Ccc Tbi Tests W/codes GMC Cab & Chassis R2500

MODEL IDENTIFICATION

BODY CODE IDENTIFICATION CHART

BASIC DIAGNOSTIC PROCEDURES

Note. Most computerized engine control problems are the result of mechanical breakdowns, poor electrical connections or damaged vacuum hoes. 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 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 the SYSTEM 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 check. This will determine if CCC system can enter "Closed Loop" mode of operation.
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 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.

ENTERING OR EXITING DIAGNOSTIC MODE

1. Turn ignition switch on but do not start engine. "SERVICE ENGINE SOON" lamp should glow. Locate Assembly Line Data Link (ALDL) connector attached to ECM wiring harness under instrument panel near steering column (under driver's seat on "G" series). Insert spade lug terminal across "TEST" terminal and "GROUND" terminal. (Scheme 85) CAUTION: Inserting spade lug in terminals of ALDL connector grounds "TEST" terminal lead. Do not ground ALDL connector until after ignition is on or engine is started.
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 ALDL connector is grounded.
4. To exit diagnostic mode, turn ignition switch off and remove spade lug terminal from ALDL connector.

ALDL Connector Terminal Locations. Scheme 85

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.

Without "SCAN" Tool

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 on a diagnostic tool ("SCAN" TOOL) connected to the ALDL connector. These special tools are faster and more accurate, but are not mandatory.

If a "SCAN" 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 code, second flashes are the second digit.

With "SCAN" Tool

A "SCAN" tool provides a visual reading of most of the ECM inputs and some of the outputs controlled by the ECM. With the "SCAN" tool in the code position, the stored code will be displayed in the display window.

By connecting the "SCAN" tool to the ALDL connector and a 12-volt power supply, valuable information can be received from the CCC system. The technician's knowledge of the system and understanding of the "SCAN" tool's limitations can provide information that would otherwise be difficult to obtain. However, "SCAN" tools cannot replace volt/ohmmeters when called for within diagnostic charts.

The "SCAN" tool can display the following information in the display window

1. "ON" when ECM has commanded A/C clutch into operation. This does not mean that A/C clutch is operating, only that ECM has instructed it to commence operation.
2. "YES" when A/C is activated.
3. Actual battery voltage reading detected at ECM ignition input.
4. "Closed Loop" or "Open Loop", depending upon mode that engine control system is operating under.
5. Diagnostic codes.
6. Coolant temperature in degrees Centigrade.
7. RPM that ECM is trying to maintain at idle.
8. "ON" when ECM has commanded EGR valve to open. This does not mean that EGR valve is open, only that ECM has instructed EGR valve to open to allow EGR flow.
9. Position of EGR valve pintle.
10. "YES" when transmission is in fourth gear.
11. A number that represents the position that the ECM thinks the idle air control valve is in at that time.
12. A number, in milliseconds, that represents the pulse width ("on" time) that the ECM is commanding to the injectors.
13. Amount of spark retard in number of degrees.
14. "YES" when knock is detected or "NO" when knock is not detected.
15. Manifold air temperature in degrees Centigrade.
16. Low voltage when the manifold pressure is low (high vacuum), or high voltage when the pressure is high (low vacuum).
17. Vehicle speed in miles per hour.
18. Oxygen sensor reading ranging from 1 to 999 millivolts. If consistently below 350 millivolts, fuel system is lean; if consistently above 550 millivolts, fuel system is rich.
19. Percent of throttle angle.

These are only some of the capabilities of the "SCAN" tool. Consult tool manufacturer's diagnostic manual for complete explanation of the tool and its limitations.

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.

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 the circuit check and will 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 System Check found a problem.
4. "Engine Cranks But Won't Run" charts.

System 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 or MEM-CAL removal and installation in this article.

Field Service Mode Check

1. This test confirms proper operation of fuel system and verifies "Closed Loop" operation. 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.

ECM Terminal ID & Voltage Values (2.5L). Scheme 86

ECM Terminal ID & Voltage Values (2.8L). Scheme 87

ECM Terminal ID & Voltage Values (4.3L, 5.0L, 5.7L & 7.4L). Scheme 88

Astro & Safari 2.5L Wiring Diagram. Scheme 89

S-10 & S-15 2.5L Wiring Diagram. Scheme 90

S-10 & S-15 2.8L Wiring Diagram. Scheme 91

Astro & Safari 4.3L Wiring Diagram. Scheme 92

"G" Series 4.3L, 5.0L & 5.7L Wiring Diagram. Scheme 93

Blazer/Jimmy "C/K" & Suburban 4.3L/5.0/5.7L Wiring Diagram. Scheme 94

"P" Series (Parcel Delivery Van) 5.7L Wiring Diagram. Scheme 95

"C" & "K" Series 7.4L Wiring Diagram. Scheme 96

2.5L VIN "E" - Astro & Safari Component Locations. Scheme 97

4.3L VIN "Z" - Astro & Safari Component Locations. Scheme 98

2.5L VIN "E" - "S/T" Series Truck Component Locations. Scheme 99

2.8L VIN "R" - "S/T" Series Truck Component Locations. Scheme 100

4.3L VIN "Z" - "C/K" Series Truck Component Locations. Scheme 101

5.0L VIN "H" & 5.7L VIN "K" - "C/K" Component Locations. Scheme 102

7.4L VIN "N" - "C/K" & "R/V" Series Truck Component Locations. Scheme 103

4.3L VIN "Z" - "R/V" Series Truck Component Locations. Scheme 104

5.0L VIN "H" & 5.7L VIN "K" - "R/V" Component Locations. Scheme 105

4.3L VIN "Z" - "G" Series Van Component Locations. Scheme 106

5.0L VIN "H"/5.7L VIN "K" "G" Series Van Component Locations. Scheme 107

7.4L VIN "K" - "P" Series Parcel Van Component Locations. Scheme 108

Computer Command Control System Schematic. Scheme 109

System Check Chart. Scheme 110

Circuit Description

There should always be a steady "SERVICE ENGINE SOON" light when ignition is turned on with engine off. Battery voltage is supplied directly to illuminate the bulb. The ECM will control the light and turn it on by providing a ground path through circuit No. 419 to the ECM. (Scheme 111)

Test Description

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

1. If fuse in holder is blown, see wiring diagram for Code 54 for complete electrical circuit.
2. Using a test light connected to 12 volts, probe each system ground circuit to ensure good ground is present. See ECM voltage pin charts for ECM pin locations of ground circuits.

Diagnostic Aids

If engine runs okay, check

1. Faulty light bulb.
2. Open in circuit No. 419. (Scheme 111)
3. Blown gauge fuse. This will result in no stop lights, oil light, generator light, seat belt reminder, etc.

Chart A1 - No "Service Engine Soon" Light. Scheme 111

There should always be a steady "SERVICE ENGINE SOON" ("SES") light when ignition is turned on with engine off. Battery voltage supplied directly to illuminate the bulb. The ECM will control the light and turn it on by providing a ground path through circuit No. 419 to ECM. (Scheme 111)

With diagnostic terminal grounded, light should flash a Code 12, followed by any trouble code(s) stored in memory.

A steady light indicates a short to ground in light control Circuit No. 419, or an open in diagnostic circuit No. 451. Refer to Fig. (Scheme 111)

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

1. If there is a problem with ECM that causes a "SCAN" tool to not read Serial Data, then ECM should flash Code 12. If Code 12 does flash, ensure "SCAN" tool is working properly by testing on another vehicle. If "SCAN" tool is functioning properly and circuit No. 461 is okay, the PROM, MEM-CAL, or ECM may be at fault for "NO ALDL" symptom.
2. If light goes off when ECM connector is disconnected, then circuit No. 419 is not shorted to ground. (Scheme 111)
3. This step checks for an open diagnostic circuit No. 451. (Scheme 111)

Chart A2 - No ALDL Data Or Won't Flash Code 12 "Service Engine Soon" Light On Steady. Scheme 112

This chart assumes that battery condition and engine cranking speed are okay, and there is enough fuel in tank. This chart should be used only on engines equipped with Model 700 throttle body unit.

