2.5l Tbi Tests W/codes GMC S15 Pickup

BASIC DIAGNOSTIC PROCEDURE - TESTING

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 64

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 64) 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 64): 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 I.D.

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.

"NON-SCAN" DIAGNOSTIC CIRCUIT CHECK

The diagnostic circuit check is an organized approach for identifying a problem caused by the Fuel Injection System. Driver complaints fall into 3 categories: steady "SERVICE ENGINE SOON" light, driveability problems, and engine "CRANKS BUT, WON'T RUN".

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

1. 1) A steady "SERVICE ENGINE SOON" light, with ignition on and engine not running, confirms battery and ignition voltage to ECM.
2. 2) Ground diagnosis terminal by jumpering terminal A to B in ALCL connector located below instrument panel. The ECM will cause "SERVICE ENGINE SOON" light to flash Code 12, indicating that ECM diagnostics are working. Code 12 will flash 3 times, followed by any other trouble codes stored in memory. Each additional code will flash 3 times starting with lowest code, and then start over again with Code 12. If there are no other codes, Code 12 will flash until diagnostic terminal jumper is disconnected or engine is started.
3. 3) Record all stored codes except Code 12.
4. 4) With engine running and diagnostic terminal grounded, ECM will respond to O2 sensor signal voltage and use the "SERVICE ENGINE SOON" light to display this information as follows
5. A) Closed loop confirms that O2 sensor voltage is being used to control fuel delivery. Signal voltage will vary from .35-.55 volt.
6. B) Open loop confirms that O2 sensor voltage to ECM is unusable. Signal voltage is a fixed value between .35 and .55 volt. System will flash open loop for 30 seconds to 2 minutes or until O2 sensor reaches operating temperature.
7. C) O2 sensor signal voltage will be less than .35 volt. Refer to Code 44.
8. D) O2 sensor signal voltage will be more than .55 volt. Refer to Code 45.
9. 5) Road test of system in field service mode must be done at steady speeds. In this mode the following conditions may be observed and should be considered normal: light on too long under acceleration (due to acceleration enrichment); light off too long under deceleration (due to decel leaning of fuel mixture or fuel cut-off); light on too long at idle with idle below 1200 RPM.
10. 6) Clearing codes. Ignition off. Disconnect battery pigtail or ECM B fuse for 10 seconds.

"Non-Scan" Diagnostic Circuit Check. Scheme 65

"Non-Scan" Diagnostic Circuit Check. Scheme 66

"SCAN" DIAGNOSTIC CIRCUIT CHECK - TESTING

The "SCAN" diagnostic circuit check is an organized approach for identifying fuel injection problems using an assembly line communication link (ALCL). This communication link can provide diagnostic information for display on any scan device or tool designed for this purpose. The tool plugs into ALCL connector located below instrument panel. 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.

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

1. If scan tool is not operating, check on another vehicle. If okay, the cigar lighter socket should be checked for 12 volts and a good ground. If scan tool reads "no data" or "no ALCL" with ignition on, check serial data wire for an open or short to ground between ALCL terminal "E" and ECM. Also check for an open diagnostic test terminal for ALCL terminal "B" and ECM. With ignition on, serial data line (ALCL terminal "E") should have a varying 2-5 volts, and diagnostic line (ALCL terminal "B") about 5 volts.

"Scan" Diagnostic Circuit Check, Schematic. Scheme 67

"Scan" Diagnostic Circuit Check, Flow Chart. Scheme 68

"Scan" Diagnostic Circuit Check, Flow Chart. Scheme 69

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.

Chart A1, No "Service Engine Soon" Light, Schematic. Scheme 70

Chart A1, No "Service Engine Soon" Light, Flow Chart. Scheme 71

Chart A1, No "Service Engine Soon" Light, Flow Chart. Scheme 72

Note. Before replacing ECM, use an ohmmeter and check resistance of each ECM controlled relay and solenoid coil. See ECM wiring diagram for coil terminal identification for solenoid(s) and relay(s) to be checked. Replace any relay or solenoid if the coil resistance measures less than 20 Ohms.

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.

Chart A2, Schematic, Won't Flash Code 12 "Serv Eng Soon" Light On Steady. Scheme 73

Chart A2, Flow Chart, Won't Flash Code 12 "Serv Eng Soon" Light On Steady. Scheme 74

Chart A2, Flow Chart, Won't Flash Code 12 "Serv Eng Soon" Light On Steady. Scheme 75

CHART A3 - ENGINE CRANKS, BUT WILL NOT RUN (1 OF 2)

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).

