Ccc Tbi Tests W/codes: Diagnosis Chevrolet RV Cutaway G30

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. Perform appropriate DIAGNOSTIC CIRCUIT CHECK for that system. If trouble codes were displayed (other than Code 12), decide whether codes are "hard" or "intermittent" trouble codes. "Hard" codes will cause the "SERVICE ENGINE SOON" light to illuminate continuously while engine is running. See ECM TROUBLE CODE DEFINITION in this article.
3. If no trouble codes were displayed, proceed to appropriate SYSTEM PERFORMANCE CHECK chart for carbureted systems or perform FIELD SERVICE MODE CHECK on fuel injected systems.
4. If no trouble is indicated by SYSTEM PERFORMANCE CHECK or FIELD SERVICE MODE check and/or a driveability problem exists, refer to SYMPTOM DIAGNOSIS and/or SCAN TESTER USAGE TROUBLESHOOTING. The comments there will send you to the proper component charts or tell you the most likely system/component to check.
5. After any repairs have been made, clear any trouble codes and perform SYSTEM PERFORMANCE CHECK or FIELD SERVICE MODE check.

Scheme 72

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 van). Insert jumper wire across terminal "B", "DIAGNOSTIC TERMINAL" and terminal "A", "GROUND". (Scheme 72) CAUTION: Inserting spade lug (jumper lead) into terminals of ALDL connector grounds "DIAGNOSTIC TERMINAL". Do not ground ALDL connector until after ignition is on (engine not running). (Scheme 72): ALDL Connector Terminal Locations NOTE: In some of the diagnostic and trouble shooting charts, the Assembly Line Data Link (ALDL) may also be referred to as the Assembly Line Communication Link (ALCL). These are referring to the same connector. It is also the test point for connection of aftermarket "Scan" testers.
2. "SERVICE ENGINE SOON" light should flash Code "12". Code "12" consists of "FLASH", pause, "FLASH", "FLASH" followed by a longer pause. Trouble Code "12" will be repeated 2 times. If any trouble codes are stored in ECM memory, they will be displayed in the same manner.
3. To exit diagnostic mode, turn ignition off and remove jumper wire from ALDL connector.

CLEARING TROUBLE CODES

Trouble codes are cleared by removing battery voltage from the ECM for at least 30 seconds. To do so, be sure ignition switch is "OFF" and remove battery positive terminal, or remove ECM fuse from fuse block on all vehicles except "C/K" Series, which has a fuse link.

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 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.

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 testers 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 testers can save a great deal of time, but are not required. Refer to tester manual for operating procedures. Also see "SCAN" charts in this section.

System Check Flow Chart. Scheme 73

System Check Flow Chart. Scheme 74

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.

Diagnostic Aids

If engine runs okay, check

1. Faulty light bulb.
2. Open in circuit No. 419.
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 75

There should always be a steady "SERVICE ENGINE SOON" 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 the ECM.

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.

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.

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 Cranks, But Will Not Run (2.5L). Scheme 76

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.

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

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.

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.

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.

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 ("C/K", "G" & "P" Series) Schematic. Scheme 77

Chart A5, Fuel Pump Relay ("C/K", "G" & "P" Series) Flow Chart. Scheme 78

From Chart A-3 (1 Of 2). Scheme 79

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.

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 Schematic. Scheme 80

Chart A6, Fuel System Pressure Test Flow Chart. Scheme 81

Chart A6, Fuel System Pressure Test. Scheme 82

ECM supplies voltage of about .45 volt between terminals "D7" and "D6". If measured with digital voltmeter with 10 megohms digital voltmeter, 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.

Normal "SCAN" voltage varies between 100-999 millivolts (.10-1.0 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 the CCC TESTS W/O CODES article in this section.

Code 13, Oxygen Sensor Circuit Flow Chart & Schematic. Scheme 83

Code 13, Oxygen Sensor. Scheme 84

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.

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

"SCAN" tester 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 the CCC TESTS W/O CODES article in this 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, Coolant Temperature Sensor (High Temperature Indicated) ("SCAN" Diagnostics) Flow Chart & Schematic. Scheme 85

Code 14, Coolant Temperature Sensor (High Temperature Indicated). Scheme 86

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.

"SCAN" tester 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 the CCC TESTS W/O CODES article in this 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 Sensor Circuit Signal Voltage High Flow Chart & Schematic. Scheme 87

Code 15, Coolant Sensor Circuit Signal Voltage High. Scheme 88

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.

