poor fuel economy, loss of power, or hesitation.

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

Scheme 144

1. Turns on fuel pump if circuit No. 120 wiring is okay. If pump runs, it is a basic fuel delivery problem, which following steps will locate. (Scheme 144): Chart A5, Fuel System Diagnosis (1 of 2) NOTE: The following step numbers refer to the numbers in the accompanying flow chart(s).

1. Pressure below 9 psi may have 2 causes: Regulated pressure less than 9 psi when amount of fuel is okay, but pressure is too low. System will run lean and may set Code 44. Will be hard to start cold with overall poor performance. Restricted flow causing pressure drop. Normally, a vehicle with a fuel pressure of less than 9 psi 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 fuel return line allows fuel pump to develop maximum pressure (dead head pressure). When battery voltage is applied to pump test terminal, pressure should be 13-18 psi.
3. This test determines if the high fuel pressure condition is due to a restricted fuel return line or a throttle body pressure regulator problem.

Chart A5, Fuel System Diagnosis (2 of 2). Scheme 145

CHART C1 - ECM REPLACEMENT CHECK CHART

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

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

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

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

ECM QDR IDENTIFICATION

Scheme 146

Chart C1A, Park Neutral Switch Diagnosis (Auto. Trans. Only). Scheme 147

Chart C1B, Crank Signal. Scheme 148

Chart C1D, MAP Output Check. Scheme 149

Chart C2C, Idle Air Control Valve Check. Scheme 150

Chart C4B, Ignition System Check. Scheme 151

Chart C5, ESC System Check. Scheme 152

Chart C6C, Air Management Check. Scheme 153

Chart C7, EGR Check. Scheme 154

Chart C8, Transmission Converter Clutch. Scheme 155

Chart C8B, Manual Transmission Shift Light Check. Scheme 156

Chart C10B, A/C Clutch Control - 2.8L Engine (1 of 2). Scheme 157

Chart C10B, A/C Clutch Control - 2.8L Engine (2 of 2). Scheme 158

CODE 13 - OXYGEN SENSOR CIRCUIT

Code 13 will set: at least 2 minutes after engine starts, with O2 sensor signal voltage between .35 and .55 volts for more than one minute, and TPS signal above 6% (.8 to 1.2 volts). ECM supplies voltage of about .45 volt between terminals "8" and "15". (If measured with a 10-megohm digital voltmeter, this may read as low as .32 volts.) The O2 sensor varies the voltage from about one volt (exhaust rich) down to about .10 volt (exhaust lean).

The sensor is like an open circuit and produces no voltage when it is below about 600°F (310°C). An open sensor circuit or cold sensor causes open loop operation.

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

1. Grounding diagnostic terminal with engine running activates "Field Service Mode", which allows ECM to confirm either open or closed loop operation using the "SERVICE ENGINE SOON" light.
2. This test simulates a lean exhaust condition. If ECM and wiring are okay, ECM will see lean condition and turn "SERVICE ENGINE SOON" light off for at least 15 seconds after engine start, then flash "open loop". Light remaining off for a longer period of time before flashing open loop is normal.

Code 13, Oxygen Sensor Circuit. Scheme 159

CODE 14 - COOLANT SENSOR (SIGNAL VOLTAGE LOW)

The Coolant Temperature Sensor (CTS) uses a thermistor to control signal voltage to ECM. ECM applies voltage on circuit No. 410 to the sensor. When engine is cold, sensor resistance is high (ECM sees high signal voltage).

As engine warms, sensor resistance becomes less, and voltage drops. At normal engine operating temperature, voltage will measure about 1-1.5 volts at ECM terminal C10.

Code 14 will set if signal voltage indicates coolant temperature above 275°F (135°C) for more than 10 seconds. Coolant temperature influences control of EGR, fuel delivery, spark timing, air management, idle speed and transmission converter clutch.

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

1. Checks to see if ECM recognized whether problem is still present or is an intermittent one.

Code 14, Coolant Temperature Sensor (Signal Voltage Low). Scheme 160

CODE 15 - COOLANT SENSOR (SIGNAL VOLTAGE HIGH)

The Coolant Temperature Sensor (CTS) uses a thermistor to control signal voltage to ECM. ECM applies voltage on circuit No. 410 to the sensor. When engine is cold, sensor resistance is high (ECM sees high signal voltage).

As engine warms, sensor resistance becomes less, and voltage drops. At normal engine operating temperature voltage will measure about 1-1.5 volts at ECM terminal C10.

ECM will set a Code 15 if signal voltage indicates a coolant temperature of less than -24°F (-35°C) for 4 seconds after engine has been running for more than one minute.