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); 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 the test light blinks while cranking (even if dim) ECM control is okay. The test light should be 1847 bulb (6.3 volts).

Chart A3, Engine Cranks, But Won't Run, Schematic. Scheme 76

Chart A3, Engine Cranks, But Won't Run, Flow Chart (1 Of 2). Scheme 77

Chart A3, Engine Cranks, But Won't Run, Flow Chart. Scheme 78

CHART A4 - ENGINE CRANKS, BUT WILL NOT RUN (2 OF 2)

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

1. 7) Circuit No. 439 supplies ignition voltage to injector. Probe each connector terminal with test light to ground. There should be a light on one terminal, confirming ignition voltage at connector. ECM injector control circuit No. 467 may be open. Reconnect injector. Using test light connected to ground, check for light at white ECM connector terminal "8". Light at this point indicates that injector drive circuit is okay.
2. 8) No blinking light indicates no ECM control of injector. With voltmeter in "AC Volts" position, and voltage scale switch in 2-volt range, voltage should be greater than .7 volt AC. If voltage reading 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.
3. 8A) ALTERNATE PROCEDURE. Disconnect distributor connector. Momentarily touch ECM side of connector terminal "B" with test light to 12 volts. Note injector as contact is made. Each time test light contacts terminal "B", injector should turn on. If so, ECM injector control circuit is okay. Contact on ST-125 and check for "spark". If on, it is faulty HEI module. No spark indicates basic HEI problem.

Chart A4, Flow Chart, Engine Cranks, But Won't Run (2 Of 2). Scheme 79

From Chart A-3. Scheme 80

CHART A5 - FUEL SYSTEM DIAGNOSIS (1 OF 3)

When ignition switch is turned to "ON" position, the ECM will turn the in-tank fuel pump on. Pump will remain on as long as engine is cranking or running, and the 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).

1. Determines if pump circuit is ECM controlled. The ECM will turn on pump relay. Engine is not cranking or running so ECM will turn off the relay within 2 seconds after ignition is turned on.
2. Turns on fuel pump if circuit No. 120 wiring is okay. If pump runs, problem is a basic fuel delivery problem, which following steps will locate.
3. Checks for battery voltage at pump relay.

Chart A5, Fuel System Diagnosis, Schematic. Scheme 81

Chart A5, Fuel System Diagnosis, Flow Chart (1 Of 3 ). Scheme 82

Note. Fuel system under pressure. To avoid fuel spillage, refer to field service procedures for testing or making repairs requiring disassembly of fuel lines or fittings.

Chart A5, Fuel System Diagnosis, Flow Chart. Scheme 83

CHART A6 - FUEL SYSTEM DIAGNOSIS (2 OF 3)

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

1. 4) Checks relay ground circuit No. 450.
2. 5) Checks for ECM control of relay through circuit No. 465.
3. 6) The fuel pump voltage control circuit includes an engine oil pressure switch with a separate set of normally open contacts. The switch closes at 4 psi of oil pressure and provides a second battery feed path to fuel pump. If the relay fails, the pump will continue to run using battery feed supplied by closed oil pressure switch. A failed pump relay will result in extended engine crank time, because of the time required to build enough oil pressure to close oil pressure switch and turn on fuel pump. There may be instances when the relay has failed but engine will not crank fast enough to build enough oil pressure to close switch. This, or a faulty oil pressure switch, can result in "engine cranks, but will not run".
4. 7) Checks oil pressure switch to ensure it provides battery feed to fuel pump should pump relay fail.
5. 8) Checks for open oil pressure switch with ignition off. Should switch stick closed, fuel pump will continue to run and discharge battery.

Chart A6, Fuel System Diagnosis, Schematic. Scheme 84

Chart A6, Fuel System Diagnosis, Flow Chart (2 Of 3). Scheme 85

From Chart A-5. Scheme 86

CHART A7 - FUEL SYSTEM DIAGNOSIS (3 OF 3)

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 high fuel pressure is due to a restricted fuel return line or a throttle body pressure regulator problem.