"SCAN" tester 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" testers read percentage of throttle angle opening (0% equals closed throttle; 100% equals wide open throttle).

See INTERMITTENT TROUBLE CODES in the CCC TESTS W/O CODES article in this 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, TPS Signal Voltage High Flow Chart & Schematic. Scheme 89

Code 21, TPS Signal Voltage High. Scheme 90

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.

"SCAN" tester 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 the CCC TESTS W/O CODES article in this 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, TPS Signal Voltage Low Flow Chart & Schematic. Scheme 91

Code 22, TPS Signal Voltage Low. Scheme 92

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.

"SCAN" tester 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 the CCC TESTS W/O CODES article in this 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 Voltage High (2.5L) Flow Chart & Schematic. Scheme 93

Code 23, MAT Sensor Voltage High (2.5L). Scheme 94

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

"SCAN" tester 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 the CCC TESTS W/O CODES article in this section.

Code 24, Vehicle Speed Sensor (VSS) Flow Chart & Schematic. Scheme 95

Code 24, Vehicle Speed Sensor (VSS). Scheme 96

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.

"SCAN" tester 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 the CCC TESTS W/O CODES article in this 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 drive- ability complaints.

Code 25, MAT Sensor Voltage Low (2.5L Engine) Flow Chart & Schematic. Scheme 97

Code 25, MAT Sensor Voltage Low (2.5L Engine). Scheme 98

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).

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 (2.5L, 4.3L (Except "S/T"), 5.0L & 5.7L Under 8500 GVW) Flow Chart & Schematic. Scheme 99

Code 32, EGR System Failure (2.5L, 4.3L (Except "S/T"), 5.0L & 5.7L Under 8500 GVW). Scheme 100

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).

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 (2.8L, 4.3L "S/T" Series, 5.7L & 7.4L Over 8500 GVW) Schematic. Scheme 101

Code 32, EGR System Failure (2.8L, 4.3L "S/T" Series, 5.7L & 7.4L Over 8500 GVW) Flow Chart. Scheme 102

Note. Before using this chart, check vacuum source to EGR solenoid. Also check hoses for leaks or restrictions, should be at least 7" Hg vacuum at 2000 RPM.

Code 32, EGR System Failure (2.8L, 4.3L "S/T" Series, 5.7L & 7.4L Over 8500 GVW). Scheme 103

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.

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 the CCC TESTS W/O CODES article in this section.

Code 33, MAP Sensor (Signal Voltage High). Scheme 104

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.

An intermittent open in circuit No. 416 will cause a Code 34. See INTERMITTENT TROUBLE CODES in the CCC TESTS W/O CODES article in this 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). Scheme 105

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

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.

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.

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

See INTERMITTENT TROUBLE CODES in the CCC TESTS W/O CODES article in this section.

Code 42, Electronic Spark Timing Schematic. Scheme 106

Code 42, Electronic Spark Timing Flow Chart. Scheme 107

Code 42, Electronic Spark Timing. Scheme 108

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.

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 the CCC TESTS W/O CODES article in this section.

Code 43, Electronic Spark Control (2.8L, 4.3L, 5.0L & 5.7L Engines) Flow Chart & Schematic. Scheme 109

Code 43, Electronic Spark Control (2.8L, 4.3L, 5.0L & 5.7L Engines). Scheme 110

The ECM supplies a voltage of about .45 volt between terminals "D6" and "D7" (If measured with a 10-megohms 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.

Using "SCAN" tester, 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.

The ECM supplies a voltage of about .45 volt between terminals "D7" (if measured with a 10 megohms 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).

The sensor is similar to an open circuit when its temperature is below about 600°F (215°C), and it produces no voltage. An open sensor circuit or cold sensor causes "Open Loop" condition.

Using "SCAN" tester, 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.

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 Flow Chart (Low Voltage). Scheme 111

Disconnect Fuel Module If Equipped. On Two Fuel Tank System

Check Pump Operation In Each Tank

Code 54, Fuel Pump Circuit (1 Of 2). Scheme 112

Code 54, Fuel Pump Circuit (1 Of 2). Scheme 113

Code 54, Fuel Pump Schematic (Astro, Safari & "S/T" Series). Scheme 114

Code 54, Fuel Pump Schematic (All Others). Scheme 115

The MAP sensor measures manifold pressure (vacuum) and sends that signal to the ECM. The MAP sensor is mainly used for fuel calculation, when ECM is running in the throttle body back-up mode.