If coolant circuit No. 410 opens with ignition off, ECM will see -40°F (-40°C) and deliver fuel for this temperature. If actual temperature is above about 20°F (-7°C), engine will not start due to rich mixture unless "Clear Flood" is used by fully depressing accelerator. However, "SERVICE ENGINE SOON" light will not come on, and code will not be stored, until engine has run for one minute.

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

1. Checks to see if code was set as a result of hard failure or intermittent condition.
2. This test simulates a Code 14. If ECM recognized low signal voltage and sets a Code 15, ECM and wiring are okay.
3. If code 15 repeats, problem is an open circuit No. 410 and 452, a poor connection at ECM or sensor, or a faulty ECM.

Code 15, Coolant Sensor (Signal Voltage High). Scheme 161

CODE 21 - THROTTLE POSITION SENSOR (TPS)(SIGNAL VOLTAGE HIGH)

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

The TPS signal is one of the most important inputs used by the ECM for fuel control and for most ECM controlled outputs.

Code 21 will set if TPS signal voltage is greater than 3.5 volts for 5 seconds with engine speed less than 1200 RPM, and MAP less than 4.5 psi (no load condition).

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

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

Code 21, Throttle Position Sensor (Signal Voltage High). Scheme 162

CODE 22 - THROTTLE POSITION SENSOR (TPS)(SIGNAL VOLTAGE LOW)

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

The TPS signal is one of the most important inputs used by the ECM for fuel control and for most ECM controlled outputs.

Code 22 will set if TPS signal is less than .2 volts for 5 seconds and engine speed is less than 1200 RPM. Possible causes of Code 22 are faulty TPS, faulty wiring or terminals, or faulty ECM.

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

1. Confirms Code 22 and that fault is present.
2. Simulates Code 21. If ECM recognizes high signal voltage and sets code 21, ECM and wiring are okay.
3. Checks for reference voltage from ECM. To prevent damage to ECM, disconnect White connector when checking circuit wiring for open or shorts to ground.

Code 22, Throttle Position Sensor (Signal Voltage Low). Scheme 163

CODE 24 - VEHICLE SPEED SENSOR

The ECM supplies a current-limited 12-volt signal on circuit No. 437. The Vehicle Speed Sensor (VSS), located in the instrument panel cluster, senses the speedometer rotating element and furnishes this information to the buffer as a pulsed signal (2 per cable revolution or 2002 pulses per mile).

The buffer assembly switches circuit No. 437 to ground for each pulse received. The ECM uses the time between pulses to determine vehicle speed.

Code 24 is set when circuit No. 437 voltage is constant (no rise and fall of voltage for 4 to 10 seconds), with engine speed between 1500 and 4400 RPM, transmission in Drive (as indicated by Park/Neutral switch), and low MAP (indicating engine deceleration).

Loss of VSS signal will affect the Torque Converter Clutch, Idle Air Control, and Fan Control (fan will not shut off above 30 MPH with A/C on).

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

1. Checks to see if there is a VSS signal to ECM while turning a drive wheel. Voltage should vary from under 3 volts to over 6 volts as wheel is turned. Rotating wheel faster reduces variation.
2. A voltage of less than one volt indicates that the circuit No. 437 wire is shorted to ground. Disconnect circuit No. 437 at vehicle speed sensor. If voltage now reads above 10 volts, VSS is faulty. If voltage remains less than 10 volts, then circuit No. 437 wire is grounded. If circuit No. 437 is not grounded, check for a faulty ECM connector or ECM.
3. A steady 8-12 volts indicated circuit No. 437 is open or a faulty vehicle speed sensor.
4. This is a normal voltage condition.
5. If "SCAN" displays vehicle speed, check park/neutral switch CHART C1A on vehicle with automatic transmission. If switch is okay, check for intermittents.

Code 24, Vehicle Speed Sensor. Scheme 164

CODE 32 EGR SYSTEM FAILURE (2.8L ENGINE)

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

1. Coolant temperature greater than 194°F (90°C).
2. EGR duty cycle commanded by ECM is greater than 50%.
3. All conditions above must be met for about 4 minutes.

If switch is detected closed during start-up, "SERVICE ENGINE SOON" light will come on within 40 seconds and stay on until switch is detected open or until engine is turned off. However, if switch is detected open when above conditions are met, "SERVICE ENGINE SOON" light will only remain on while all conditions are met.

The EGR vacuum control used an ECM controlled pulse width modulated EGR solenoid. The valve is normally closed and vacuum source is a ported signal. The ECM will turn EGR on and off (Duty Cycle) by grounding circuit No. 435. The duty cycle is calculated by ECM, based on information from coolant, MAP and TPS sensors, and engine RPM.

The duty cycle should be 0% (No EGR) when in park or neutral, TPS input below a specified value or indicating WOT. With ignition on, and engine stopped, EGR solenoid is de-energized if diagnostic terminal is grounded under the same conditions.