Chart A7, Fuel System Diagnosis, Schematic. Scheme 87

Chart A7, Fuel System Diagnosis, Flow Chart (3 Of 3). Scheme 88

From Chart A-5. Scheme 89

CODE 13 - OXYGEN (O2) 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, about 1200 RPM). 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. Verifies no additional codes stored, and that Code 13 is intermittent.
3. Simulates lean exhaust. 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, Schematic. Scheme 90

Code 13, Oxygen Sensor Circuit, Flow Chart. Scheme 91

Start Non-Scan. Scheme 92

CODE 14 - COOLANT SENSOR CIRCUIT (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 4.

Code 14 will set if signal voltage indicates coolant temperature above 275°F (135°C) for more than two seconds.

Coolant temperature influences control of fuel delivery, engine timing, idle speed and transmission converter clutch.

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

1. If voltage is above 4 volts, ECM and wiring are okay.

Code 14, Coolant Sens Ckt (Signal Voltage Low), Schematic. Scheme 93

Code 14, Coolant Sens Ckt (Signal Voltage Low), Flow Chart. Scheme 94

Start Non-Scan. Scheme 95

Code 14 Coolant Sensor Table. Scheme 96

CODE 15 - COOLANT SENSOR CIRCUIT (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 4.

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. If voltage is above 4 volts, ECM and wiring are okay. If location of sensor makes it hard to check, disconnect Black ECM connector and check resistance between connector terminals 4 and 11.

Code 15, Coolant Sens Ckt (Signal Voltage High), Flow Chart. Scheme 97

Start Non-Scan. Scheme 98

Code 15 Coolant Sensor Table. Scheme 99

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

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 21 will set if TPS signal voltage is greater than 2.5 volts for 2 seconds with engine speed less than 1600 RPM, and MAP less than 9 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, TPS Sensor (Signal Voltage High), Schematic. Scheme 100

Code 21, TPS Sensor (Signal Voltage High), Flow Chart. Scheme 101

Start Non-Scan. Scheme 102

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 2 seconds and engine speed is less than 1600 RPM.

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, TPS Sensor (Signal Voltage Low), Flow Chart. Scheme 103

Start Non-Scan. Scheme 104

CODE 24 - VEHICLE SPEED SENSOR (VSS)

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 reading of less than one volt at ECM connector indicates that circuit No. 437 wire is shorted to ground. Disconnect circuit No. 437 at VSS. If voltage now reads above 10 volts, VSS is faulty. If voltage remains less than 10 volts, circuit No. 437 wire is grounded. If circuit No. 437 is not grounded, check for faulty ECM connector or ECM.
3. A steady 8-12 volts at ECM connector indicates circuit No. 437 is open or a faulty VSS.
4. This is a normal voltage condition and indicates a possible intermittent 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, Schematic. Scheme 105

Code 24, Vehicle Speed Sensor, Flow Chart. Scheme 106

Start Non-Scan. Scheme 107

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 8 seconds while TPS voltage indicates throttle is closed.

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), Schematic. Scheme 108

Code 33, MAP Sensor (Signal Voltage High), Flow Chart. Scheme 109

Start Non-Scan. Scheme 110

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), Flow Chart. Scheme 111

Start Non-Scan. Scheme 112

CODE 35, IDLE AIR CONTROL (IAC)

Code 35 will set when closed throttle engine speed is 50 RPM above or below correct idle speed for 30 seconds. Following are nominal warm engine idle speeds

NOMINAL IDLE SPEEDS (RPM)

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

1. Continue with test even if engine will not idle. If idle is too low, "SCAN" will display 80 or more counts or steps. If idle is high and a visual check of idle air passage shows a seated valve, locate and correct vacuum leak. If idle is very high, usually above 1400 RPM and IAC valve is not visible in air passage, follow left side of chart. 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. There is a system problem. Proceed to step 3).
2. When engine was stopped, IAC valve retracted (more air) to fixed park position to provide increased airflow during next engine start. A "SCAN" will display 95 or more counts and valve should not be visible in idle air passage. Disconnecting IAC will hold valve in retracted or open position, and cause a closed throttle idle speed above 1500 RPM. A "SCAN" will now display "0" counts because ECM has tried to reduce idle speed by extending valve. The IAC is okay. The Code 35 is likely a thermac or cruise control vacuum hose disconnect.
3. A slow unstable idle may be caused by a system problem that cannot be overcome by IAC. If IAC is visible in air passage, the chart should locate problem. If valve is not visible, IAC is probably okay. In both cases "SCAN" counts will be above 60 counts.