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

1. "SERVICE ENGINE SOON" light on is basic check for ignition and battery supply to ECM. No "ALDL" may be due to an ECM problem. CHART A2 will diagnose the ECM. If TPS is over 2.5 volts, engine may be in "clear flood" mode, that will cause starting problems. If coolant sensor is below -22°F (-30°C), ECM will provide fuel for extremely cold temperatures that will completely flood engine.
2. Check voltage at spark plugs with ST-125 (J-26792). No spark indicates basic ignition problem.
3. While cranking engine, there should be no fuel spray with injector disconnected. Replace injector if it sprays or drips excessively.
4. Using Injector Test Light (J-34730, BT-8329A), test injector circuit. A blinking light indicates ECM is controlling injector.
5. This determines if fuel pressure is present at injector and that injector is operating properly.

If no trouble is found in ECM, fuel pump, and/or ignition system, check for fouled spark plugs, EGR valve stuck in open position, and/or low fuel pressure. See CHART A6.

Chart A3 - Engine Cranks, But Will Not Run (2.5L). Scheme 113

This chart assumes that battery condition and engine cranking speed are okay, and there is enough fuel in tank. This chart should be used only on engines equipped with Model 220 throttle body unit.

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

1. "SERVICE ENGINE SOON" light on is basic check for ignition and battery supply to ECM. No "ALDL" may be due to an ECM problem. CHART A2 will diagnose the ECM. If TPS is over 2.5 volts, engine may be in "clear flood" mode, that will cause starting problems. If coolant sensor is below -22°F (-30°C), ECM will provide fuel for extremely cold temperatures that will completely flood engine.
2. Check voltage at spark plugs with ST-125 (J-26792). No spark indicates basic ignition problem.
3. While cranking engine, there should be no fuel spray with injector disconnected. Replace injector if it sprays fuel or drips excessively.

Chart A3 - Engine Cranks, But Won't Run (2.8L, 4.3L, 5.0L, 5.7L & 7.4L Engines). Scheme 114

Use this chart ONLY if CHART A3 indicated an injector circuit problem.

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

1. These tests will determine if the ignition module is generating a reference pulse, if wiring is at fault or if ECM is at fault. Touch and connect a 12-volt test light to circuit No. 430, a reference pulse should be generated. If injector test light blinks, ECM and wiring are okay.
2. This tests for 12 volts to injector. It will also determine if there is a short to voltage on ECM side of circuit.
3. This tests for continuity to ECM.

Chart A4, Injector Circuit Diagnosis (2.5L). Scheme 115

Use this chart ONLY if CHART A3 indicated an injector circuit problem. If both injector circuits fail to blink when tested, diagnose one injector circuit at a time.

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

1. These tests will determine if the ignition module is generating a reference pulse, if wiring is at fault or if ECM is at fault. Touch and connect a 12-volt test light to circuit No. 430, a reference pulse should be generated. If injector test light blinks, ECM and wiring are okay.
2. This tests for 12 volts to injector. It will also determine if there is a short to voltage on ECM side of circuit.
3. This tests for continuity to ECM.

Chart A4, Injector Circuit Diagnosis (All Other Models). Scheme 116

When ignition is turned on, ECM will activate the in-tank fuel pump. The pump will remain on as long as engine is cranking or running, and ECM is receiving distributor reference pulses. If there are no reference pulses, ECM will de-activate the fuel pump about 2 seconds after ignition is turned on, or when engine stops. The pump will deliver fuel to the TBI unit where the pressure regulator maintains system pressure at about 9-13 psi (.63-.91 kg/cm 2 ). Excess fuel is returned to fuel tank.

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

1. Fuel pump is activated if circuit No. 120 wiring is okay. If pump runs, problem may be located in fuel pump relay circuit. The following steps will locate this problem.
2. Next 2 steps check for power and ground circuits to fuel pump relay.
3. This determines if ECM can control the fuel pump relay.
4. Oil pressure switch serves as back-up for fuel pump relay to help prevent a "NO START CONDITION". If fuel pump relay was found to be inoperative, oil pressure switch circuit should also be tested to determine why it did not operate fuel pump.

Chart A5 - Fuel Pump Relay Circuit Diagnosis ("M" Series & "S/T" Series). Scheme 117

When ignition is turned on, ECM will activate the in-tank fuel pump. The pump will remain on as long as engine is cranking or running, and ECM is receiving distributor reference pulses. If there are no reference pulses, ECM will de-activate the fuel pump about 2 seconds after ignition is turned on, or when engine stops. The pump will deliver fuel to the TBI unit where the pressure regulator maintains system pressure at about 9-13 psi (.63-.91 kg/cm 2 ). Excess fuel is returned to fuel tank.

A fuel module is used on all 7.4L and some 5.7L engines to correct hot start (vapor lock) during high ambient temperature conditions. It is designed to override the ECM 2-second pump operation and run fuel pump for 20 seconds after initial ignition on condition.

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

1. Fuel pump is activated if circuit No. 120 wiring is okay. If pump runs, problem may be located in fuel pump relay circuit. The following steps will locate this problem.
2. Next 2 steps check for power and ground circuits to fuel pump relay.
3. This determines if ECM can control the fuel pump relay.
4. Oil pressure switch serves as back-up for fuel pump relay to help prevent a "NO START CONDITION". If fuel pump relay was found to be inoperative, oil pressure switch circuit should also be tested to determine why it did not operate fuel pump.

An inoperative fuel module may be cause of "HOT STALL/NO START" condition. Check for power and ground circuit to fuel module and a complete circuit to pump from terminal "A". If okay, and pump does not run for specified 20 seconds at initial ignition on, replace fuel module.

Chart A5, Fuel Pump Relay Ckt. ("C", "K", "G" & "P" Series). Scheme 118

When fuel pump is running, fuel is delivered to injector(s) and then to pressure regulator where system pressure is maintained at about 9-13 psi (.63-.91 kg/cm 2 ). Excess fuel is returned to fuel tank.

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

1. Pressure is present, but is less than 9 psi (.63 kg/cm 2 ), fall into 2 categories: Regulated Pressure, But Less Than 9 PSI (.63 kg/cm 2 ) Amount of fuel delivered to injector is adequate, but pressure is too low. System will run lean and may set Code Restricted Flow Causing Pressure Drop Normally, a vehicle with fuel pressure of less than 9 psi (.63 kg/cm 2 ) at idle will not be driveable. However, if the pressure drop occurs only while driving, 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 (.91-1.27 kg/cm 2 ).
3. This determines if high fuel pressure is due to restricted fuel return line or a faulty pressure regulator on throttle body unit.

If vehicle is equipped with fuel module, module must first be disconnected before performing fuel system pressure test. Fuel system is under pressure. Ensure pressure is relieved before testing or making repairs requiring disassembly of fuel lines or fittings.

Chart A6 - Fuel System Pressure Test Flow Chart. Scheme 119

Chart A6 - Fuel System Pressure Test. Scheme 120

Chart A6 - Fuel System Pressure Test Schematic. Scheme 121

ECM supplies voltage of about .45 volt between terminals "D7" and "E15". If measured with digital voltmeter with 10 megohm impedance, this may read as low as .32 volts. The oxygen sensor varies voltage within range of about one volt if exhaust gases are rich, down to about .10 volt if exhaust gases are lean. When sensor is below 600°F (315°C), no voltage signal is produced. 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 under the following conditions: Engine at normal operating temperature. At least 2 minutes of operation after engine starts. Oxygen sensor voltage signal steady between .35 and .55 volt. Throttle position sensor signal above 4%. All of these conditions must be sustained for about one minute. If conditions for Code 13 exist, system will not go into "Closed Loop" mode of operation.
2. This determines if sensor is at fault, or if wiring or ECM is cause of Code 13.
3. Use only a high impedance digital volt/ohmmeter. Test checks continuity of circuit No. 412 and circuit No. 413. If circuit No. 413 is open, ECM voltage on circuit No. 412 will be over .6 volt (600 millivolts).