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

1. Checks for circuit No. 935 being shorted to ground during start-up or temperature switch stuck closed.
2. Checks to see if fault is switch, faulty wiring or ECM.
3. Checks for an open in circuit No. 935.
4. This test determines whether control solenoid, wiring or ECM is faulty.

Code 32, EGR System Failure (2.8L Engine). Scheme 165

CODE 32 - EGR SYSTEM FAILURE (4.3L ENGINE)

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

1. Coolant temperature greater than 194°F (90°C).
2. EGR duty cycle commanded by ECM is greater than 50%.
3. All conditions above must be met for about 4 minutes.

If switch is detected closed during start-up, "SERVICE ENGINE SOON" light will come on within 40 seconds and stay on until switch is detected open or until engine is turned off. However, if switch is detected open when above conditions are met, "SERVICE ENGINE SOON" light will only remain on while all conditions are met.

The EGR vacuum control used an ECM controlled pulse width modulated EGR solenoid. The valve is normally closed and vacuum source is a ported signal. The ECM will turn EGR on and off (Duty Cycle) by grounding circuit No. 435. The duty cycle is calculated by ECM, based on information from coolant, MAP and TPS sensors, and engine RPM.

The duty cycle should be 0% (No EGR) when in park or neutral, TPS input below a specified value or indicating WOT. With ignition on, and engine stopped, EGR solenoid is de-energized if diagnostic terminal is grounded under the same conditions.

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

1. Checks for circuit No. 935 being shorted to ground during start-up or temperature switch stuck closed.
2. Checks to see if fault is switch, faulty wiring or ECM.
3. Checks for an open in circuit No. 935.
4. This test determines whether control solenoid, wiring or ECM is faulty.

Code 32, EGR System Failure (4.3L Engine). Scheme 166

CODE 33 - MAP SENSOR SIGNAL VOLTAGE HIGH

The MAP sensor responds to changes in manifold pressure (vacuum). The ECM receives this information as a signal voltage that varies from 1-1.5 volts at idle to 4-4.5 volts at wide open throttle.

If MAP sensor fails, ECM substitutes a fixed MAP value and uses the TPS signal to control fuel delivery.

Code 33 will set when signal is too high for more than 5 seconds while TPS voltage indicates throttle is closed, and vehicle speed is zero MPH.

Engine misfire or a low and unstable idle may set Code 33. Disconnect MAP sensor and system will go into back-up mode. If misfire or idle condition remains, MAP sensor is good.

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

1. Confirms Code 33 and that fault is present.
2. If ECM recognizes and sets Code 34 (low MAP signal), ECM and wiring are okay.

Code 33, MAP SENSOR (Signal Voltage High). Scheme 167

CODE 34 - MAP SENSOR SIGNAL VOLTAGE LOW

The MAP sensor responds to changes in manifold pressure (vacuum). The ECM receives this information as a voltage signal that varies from about 1-1.5 volts at idle to 4-4.5 volts at wide open throttle.

If MAP sensor fails, ECM substitutes a fixed MAP value and uses the TPS signal to control fuel delivery.

Code 34 will set when signal is too low and ignition is turned on.

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

1. Confirms Code 34, and that fault is present.
2. If ECM recognizes and sets Code 33 (high MAP signal), ECM and wiring are okay.

Code 34, MAP Sensor (Signal Voltage Low). Scheme 168

CODE 42 - ELECTRONIC SPARK TIMING

Code 42 means ECM has seen an open or short to ground in the Electronic Spark Timing (EST) or by-pass circuits.

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

1. Confirms Code 42 and that fault causing code is present.
2. Checks for normal EST ground path through ignition module. An EST circuit No. 423 shorted to ground will also read less than 500 ohms (will be checked later).
3. As test light voltage touches terminal 10 (circuit No. 424), module should switch, causing ohmmeter to "overrange" (meter is in the 1000-2000 ohms position). Selecting 10-20,000 ohms position will indicate above 5000 ohms. The important thing is that the module "switched".
4. The module did not switch and this step checks for: EST circuit No. 423 shorted to ground. By-pass circuit No. 424 open. Faulty ignition module connection or module.
5. Confirms that Code 42 is a faulty ECM and not an intermittent in circuit Nos. 423 or 424.

Code 42, Electronic Spark Timing. Scheme 169

CODE 43 - ELECTRONIC SPARK CONTROL (ESC)

Code 43 means ECM has low voltage at circuit No. 485, terminal B7 for longer than 4 seconds with engine running. The voltage drops at terminal B7 when ESC module shuts off because it receives a knock signal. This system also performs a functional check once per start-up to check ESC system. To perform this test, ECM will advance spark when coolant is above 194°F (90°C) and at WOT. The ECM then checks signal at B7 to see if knock is detected. The functional check is performed once per start-up. If knock is detected when coolant is below 194°F (90°C), the test has passed. If the functional check fails, the "SERVICE ENGINE NOW" light will remain on until ignition is turned off or until a knock signal is detected.