Code 35, Idle Air Control (IAC), Schematic. Scheme 113

Code 35, Idle Air Control (IAC), Flow Chart. Scheme 114

Code 35, Idle Air Control (IAC), Flow Chart. Scheme 115

CODE 42 - ELECTRONIC SPARK TIMING (EST)

Code 42 means ECM has seen an open or short to ground in the Electronic Spark Timing (EST) or by-pass circuit Nos. 423 or 424.

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, 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, Schematic (S-10/15 - S & T Models). Scheme 116

Code 42, Electronic Spark Timing, Schematic (Astro - M Models). Scheme 117

Code 42, Electronic Spark Timing, Flow Chart. Scheme 118

Code 42, Electronic Spark Timing, Flow Chart. Scheme 119

CODE 44 - LEAN EXHAUST INDICATION

The ECM supplies a voltage of about .45 volt between terminals "8" and "15" (measured with a one-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 ECM remains below .2 volt for 8 seconds or more, 2 minutes 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, Schematic. Scheme 120

Code 44, Lean Exhaust Indication, Flow Chart. Scheme 121

Start Non-Scan. Scheme 122

CODE 45 - RICH EXHAUST INDICATION

The ECM supplies a voltage of about .45 volt between terminals "8" and "15" (measured with a one 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 .0 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 "8" 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 A low O2 sensor signal voltage. If ECM and wiring are okay, ECM should recognize low voltage and confirm lean signal by turning off the "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, leaking injector, HEI shielding, canister purge, coolant sensor, MAP sensor, or TPS.

Code 45, Rich Exhaust Indication, Schematic. Scheme 123

Code 45, Rich Exhaust Indication, Flow Chart. Scheme 124

Start Non-Scan. Scheme 125

CODE 51 - FAULTY PROM

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

CODE 55 - FAULTY ECM

Replace ECM. Clear codes, confirm closed loop operation and no "SERVICE ENGINE SOON" light.

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

ECM QDR IDENTIFICATION

ECM QDR IDENTIFICATION

ECM QDR IDENTIFICATION

ECM QDR IDENTIFICATION

Scheme 126

Chart C1A, Park/Neutral Switch Auto Transmission Only. Scheme 127

Chart C1A, Park/Neutral Switch Auto Transmission Only. Scheme 128

Chart C1B, Fuel Injection (TBI) Crank Signal. Scheme 129

Chart C1B, Fuel Injection (TBI) Crank Signal. Scheme 130

Chart C1D, Fuel Injection (TBI) MAP Output Check. Scheme 131

Chart C1D, Fuel Injection (TBI) MAP Output Check. Scheme 132

Chart C1E, Fuel Injection (TBI) P/S Pressure Switch. Scheme 133

Chart C1E, Fuel Injection (TBI) P/S Pressure Switch. Scheme 134

Chart C4B, Fuel Injection (TBI) Ign. Sys Check, Schematic. Scheme 135

Chart C4B, Fuel Injection (TBI) Ign. Sys Check, Flow Chart. Scheme 136

Chart C7A, Fuel Injection (TBI) EGR Valve Check. Scheme 137

Chart C7A, Fuel Injection (TBI) EGR Valve Check. Scheme 138

Chart C8A, Fuel Injection (TBI) 700-4R TCC. Scheme 139

Chart C8A, Fuel Injection (TBI) 700-4R TCC. Scheme 140

Chart C8B, Fuel Injection (TBI) Manual Transaxle Shift Light. Scheme 141

Chart C8B, Fuel Injection (TBI) Manual Transaxle Shift Light. Scheme 142

Chart C10, Fuel Injection (TBI) A/C Clutch Control (1 Of 2). Scheme 143

Chart C10, Fuel Injection (TBI) A/C Clutch Control (1 Of 2). Scheme 144

Chart C10, Fuel Injection (TBI) A/C Clutch Control (2 Of 2). Scheme 145

Chart C10, Fuel Injection (TBI) A/C Clutch Control (2 Of 2). Scheme 146

ECM Terminal ID & Voltage Values (2.5L TBI Engine). Scheme 147

Wiring Diagram (2.5L TBI Engine). Scheme 148