Normal "SCAN" voltage varies between 100-999 millivolts (.10-l.0 volt) while in "Closed Loop". Code 13 sets in one minute if voltage remains between .35-.55 volt, but system will go "Open Loop" in about 15 seconds. See INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING Section.

Code 13, Oxygen (O2) Sensor Circuit ("SCAN" Diagnostics). Scheme 122

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 should measure about 1.5-2.0 volts.

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

1. Code 14 will set if signal voltage indicates coolant temperature above 266°F (130°C) for more than three seconds.
2. Tests if circuit No. 410 is shorted to ground. This will cause conditions for Code 14.

Check harness routing for potential short to ground in circuit No. 410.

"SCAN" tool displays engine temperature in degrees Centigrade. After engine is started, temperature should rise steadily to about 90°C, then stabilize when thermostat opens. See INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section.

The TEMPERATURE-TO-RESISTANCE VALUE chart may be used to test coolant sensor at various temperatures to evaluate possibility of "slewed" (mis-scaled) sensor. A "slewed" sensor could result in poor driveability complaints.

Code 14, CTS (High Temp Indicated) ("SCAN" Diagnostics). Scheme 123

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 should measure about 1.5-2.0 volts.

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

1. Code 15 will set if signal voltage indicates coolant temperature of less than -47°F (-44°C) than three seconds.
2. Test simulates Code 14. If ECM recognizes low signal voltage, (high temperature) and "SCAN" tool reads 266°F (130°C) or above, ECM and wiring are all okay.
3. Test will determine if circuit No. 410 is open. There should be 5 volts present at sensor connector if measured with DVOM.

"SCAN" tool displays engine temperature in degrees Centigrade. After engine is started, temperature should rise steadily to about 90°C, then stabilize when thermostat opens. A faulty connection, or open in circuit No. 410 or circuit No. 452 will result in Code 15. See INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section. The TEMPERATURE-TO-RESISTANCE VALUE chart may be used to test coolant sensor at various temperatures to evaluate possibility of "slewed" (mis-scaled) sensor. A "slewed" sensor could result in poor driveability complaints.

Code 15 - Coolant Temp Sensor (Low Temperature Indicated) ("SCAN" Diagnostics). Scheme 124

The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to throttle valve position. Signal voltage will vary from about .50 volts at idle to about 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.

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

1. Code 21 will set under the following conditions: Engine running. TPS signal voltage greater than about 3.5 volts. All conditions must be present for at least 5 seconds. OR TPS signal voltage over 4.5 volts with ignition on, engine off. With throttle closed, TPS should read less than .70 volt. If not, check adjustment (if applicable).
2. With TPS disconnected, TPS voltage should go low if ECM and wiring are okay.
3. Probing circuit No. 452 with a test light checks 5-volt signal return circuit. Faulty 5-volt return circuit will cause a Code 21.

"SCAN" tool reads throttle position in volts. Reading should be about .65-.81 volt for 2.5L, .42-.54 volt for 2.8L, and .52-.68 volts for all other engines with throttle closed and ignition on, or at idle. Voltage should increase at a steady rate as throttle is moved toward wide open throttle.

Some "SCAN" tools read percentage of throttle angle opening (0% equals closed throttle; 100% equals wide open throttle). See INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section. "SCAN" TPS while depressing accelerator pedal with engine stopped and ignition on. Display should vary from below 2.5 volts (2500 millivolts) when throttle was closed, to over 4.5 volts (4500 millivolts) when throttle is held at wide open throttle position.

Code 21, Throttle Position Sensor (Signal Voltage High) ("SCAN" Diagnostics). Scheme 125

The Throttle Position Sensor (TPS) provides a voltage signal that changes relative to throttle valve position. Signal voltage will vary from about .50 volts at idle to about 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.

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

1. Code 22 will set when engine is running and TPS signal voltage is less than about .2 volt for 3 seconds.
2. Simulates Code 21 (high voltage). If ECM recognizes high signal voltage, ECM and wiring are okay.
3. TPS adjustment (2.8L engine only). With throttle closed, TPS voltage reading should be .42-.54 volt.
4. This simulates high signal voltage to check for open in circuit No. 417.

"SCAN" tool reads throttle position in volts. Reading should be about .65-.81 volt for 3.5L, .42-.54 volt for 2.8L, and .52-.68 volts for all other engines with throttle closed and ignition on, or at idle. Voltage should increase at a steady rate as throttle is moved toward wide open throttle. An open or short to ground in circuit No. 416 or circuit No. 417 will result in Code 22. See INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section. "SCAN" TPS while depressing accelerator pedal with engine stopped and ignition on. Display should vary from below 2.5 volts (2500 millivolts) when throttle was closed, to over 4.5 volts (4500 millivolts) when throttle is held open throttle position.

Code 22 - Throttle Position Sensor (Signal Voltage Low) ("SCAN" Diagnostics). Scheme 126

The Manifold Air Temperature (MAT) sensor uses a thermistor to control signal voltage to ECM. The ECM applies 4-6 volts on circuit No. 472 to sensor. When air is cold, sensor resistance is high; there, ECM will see high voltage signal. If air is warm, sensor resistance is low; therefore, ECM will see low voltage.

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

1. Code 23 will be set if the following conditions are met: Signal voltage indicates manifold air temperature below -22°F (-30°C) for 12 seconds. Time since engine start is one minute or longer. No vehicle speed sensor reading (vehicle stationary).
2. A Code 23 will set due to an open sensor, wire, or connection. This test will determine if wiring and ECM are okay.
3. This determines if signal circuit No. 472 or the 5-volt return circuit (No. 452) is open.

"SCAN" tool reads temperature of incoming air. This reading should be closed to ambient air temperature when engine is cold, and rise as underhood air temperature increases.

Carefully check harness and connections for possible open circuit No. 472 or circuit No. 452.

See INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section.

The TEMPERATURE-TO-RESISTANCE VALUE chart may be used to test MAT sensor at various temperatures to evaluate possibility of "slewed" (mis-scaled) sensor. A "slewed" sensor could result in poor driveability complaints.

Code 23 - MAT Sensor (Low Temp Indicated) (2.5L) ("SCAN" Diagnostics). Scheme 127

The ECM supplies and monitors 12-volt signal on circuit No. 437. Circuit No. 437 connects Vehicle Speed Sensor (VSS), that alternately grounds circuit No. 437 when drive wheels are turning. This pulsing action takes place about 2000 times per mile, and the ECM calculates vehicle speed based on time between "pulses".

"SCAN" tool reading should closely match speedometer reading with drive wheels turning.

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

Code 24 will set if the following conditions exist for at least 3 seconds

1. Circuit No. 437 voltage is constant.
2. Engine speed is 900-4400 RPM.
3. Less than 2% throttle opening.
4. Low load condition (low airflow).
5. Transmission not in Park or Neutral.

The above conditions are met during a road load deceleration.

1. This monitors ECM voltage on circuit No. 437. With wheels turning, pulsing action will result in varying voltage. Variation will be greater at low wheel speeds to an average of 4-6 volts at about 20 MPH.
2. A voltage of less than one volt at ECM connector indicates that circuit No. 437 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 faulty VSS.
4. This is a normal voltage that indicates a possible intermittent condition.

"SCAN" tool reading should closely match speedometer reading with drive wheels turning.

See PARK/NEUTRAL SWITCH DIAGNOSIS chart if vehicle is equipped with automatic transmission.