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

1. Checks to see if ECM recognized a problem.
2. This will determine if system is functioning at this time.
3. The ESC module supplies voltage to ECM. It should always be over 6 volts, unless system is sensing engine detonation.
4. Checks for intermittent ESC operation. If voltage is now over 6 volts, it is a faulty ESC terminal C connection or ESC module.
5. Checks for grounded ECM.
6. Checks for open ignition circuit.

Code 43, Electronic Spark Control (ESC). Scheme 170

CODE 44 - LEAN EXHAUST INDICATION

The ECM supplies a voltage of about .45 volt between terminals "8" and "15" (circuit Nos. 412 and 413). If measured with a 1-megohm digital voltmeter, this may read as low as .32 volts. The O2 sensor varies voltage from about one volt (exhaust rich), to about .10 volt (exhaust lean). An open sensor circuit or cold sensor causes open loop operation.

Code 44 is set when the O2 sensor signal to the ECM remains below .2 volt for 8 seconds or more, 1 1/2 minutes or more after engine start.

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

1. Grounding the diagnostic terminal with the engine running enables the "Field Service Mode" and allows ECM to confirm either open or closed loop operation.
2. A light out or "Open Loop" indicates fault is present. Disconnecting O2 sensor raises signal voltage above .2 volt. If ECM and wiring are okay, ECM should recognize higher voltage (.35-.55) and flash open loop when engine is started.
3. Code 44 may be set by any of the conditions listed in chart. If these systems are operating properly, O2 sensor is faulty.

Code 44, Lean Exhaust Indication. Scheme 171

CODE 45 - RICH EXHAUST INDICATION

The ECM supplies a voltage of about .45 volt between terminals "D7" and "D6" (if measured with a 1-megohm digital voltmeter, this may read as low as .32 volts). The O2 sensor varies the voltage from about one volt (exhaust rich), to about .10 volt (exhaust lean). An open sensor circuit or cold sensor causes open loop operation.

Code 45 is set when the O2 signal voltage at the ECM Black connector terminal "D7" remains above .7 volts for one minute, one minute or more after engine start.

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

1. Test confirms open or closed loop operation.
2. A steady light or open loop indicates presence of fault. Grounding circuit No. 412 causes low O2 signal voltage. If ECM and wiring are okay, ECM should recognize low voltage and confirm lean signal by turning off "SERVICE ENGINE SOON" light for at least 15 seconds.
3. A Code 45 WILL NOT be caused by a faulty O2 sensor. Code 45 indicates a rich exhaust and diagnosis should begin with these items: fuel pressure, rich injector, leaking injector, HEI shielding, canister purge, coolant sensor, MAP sensor, or TPS. Also, inspect oxygen sensor for silicone contamination from fuel, or use of improper RTV sealant. The sensor may have a White, powdery coating and result in a high but false signal voltage (rich exhaust indication). The ECM will then reduce the amount of fuel delivered to engine, causing a severe driveability problem.

Code 45, Rich Exhaust Indication. Scheme 172

CODE 51 - PROM FAULT

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

CODE 52 - FUEL CALPAK MISSING

Install missing or faulty CALPAK.

CODE 54 - FUEL PUMP CIRCUIT VOLTAGE LOW

When key is first turned on without engine running, ECM will turn fuel pump relay on for 2 seconds. This builds up fuel pressure quickly. If engine is not started within 2 seconds, ECM will shut fuel pump off. As soon as engine is cranking, ECM will turn relay on and run fuel pump.

As a back-up system to fuel pump relay, the fuel pump can also be turned on by oil pressure switch. The oil pressure switch is a normally open switch that closes when oil pressure reaches about 4 psi. If fuel pump relay fails, oil pressure switch will run fuel pump.

An inoperative fuel pump relay can result in long cranking times, particularly if engine is cold. The oil pressure switch will turn on fuel pump. However, there is an extended crank time caused by time necessary for oil pressure to reach 4 psi and close switch.

When ignition switch is turned to "ON" position, ECM terminal B2 looks for voltage on pump circuit No. 120 for .2 seconds. If voltage is not present, it will set Code 54 and turn on "SERVICE ENGINE SOON" light after engine starts.

Code 54, Fuel Pump Circuit Low Voltage. Scheme 173

CODE 55 - ECM FAULT

Replace electronic control module (ECM).

ECM Terminal ID & Voltage Values (2.8L TBI Engine). Scheme 174

ECM Terminal ID & Voltage Values (4.3L TBI Engine). Scheme 175

Wiring Diagram (2.8L TBI Engine). Scheme 176

Wiring Diagram (4.3L TBI Engine). Scheme 177