If Park/Neutral switch is okay, see INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section.

Code 24 - Vehicle Speed Sensor Fault ("SCAN" Diagnostics). Scheme 128

The Manifold Air Temperature (MAT) sensor uses a thermistor to control signal voltage to ECM. The ECM applies 4-6 volts on circuit No. 472 to sensor. When air is cold, sensor resistance is high; therefore, ECM will see high voltage signal. If air is warm, sensor resistance is low; therefore, ECM will see low voltage.

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

1. Code 25 will be set if the following conditions are met: Signal voltage indicates manifold air temperature below 302°F (150°C) for 2 seconds. Time since engine start is one minute or longer. Vehicle speed sensor reading is present (vehicle moving).

"SCAN" tool reads temperature of incoming air. This reading should be closed ambient air temperature when engine is cold, and rise as underhood air temperature increases.

Carefully check harness and connections for possible short to ground in circuit No. 472.

See INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section.

The TEMPERATURE-TO-RESISTANCE VALUE chart may be used to test MAT sensor at various temperatures to evaluate possibility of "slewed" (mis-scaled) sensor. A "slewed" sensor could result in poor driveability complaints.

Code 25 - MAT Sensor (High Temperature Indicated) (2.5L) ("SCAN" Diagnostics). Scheme 129

The EGR valve is controlled by an ECM-operated solenoid. The solenoid 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 (on-off sequence) is calculated by ECM, based upon information from coolant temperature sensor, MAP sensor, and engine RPM. The duty cycle should be 0% (no EGR) when in Park or Neutral with TPS input below a specified value, or TPS indicating wide open throttle.

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

1. Code 32 means that EGR vacuum switch was closed during start-up or that vacuum switch was not detected closed under the following conditions: Coolant temperature above 185°F (85°C). EGR duty cycle commanded by ECM is greater than 70%. TPS less than half throttle, but not at idle. All above conditions must be present for 5 seconds. If vacuum switch is detected closed during start-up or, if switch is detected open when the above conditions are met, the "SERVICE ENGINE SOON" light will remain on unless the vacuum switch changes position.
2. If step 1) caused Code 32 to set, then the ECM has recognized a closed vacuum switch on start-up. This step will determine whether the EGR control valve is the cause, or if the wiring or ECM is the cause.
3. With ignition on, solenoid should not be energized and vacuum should not flow to EGR valve.
4. The EGR solenoid and valve are okay and the next step will check the vacuum switch portion of the system.
5. The switch should close at about 2 in. Hg. With vacuum applied, the switch should close and resistance should be near zero ohms and vacuum should hold.

With ignition on and engine stopped, EGR solenoid is de-energized unless diagnostic terminal is grounded.

Code 32 - EGR System Failure (2.8L). Scheme 130

The EGR valve is controlled by an ECM-operated solenoid. The solenoid is normally closed. The ECM provides a ground to energized the solenoid which allows vacuum to flow to the EGR valve.

The ECM monitor EGR effectiveness by de-energizing the EGR control solenoid, thereby shutting off vacuum to the EGR valve diaphragm. With the EGR valve closed, fuel integrator counts will be greater than they were during normal EGR operation. If the change is not within the "calibrated window", Code 32 will be set.

The ECM checks EGR operation when vehicle speed is over 50 MPH, engine vacuum is 11.8-51.1 in. Hg, and throttle position is constant (no change during check sequence).

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

1. Checks for solenoid being stuck in open position.
2. Checks for solenoid being energized all the time.
3. Grounding test terminal should energize solenoid and vacuum should drop.
4. Negative backpressure valve should hold vacuum when engine is not running.
5. When engine is started, exhaust backpressure should cause vacuum to bleed off and valve to fully close.

Before replacing ECM, use an ohmmeter to check the resistance of each ECM-controlled relay and solenoid coil. See the appropriate WIRING DIAGRAM for identification of coil terminal on solenoids and relays to be checked. Replace any solenoid where resistance is less than 20 ohms.

Code 32 - EGR System Failure (2.5/4.3/5.0/5.7L Under 8500 GVW). Scheme 131

The EGR valve is controlled by an ECM-operated solenoid. The solenoid is normally closed. The ECM provides a ground to energized the solenoid which allows vacuum to flow to the EGR valve.

The ECM monitor EGR effectiveness by de-energizing the EGR control solenoid, thereby shutting off vacuum to the EGR valve diaphragm. With the EGR valve closed, fuel integrator counts will be greater than they were during normal EGR operation. If the change is not within the "calibrated window", Code 32 will be set.

The ECM checks EGR operation when vehicle speed is over 50 MPH, engine vacuum is 11.8-51.1 in. Hg, and throttle position is constant (no change during check sequence).

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

1. With ignition on, engine off, solenoid should not be energized and vacuum should not flow to EGR valve. Grounding test terminal will energize the solenoid and vacuum should flow to EGR valve.
2. Checks for plugged EGR passages. If passages are plugged, engine may have severe detonation on acceleration.
3. The EGR solenoid will not be energized in Park or Neutral positions. This step will determine if the P/N switch input is being received by the ECM.

Before replacing ECM, use an ohmmeter to check the resistance of each ECM-controlled relay and solenoid coil. See the appropriate WIRING DIAGRAM for identification of coil terminal on solenoids and relays to be checked. Replace any solenoid where resistance is less than 20 ohms.

Code 32 - EGR System Failure (7.4L & 5.7L Over 8500 GVW). Scheme 132

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.

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

1. Code 33 will be set under the following conditions: When signal is too high for more than 6 seconds. Engine misfire or a low, unstable idle. Manifold pressure greater than 22.3 in. Hg with A/C off, or greater than 24 in. Hg with A/C on with engine running. Throttle angle less than 2% with engine running. The above 2 conditions must be present for 2 seconds before ECM will store Code 32.
2. With MAP sensor disconnected, the ECM should recognize low voltage if ECM and wiring are okay.

The ALTITUDE-TO-VOLTAGE chart may be used to test MAP sensor at various altitudes to evaluate possibility of "slewed" (mis-scaled) sensor. A "slewed" sensor could result in poor driveability complaints.

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, see TROUBLE SHOOTING section.

Code 33 - MAP Sensor (Signal Voltage High) ("SCAN" Diagnostics). Scheme 133

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.

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

1. Code 34 will set under the following conditions: Engine running at less than 600 RPM. Manifold pressure is less than 3.8 in. Hg. Above conditions must be present for at least one second. OR Engine running above 600 RPM. Throttle angle greater than 20%. Manifold pressure greater than 3.8 in. Hg. Above conditions must be present for at least one second.
2. Tests to see if sensor is cause of low voltage, or if it is an ECM fault, or wiring problem.
3. Simulates high signal voltage to check for an open in circuit No. 432. If test light is bright during test, circuit No. 432 is probably shorted to ground. If "SCAN" tool reads over 4 volts, circuit No. 416 can be checked for open or short to ground by measuring voltage at terminal "C" (should be 5 volts).

An intermittent open in circuit No. 416 will cause a Code 34.

See INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section.

The ALTITUDE-TO-VOLTAGE chart may be used to test MAP sensor at various altitudes to evaluate possibility of "slewed" (mis-scaled) sensor. A "slewed" sensor could result in poor driveability complaints.

Code 34 - MAP Sensor (Signal Voltage Low) ("SCAN" Diagnostics). Scheme 134

Code 35 will set when closed throttle engine speed is 100 RPM above or below correct idle speed for 45 seconds.

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

1. Continue with test even if engine will not idle. If idle is too low, "SCAN" will display 80 or more counts or steps. If idle is high, "SCAN" will display zero counts (steps). 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. "SCAN" will display 100 or more counts.
3. IAC MUST be disconnected before performing this step. Test light will confirm ECM signals by a steady or flashing light on all circuits.
4. There is a remote possibility that one of the circuits is shorted to voltage that would have been indicated by a steady light. Disconnect ECM and turn 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 IAC. The "SCAN" counts will be above 60 counts (steps) if too low, and zero counts (steps) if too high.

If idle is too high, stop engine. Turn ignition on. Ground diagnostic terminal. Wait a few seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is more than 750-850 RPM, locate and correct vacuum leak.

System Too Lean (High Air/Fuel Ratio)

Idle speed may be too high or too low. Engine speed may vary up and down. Disconnecting IAC does not help. May set Code 44.

"SCAN" and/or voltmeter will read an oxygen sensor output of less than .30 volt (300 millivolts). Check for low regulated fuel pressure, or water contaminated fuel. A lean exhaust with an oxygen sensor output fixed above .80 volt (800 millivolts) indicates a contaminated sensor, usually silicon. This may set a Code 45.

System Too Rich (Low Air/Fuel Ratio)

Idle speed too low. "SCAN" counts usually above 80. System obviously rich, and may exhibit black smoke exhaust.

"SCAN" and/or voltmeter will read an oxygen sensor signal fixed above .80 volt (800 millivolts). Check for high fuel pressure, and/or leaking or sticking injector.

Throttle Body

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

A/C Compressor Relay Failure

Diagnose A/C system if shorted to ground. If relay is faulty, an idle problem may exist. See ROUGH, UNSTABLE, INCORRECT IDLE, STALLING in TROUBLE SHOOTING section.

Code 35 - Idle Speed Error (2.5L). Scheme 135

When system is running on ignition module (no voltage on by-pass line), the ignition module grounds the EST signal line. The ECM expects to see no voltage on the EST signal line during this condition. If the ECM sees voltage on this line, Code 42 will be set and the system will not go into "EST" mode.

When engine RPM for EST is reached (about 400 RPM), and by-pass voltage is applied, EST should no longer be grounded in ignition module, so EST voltage should be varying.

If the by-pass line is open or grounded, the ignition module will not switch to "EST" mode. EST voltage will be low and Code 42 will be set.

If the EST line is grounded, the ignition module will switch to EST, but because the line is grounded, no EST signal will be present. Code 42 will be set.

Code 42 sets if there is an open, or a short to ground, in the EST or by-pass circuit.

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

1. Code 42 means the ECM has seen an open, or short to ground, in EST or by-pass circuit. This confirms Code 42, and confirms that fault causing the code is also 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 circuit No. 424, module should switch, causing ohmmeter to "overrange" if meter is in the 100-200 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 No. 423 or circuit No. 424.

"SCAN" tool does not have any ability to help diagnose Code 42 problem.

See INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section.

Code 42 - Electronic Spark Timing Flow Chart. Scheme 136

Code 42 - Electronic Spark Timing. Scheme 137

Code 42 - Electronic Spark Timing Schematic. Scheme 138

Electronic spark control is accomplished with the ESC module. The module sends a voltage signal to the ECM. As knock sensor detects engine knock, voltage from ESC module to ECM drops. This signals ECM to retard timing. ECM will retard timing when knock is detected, and RPM is above about 900 RPM.

Code 43 means the ECM has seen low voltage at circuit No. 485 (terminal "B7") for more than 5 seconds with engine running, or system has failed functional check.

This system performs a functional check once per start-up to check ESC system. To perform this test, ECM will advance spark when coolant temperature is above 203°F (95°C), and a high load condition is present (near wide open throttle). The ECM then checks signal at terminal "B7" to determine if knock is detected. If knock is detected when coolant temperature is below 203°F (95°C), the test has passed and the functional check will not be run. 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. Test numbers refer to test numbers on diagnostic chart.

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

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

See INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section.

Code 43 - Electronic Spark Control (2.8/4.3/5.0/5.7L Only) ("SCAN" Diagnostics). Scheme 139

The ECM supplies a voltage of about .45 volt between terminals "D6" and "D7" (If measured with a 10-megohm digital voltmeter, this may read as low as .32 volts). The O2 sensor varies voltage from about one volt (rich exhaust), to about .10 volt (lean exhaust).

The sensor is similar to an open circuit when its temperature is below about 600°F (315°C), and it produces no voltage. An open sensor circuit or cold sensor causes open loop operation.

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

1. Code 44 is set when the O2 sensor signal on circuit No. 412 remains below .2 volt for 20 seconds, and the system is operating in "Closed Loop".

Using "SCAN" tool, observe block learn values at different RPM and airflow conditions. "SCAN" also displays the block cells, so the block learn values can be checked in each of the cells to determine when Code 44 may have been set. If the conditions for Code 44 exist, the block learn values will be around 150.

O2 Sensor Wire

Sensor pigtail may be mispositioned and contacting exhaust manifold. Check for intermittent ground in wire between connector and sensor.

Fuel Contamination

Water, even in small amounts, near in-tank fuel pump inlet can be delivered to fuel injectors. Water causes a lean exhaust and can set Code 44.

Fuel Pressure

System will be lean if pressure is too low. It may be necessary to monitor fuel pressure while driving the vehicle at various road speeds and/or loads to confirm. See CHART A6.

Air System

Be sure air is not being directed to exhaust ports while in "Closed Loop". If block learn value goes down while squeezing air hose to left side of exhaust ports, service AIR management system. If okay, oxygen sensor is faulty.

Circuit No. 413

If circuit No. 413 is open, voltage at terminal "D7" will be over one volt.

Sensor Harness

Sensor pigtail may be mispositioned and contacting exhaust manifold.

Code 44 - Lean Exhaust Indication ("SCAN" Diagnostics). Scheme 140

The ECM supplies a voltage of about .45 volt between terminals "D7" (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 (rich exhaust), to about .10 volt (lean exhaust).

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

1. Code 45 is set when the O2 signal on circuit No. 412 remains above .7 volts for 50 seconds, the system is operating in "Closed Loop", time since engine start has been one minute or longer, and throttle angle is greater than 2% (about .2 volt above idle voltage).

Using "SCAN" tool, observe block learn values at different RPM and airflow conditions. "SCAN" also displays the block cells, so the block learn values can be checked in each of the cells to determine when Code 45 may have been set. If the conditions for Code 45 exist, the block learn values will be around 115.

System will go rich if pressure is too high. The ECM can compensate for some increase. However, if pressure is too high, Code 45 may be set. See Chart A6 for fuel pressure. See CHART A4 for leaking injector. Check for fuel contaminated oil.

HEI Shielding

An open ground circuit may result in interference, or induced electrical "noise". The ECM looks at this "noise" as a reference pulse. The additional reference pulse results in higher than actual engine speed signal. The ECM then delivers too much fuel, causing the system to go rich. Engine tachometer will also show higher than actual engine speed which can help in diagnosing this problem.

Canister Purge

Check for fuel saturation. If full of fuel, check canister control and hoses.

MAP Sensor

An output that causes the ECM to receive a higher than normal manifold pressure (low vacuum) can cause the system to go rich. Disconnecting the MAP sensor will allow the ECM to set a fixed value for the MAP reading. Substitute a different MAP sensor if the rich condition is gone while the MAP sensor is disconnected.

Pressure Regulator

Check for leaking fuel pressure regulator diaphragm by checking for presence of liquid fuel in vacuum line to regulator.

TPS

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

Code 45 - Rich Exhaust Indication ("SCAN" Diagnostics). Scheme 141

CODE 51 - FAULTY MEM-CAL (2.5L ENGINE ONLY)

Check that all pins are fully inserted into socket. If okay, replace MEM-CAL, clear memory and recheck. If Code 51 reappears, replace ECM.

CODE 51 - PROM PROBLEM (2.8/4.3/5.0/5.7/7.4L)

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

CODE 52 - FUEL CALPAK MISSING (2.8/4.3/5.0/5.7/7.4L)

Install missing or faulty CALPAK.

Status of fuel pump circuit No. 120 is monitored by the ECM at terminal "B2" and is used to compensate fuel delivery based on system voltage. This signal is also used to store a trouble code if the fuel pump relay is defective or fuel pump voltage is lost while the engine is running. There should be about 12 volts on circuit No. 120 for at least 2 seconds after the ignition is turned on, or any time reference pulses are being received by the ECM.

Code 54 will set if the voltage at terminal "B2" is less than 2 volts for 1.5 seconds, since the last reference pulse was received. This code is designed to detect a faulty relay, causing an extended crank time, and the code will help in the diagnosis of "ENGINE CRANKS BUT WILL NOT RUN" condition.

If a fault is detected during start-up, the "SERVICE ENGINE SOON" light will stay on until the ignition is cycled off.

Code 54 - Fuel Pump Circuit (Low Voltage) Flow Chart. Scheme 142

Disconnect Fuel Module If Equipped. On Two Fuel Tank System

Check Pump Operation In Each Tank

Code 54 - Fuel Pump Circuit (Low Voltage) (1 Of 2). Scheme 143

Code 54 - Fuel Pump Circuit (Low Voltage) (2 Of 2). Scheme 144

Code 54 - Fuel Pump Ckt (Low Voltage) Schematic ("M" & "S/T" Series). Scheme 145

Code 54 - Fuel Pump Ckt (Low Voltage) Schematic (All Others). Scheme 146

CODE 55 - ECM PROBLEM

Be sure ECM grounds are okay and that MEM-CAL, PROM, and/or CALPAK are properly latched. If okay, replace ECM. Clear codes, confirm "Closed Loop" operation, and no "SERVICE ENGINE SOON" light.

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

Note. Use this chart only after the normal diagnostic charts have determined that there is an ECM failure.

ECM Replacement Check Chart C-1. Scheme 147

Scheme 148

The P/N switch contacts are closed to ground in Park or Neutral, and open in all Drive ranges. The ECM supplies ignition voltage, through a current limiting resistor, to circuit No. 434. The ECM senses a closed switch when voltage on circuit No. 434 drops to less than one volt. The ECM uses P/N signal as one of the inputs to control idle air, VSS diagnostics, and EGR flow.

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

1. Checks for closed switch to ground in Park position. Different makes of "SCAN" tools will read P/N signal differently. Refer to tool's instruction manual for type of display used for specific Tool.
2. Checks for an open switch in Drive or Reverse.
3. Be sure "SCAN" tool indicates Drive, even while wiggling shifter, to test for an intermittent or misadjusted switch in Drive.

If circuit No. 434 indicates P/N switch grounded while in Drive, EGR would be inoperative, resulting in possible detonation.

If circuit No. 434 always indicates Drive (open), a drop in idle may exist when gear selector is moved into Drive.

Park/Neutral Switch Diagnosis ("SCAN" Diagnosis). Scheme 149

The crank signal is a 12-volt signal to the ECM during cranking to allow enrichment, and to cancel diagnostics until engine is running, or 12 volts on circuit is no longer present.

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

1. Checks for normal (cranking) voltage to terminal "C9" of ECM. Test light should be on during cranking and then go off when engine is running.
2. Checks to determine if source of open fuse or fuse link was a faulty ECM.

Crank Signal Diagnosis. Scheme 150

The power steering pressure switch is normally open to ground, and circuit No. 495 will be near battery voltage. Turning steering wheel increases power steering oil pressure, and its load on an idling engine. The pressure switch will close before the 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.

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

1. Checks for ECM signal voltage on circuit No. 495, and confirms ground circuit No. 450 is okay.
2. Maximum resistance, or infinity, indicates an open switch.
3. Less than one ohm indicates that switch is closed when power steering pressure is high. Switch is okay.

A pressure switch that will not close, or an open circuit No. 495 or circuit No. 450, may cause the engine to stop when power steering loads are high.

A switch that will not open, or a circuit No. 450 or circuit No. 495 shorted to ground, will cause timing to retard at idle, and may affect idle quality.

Power Steering Pressure Switch (2.5L). Scheme 151

The ECM control of the A/C clutch improves idle quality and performance by delaying clutch application until idle air rate is increased, releasing clutch when idle speed is too low, releasing clutch at wide open throttle, and smoothing cycling of compressor by providing additional fuel at instant clutch is applied.

Turning on A/C supplies circuit No. 459 battery voltage to the clutch control relay and terminal "B8" of ECM connector. After a time delay of about .5 second, the ECM will ground terminal "A4" (circuit No. 458), and close the control relay. A/C compressor clutch will engage.

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

1. Checks for low refrigerant as cause for no A/C.
2. This and following tests check for faulty A/C control relay.
3. Checks for faulty cycling switch. Solenoids and relays are turned on and off by the ECM, using internal electronic switches (drivers). Each driver is part of a group of 4, called "Quad-Drivers". Failure of one driver can damage any other driver in set. Solenoid and relay coil resistance must measure more than 20 ohms. Less resistance will cause early failure of ECM driver. Using an ohmmeter, check coil resistance of A/C relay before replacing ECM.

Before replacing ECM, use ohmmeter to check resistance of each ECM-controlled relay or solenoid. Replace any relay or solenoid that measures less than 20 ohms.

A/C Clutch Control Diagnosis Schematic (2.5L). Scheme 152

A/C Clutch Control Diagnosis Flow Chart (1 of 2) (2.5L). Scheme 153

A/C Clutch Control Diagnosis Flow Chart (2.5L) (1 of 2). Scheme 154

A/C Clutch Control Diagnosis Flow Chart (2 of 2) (2.5L). Scheme 155

From A/C Clutch Control Diagnosis Chart (1 Of 2). Scheme 156

The ECM control of the A/C clutch improves idle quality and performance by delaying clutch application until idle air rate is increased, releasing clutch when idle speed is too low, releasing clutch at wide open throttle, and smoothing cycling of compressor by providing additional fuel at instant clutch is applied.

Turning on A/C supplies circuit No. 59 battery voltage to the clutch control relay and terminal "B8" of ECM connector. After a time delay of about .5 second, the ECM will ground terminal "A2" (circuit No. 459), and close the control relay. A/C compressor clutch will engage.

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

1. Checks for low refrigerant as cause for no A/C.
2. This and following tests check for faulty A/C control relay.
3. Checks for faulty cycling switch. Solenoids and relays are turned on and off by the ECM, using internal electronic switches (drivers). Each driver is part of a group of 4, called "Quad-Drivers". Failure of one driver can damage any other driver in set. Solenoid and relay coil resistance must measure more than 20 ohms. Less resistance will cause early failure of ECM driver. Using an ohmmeter, check coil resistance of A/C relay before replacing ECM.

Before replacing ECM, use ohmmeter to check resistance of each ECM-controlled relay or solenoid. Replace any relay or solenoid that measures less than 20 ohms.

A/C Clutch Control Diagnosis Schematic (2.8L). Scheme 157

A/C Clutch Control Diagnosis Flow Chart (1 of 2) (2.8L). Scheme 158

A/C Clutch Control Diagnosis Flow Chart (1 of 2) (2.8L). Scheme 159

A/C Clutch Control Diagnosis Flow Chart (2 of 2) (2.8L). Scheme 160

From A/C Clutch Control Diagnosis Chart (1 Of 2). Scheme 161

Circuit Test Description

Turning on A/C supplies battery voltage to circuit No. 59 A/C compressor clutch, and to terminal "B8" of ECM connector to increase idle air rate and maintain idle speed.

The ECM does not control the A/C compressor clutch; therefore, if A/C does not function, service A/C system.

If A/C is operating properly and idle speed dips too low when A/C compressor turns on, or flares too high when A/C compressor turns off, check for an open circuit No. 59 to ECM. If circuit is okay, it is a faulty ECM connector terminal "B8" or ECM.

A/C "On" Signal Diagnosis (4.3/5.0/5.7/7.4L). Scheme 162

The ECM sends voltage pulses to proper IAC motor winding that causes the motor shaft and valve to move "in" and "out" a given distance for each pulse (called counts) received. This movement controls airflow around the throttle plate, that in turn controls engine idle speed.

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

1. Continue with test even if engine will not idle. If idle is too low, "SCAN" will display 80 or more counts (steps). 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. "SCAN" will display 100 or more counts.
3. IAC MUST be disconnected before performing this step. Test light will confirm ECM signals by a steady or flashing light on all circuits.
4. There is a remote possibility that one of the circuits is shorted to voltage that would have been indicated by a steady light. Disconnect ECM and turn 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. "SCAN" counts will be above 60 counts (steps) if too low, and zero counts (steps) if too high.

If idle is too high, stop engine. Turn ignition on. Ground diagnostic terminal. Wait a few seconds for IAC to seat, then disconnect IAC. Start engine. If idle speed is more than 750-850 RPM, locate and correct vacuum leak.

Idle speed may be too high or too low. Engine speed may vary up and down. Disconnecting IAC does not help. May set Code 44.

"SCAN" and/or voltmeter will read an oxygen sensor output of less than .30 volt (300 millivolts). Check for low regulated fuel pressure, or water contaminated fuel. A lean exhaust with an oxygen sensor output fixed above .80 volt (800 millivolts) indicates a contaminated sensor, usually silicon. This may set a Code 45.

Idle speed too low. "SCAN" counts usually above 80. System obviously rich, and may exhibit Black smoke exhaust.

"SCAN" and/or voltmeter will read an oxygen sensor signal fixed above .80 volt (800 millivolts). Check for high fuel pressure, and/or leaking or sticking injector.

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

A/C Compressor Or Relay Failure

Diagnose A/C system if shorted to ground. If relay is faulty, an idle problem may exist. See ROUGH, UNSTABLE, INCORRECT IDLE, STALLING in TROUBLE SHOOTING section.

Idle Speed Error Flow Chart (2.8L, 4.3L, 5.0L, 5.7L & 7.4L). Scheme 163

"Scan" Tool Must Be In Open Mode During This Check. Scheme 164

Idle Speed Error Schematic (2.8L, 4.3L, 5.0L, 5.7L & 7.4L). Scheme 165

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

1. 1) Two wires are checked to ensure that an open is not present in a spark plug wire.
2. 1a) If spark occurs with 4-terminal distributor connector disconnected, pick-up coil output is too low for EST operation.
3. 2) A spark indicates the problem must be distributor cap or rotor.
4. 3) Normally, there should be battery voltage at "C" and "+" terminals. Low voltage would indicate an open or a high resistance circuit from distributor to coil or ignition switch. If "C" terminal voltage was low, but "+" terminal voltage is 10 volts or more, circuit from "C" terminal to ignition coil, or ignition coil primary winding, is open.
5. 4) Checks for a shorted module or grounded circuit from ignition coil to module. Distributor module should be turned "Off", so normal voltage should be about 12 volts. If module is turned "ON", voltage would be low, but above one volt. This could cause ignition coil failure from excessive heat. With open ignition coil primary winding, small amount of voltage will leak through module from "Bat" terminal to "Tach" terminal.
6. 5) Applying 1.5-8 volts to module terminal "P" should turn module "ON" and "Tach" terminal voltage should drop to about 7-9 volts. This test will determine whether the module or coil is faulty, or if pick-up coil is not generating proper signal to turn module "ON". This test can be performed by using a DC battery with a rating of 1.5-8 volts. The use of the test light is mainly to allow the "P" terminal to be probed more easily. Some digital multimeters can also be used to trigger the module by selecting ohms, usually the diode position. In this position, the meter may have a voltage across its terminals which can be used to trigger the module. The voltage in the ohms position can be checked by using a second meter, or by checking the manufacturer's specification of the tool being used.
7. 6) This should turn module "OFF" and cause a spark. If no spark occurs, fault is most likely in ignition coil because most module problems would have been found before this point in the procedure. A Module Tester (J-24642) could determine which is at fault.

"SCAN" tool does not have any ability to help diagnose ignition system.

See INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section.

Ign. System Check (Remote Coil) Flow Chart (2.5L "S" Series). Scheme 166

Ignition System Check (Remote Coil) (2.5L "S" Series) (1 Of 2). Scheme 167

Ignition System Check (Remote Coil) (2.5L "S" Series) (2 Of 2). Scheme 168

Ign. System Check (Remote Coil) Schematic (2.5L "S" Series). Scheme 169

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

1. 1) Two wires are checked to ensure that an open is not present in a spark plug wire.
2. 1a) If spark occurs with 4-terminal distributor connector disconnected, pick-up coil output is too low for EST operation.
3. 2) A spark indicates the problem must be distributor cap or rotor.
4. 3) Normally, there should be battery voltage at "C" and "+" terminals. Low voltage would indicate an open or a high resistance circuit from distributor to coil or ignition switch. If "C" terminal voltage was low, but "+" terminal voltage is 10 volts or more, circuit from "C" terminal to ignition coil, or ignition coil primary winding, is open.
5. 4) Checks for shorted module or grounded circuit from ignition coil to module. Distributor module should be turned "OFF", so normal voltage should be about 12 volts. If module is turned "ON", voltage would be low, but above one volt. This could cause ignition coil failure from excessive heat. With open ignition coil primary winding, small amount of voltage will leak through module from "Bat" terminal to "Tach" terminal.
6. 5) Checks for open module, or circuit to module. Applying 12 volts to "P" terminal or module should turn module "ON" and voltage should drop to about 7-9 volts.
7. 6) This should turn module "OFF" and cause a spark. If no spark occurs, fault is most likely in ignition coil because most module problems would have been found before this point in the procedure. A Module Tester (J-24642) could determine which is at fault.

"SCAN" tool does not have any ability to help diagnose ignition system.

See INTERMITTENT TROUBLE CODES in TROUBLE SHOOTING section.

Ignition System Check Flow Chart (Remote Coil/Sealed Connector Distributor) (Except 2.5L "S" Series). Scheme 170

Ignition System Check (Remote Coil/Sealed Connector Distributor) (Exc. 2.5L "S" Series) (1 Of 2). Scheme 171

Ignition System Check (Remote Coil/Sealed Connector Distributor) (Exc. 2.5L "S" Series) (2 Of 2). Scheme 172

Ignition System Check Schematic (Remote Coil/Sealed Connector Distributor) (Except 2.5L "S" Series). Scheme 173

Electronic spark control is accomplished with a module that sends a voltage signal to the ECM. As the knock sensor detects engine knock, voltage from ESC module to ECM is shut off. This signals the ECM to retard timing, if engine RPM is over about 900 RPM.

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

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

"SCAN" tools have two positions to diagnose the ESC system. The knock signal can be monitored to see if the knock sensor is detecting a knock condition, and if the ESC module is functioning. Knock signal should display "YES" on "SCAN" tool whenever detonation is present. The knock retard position on the "SCAN" displays the amount of spark retard the ECM is commanding. The ECM can retard timing up to 20 degrees.

If the ESC system checks okay, but detonation is the complaint, see DETONATION/SPARK KNOCK in TROUBLE SHOOTING section.

This check should be used after other causes of spark knock have been checked; such as, engine timing, EGR systems, engine temperature, or excessive engine noise.

Electronic Spark Control Check (Except 2.5L & 7.4L). Scheme 174

An electronic air control (EAC) valve solenoid directs air into the exhaust ports, or routes air to the air cleaner. During cold start on all engines except 2.8L Federal engine. The ECM completes the ground circuit. This energizes the EAC valve solenoid to direct air to the exhaust ports. As coolant temperature increases, or system goes into "Closed. Loop" mode, the ECM opens the ground circuit. This de-energizes the EAC valve solenoid to direct air to the air cleaner.

On 2.8L Federal engines, the ECM completes the ground circuit, energizing the EAC valve solenoid to direct air to the exhaust ports whenever the engine is started. The only time air is directed to the air cleaner is during deceleration. If the system is not operating properly, check manifold vacuum signal (10 in. Hg) at the valve and check the electrical circuit from the solenoid to the ECM.

Note. Test does not apply to 2.8L Federal engines. See CIRCUIT DESCRIPTION for operation of this system.

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

1. This is a system performance test. When vehicle goes to "Closed Loop", air will switch from the exhaust ports to divert air to the air cleaner.
2. Tests for a grounded electric divert circuit. Normal system light will be off.
3. Checks for an open control circuit. Grounding diagnostic terminal will energize the solenoid. If the ECM and circuits are okay. In this step, if test light is on, circuits are okay and fault is in valve connections or valve.
4. Checks for voltage from battery through a fuse to the solenoid.

AIR Management Check Flow Chart (Elect. Air Control Valve). Scheme 175

AIR Management Check. Scheme 176

AIR Management Check Schematic (Elect. Air Control Valve). Scheme 177

The ECM operates a solenoid to control EGR valve. This solenoid is normally closed. By providing the ground path, the ECM energizes the solenoid to allow vacuum to flow to the EGR valve. The ECM control of EGR is based upon the following inputs: engine coolant temperature above 77°F (25°C), throttle position off idle, and MAP sensor reading.

If Code 24 is present, use that chart first.

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

1. Checks for solenoid being stuck in open position.
2. Checks for solenoid always being energized.
3. Grounding test terminal should energize the solenoid, causing vacuum to drop.
4. Negative backpressure valve should hold vacuum with engine off.
5. When engine is started, exhaust backpressure should cause vacuum to bleed off, and valve to fully close.

Before replacing ECM, use an ohmmeter to check resistance of each ECM-controlled relay and solenoid coil. Replace any solenoid if resistance measures less than 20 ohms.

EGR System Check Flow Chart (2.5/4.3/5.0/5.7L Under 8500 GVW). Scheme 178

EGR System Check (2.5/4.3/5.0/5.7L Under 8500 GVW). Scheme 179

EGR System Check Schematic (2.5/4.3/5.0/5.7L Under 8500 GVW). Scheme 180

The EGR valve is controlled by a normally closed solenoid (allows vacuum to flow when energized). The ECM pulses the solenoid to turn on and regulate EGR. On 2.8L engines, the ECM monitors vacuum to the EGR valve with a vacuum switch. On 5.7L and 7.4L engines, the ECM diagnoses the system using an internal EGR test procedure.

Code 32 will detect a faulty solenoid, vacuum switch (2.8L engine), vacuum supply, or EGR valve (5.7L and 7.4L engines). This chart checks for plugged EGR passages, sticking EGR valve, or a solenoid that is inoperative or stuck open.

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

1. With ignition on (engine off), the solenoid should not be energized and vacuum should not flow to EGR valve.
2. Grounding the diagnostic terminal will energize the solenoid and allow vacuum to flow to the EGR valve.
3. Checks for plugged EGR passages. If passages are plugged, the engine may have severe detonation upon acceleration.
4. The EGR solenoid will not be energized in Park or Neutral. This test will determine if the P/N switch input is being received by the ECM.

Before replacing ECM, use an ohmmeter to check resistance of each ECM-controlled relay and solenoid coil. Replace any solenoid if resistance measures less than 20 ohms.

EGR System Check (2.8L, 7.4L & 5.7L Over 8500 GVW). Scheme 181

The purpose of TCC is to eliminate the power loss of torque converter stage when vehicle is in "cruise" condition. This allows the use of an automatic transmission with the fuel economy of a manual transmission. Fused battery ignition power is supplied to TCC solenoid through the TCC brake switch. The ECM will engage TCC by grounding circuit No. 422 to energize the TCC solenoid.

Engagement of torque converter clutch will be executed under the following conditions

1. Vehicle speed above 24 MPH.
2. Engine temperature above 149°F (65°C).
3. Steady TPS reading (not changing-steady road speed).
4. Brake switch closed.
5. Transmission in 3rd or 4th gear.

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

1. Check continuity through the brake switch and TCC solenoid.
2. Check ability of ECM to energize the TCC solenoid. Grounding the diagnostic connector should energize the relay and cause the light to go out.
3. This test by-passes the TCC solenoid and checks for an open or short in circuit No. 422.

Solenoid coil resistance must measure more than 20 ohms. Less resistance ill cause early failure of the ECM "DRIVER". Using an ohmmeter, check solenoid coil resistance of all ECM-controlled solenoids and relays before replacing ECM. Replace any solenoid or relay that measures less than 20 ohms resistance.

Torque Converter Clutch Electrical Diagnosis (2.5L & 2.8L). Scheme 182

The purpose of TCC is to eliminate the power loss of torque converter stage when vehicle is in "cruise" condition. This allows the use of an automatic transmission with the fuel economy of a manual transmission.

Fused battery ignition power is supplied to TCC solenoid through the TCC brake switch.

The ECC will engage TCC by grounding circuit No. 422 to energize the TCC solenoid.

Engagement of torque converter clutch will be executed under the following conditions

1. Vehicle speed above 30 MPH.
2. Engine temperature above 149°F (65°C).
3. Steady TPS reading (not changing-steady road speed).
4. Brake switch closed.
5. Transmission in 3rd or 4th gear.

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

1. Glowing test light indicates that battery voltage and continuity through TCC Solenoid are okay.
2. Checks for vehicle speed sensor (VSS) signal to ECM, using a "SCAN" tool.
3. Checks for 4th gear signal to ECM. This signal will not prevent TCC engagement, but could cause a change in engage/disengage speed points.

Solenoid coil resistance must measure more than 20 ohms. Less resistance will cause early failure of the ECM "DRIVER". Using an ohmmeter, check solenoid coil resistance of all ECM-controlled solenoids and relays before replacing ECM. Replace any solenoid or relay that measures less than 20 ohms resistance.

TCC Electrical Diagnosis (4.3/5.0/5.7L Under 8500 GVW). Scheme 183

When accelerator pedal is fully depressed, manifold vacuum drops, causing MAP sensor signal voltage to increase to about 4 volts. The ECM responds by grounding circuit No. 422 to energize downshift control relay. The downshift control relay then sends battery voltage to detent solenoid that causes a forced transmission downshift.

If problem is diagnosed as being an internal transmission problem, perform transmission service.

Relay coil resistance must measure more than 20 ohms. Less resistance will cause early failure of the ECM "DRIVER". Using an ohmmeter, check solenoid coil resistance of all ECM-controlled solenoids and relays before replacing ECM. Replace any solenoid or relay that measures less than 20 ohms resistance.

THM 400 Downshift Control Electrical Diagnosis Flow Chart. Scheme 184

THM 400 Downshift Control Electrical Diagnosis. Scheme 185

THM 400 Downshift Control Electrical Diagnosis Schematic. Scheme 186

The ECM uses information from the following inputs to control the shift light; coolant temperature, throttle position, vehicle speed, and engine RPM. The ECM uses the measured RPM and vehicle speed to calculate what gear the vehicle is in. This calculation determines when shift light is energized.

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

1. This should not turn the shift light on. If light is on, there is a short to ground in circuit No. 456 wiring, or a fault in the ECM.
2. This should turn shift light on.
3. This check for an open in the shift light circuit, or a faulty ECM.

Manual Transmission Shift Light Check Flow Chart. Scheme 187

Manual Transmission Shift Light Check. Scheme 188

Manual Transmission Shift Light Check Schematic. Scheme 189