The MAP sensor is also used to determine barometric pressure and to help calculate fuel delivery.

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.

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 - Flow Chart & Schematic. Scheme 116

Park/Neutral Switch Diagnosis. Scheme 117

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.

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.

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) - Flow Chart & Schematic. Scheme 118

Power Steering Pressure Switch (2.5L). Scheme 119

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.

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.

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 Flow Chart (2.5L) (3 Of 3). Scheme 120

From A/C Clutch Control Diagnosis Chart (1 Of 2). Scheme 121

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.

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.

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 Flow Chart (2.8L) (3 Of 3). Scheme 122

From A/C Clutch Control Diagnosis Chart (1 Of 2). Scheme 123

ECM QDR (Quad-Driver) Check. Scheme 124

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.

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.

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

See INTERMITTENT TROUBLE CODES in the CCC TESTS W/O CODES article in this section.

Ignition System Check (Remote Coil) Flow Chart (2.5L "S/T" Series). Scheme 125

Ignition System Check (Remote Coil) Flow Chart (2.5L "S/T" Series) (1 Of 2). Scheme 126

Ignition System Check (Remote Coil) Flow Chart (2.5L "S/T" Series) (2 Of 2). Scheme 127

Ignition System Check (Remote Coil) Schematic (2.5L "S/T" Series). Scheme 128

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

See INTERMITTENT TROUBLE CODES in the CCC TESTS W/O CODES article in this section.

Ignition System Check Flow Chart (Remote Coil/Sealed Connector Distributor) (Except 2.5L "S/T" Series). Scheme 129

Ignition System Check Flow Chart (Remote Coil/Sealed Connector Distributor) (Except 2.5L "S/T" Series)(1 Of 2). Scheme 130

Ignition System Check Flow Chart (Remote Coil/Sealed Connector Distributor) (Except 2.5L "S/T" Series)(2 Of 2). Scheme 131

Ignition System Check Circuit Diagram (Remote Coil/Sealed Connector Distributor) (Except 2.5L "S/T" Series). Scheme 132

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.

"SCAN" testers 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" tester 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 the CCC TESTS W/O CODES article in this 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 Engines) - Flow Chart & Schematic. Scheme 133

Electronic Spark Control Check (Except 2.5L & 7.4L Engines). Scheme 134

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.

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.

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 reading.

If Code 24 is present, use that chart first.

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 (2.5L, 4.3L, 5.0L & 5.7L Under 8500 GVW). Scheme 135

EGR System Check Flow Chart (2.5L, 4.3L, 5.0L & 5.7L Under 8500 GVW. Scheme 136

EGR System Check Schematic (2.5L, 4.3L, 5.0L & 5.7L Under 8500 GVW). Scheme 137

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. The ECM diagnoses the system using an internal EGR test procedure. The ECM control of EGR is based upon the following outputs: engine coolant temperature greater than 77°F (25°C), throttle position off idle, and MAP reading.

If Code 24 is present, use that chart first. Code 32 will detect a faulty solenoid, vacuum supply, EGR valve or plugged passage. This chart checks for plugged EGR passages, a sticking EGR valve, or a stuck open or inoperative solenoid.

Before replacing ECM, use an ohmmeter to check 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 if resistance measures less than 20 ohms.

EGR System Check Flow Chart (2.8/4.3L "S/T" & 5.7/7.4L Over 8500 GVW). Scheme 138

EGR System Check Flow Chart (2.8/4.3L "S/T" & 5.7/7.4L Over 8500 GVW). Scheme 139

EGR System Check Schematic (2.8/4.3L "S/T" & 5.7/7.4L Over 8500 GVW). Scheme 140

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.

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.

TCC Electrical Diagnosis Flow Chart (2.5/2.8L Engines). Scheme 141

TCC Electrical Diagnosis Flow Chart (2.5/2.8L Engines). Scheme 142

TCC Electrical Diagnosis Schematic (2.5L/2.8L Engines). Scheme 143

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.

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 Elect. Diagnosis Flow Chart (4.3/5.0/5.7L Under 8500 GVW). Scheme 144

Use A "Scan" Tool To Check The Following & Correct If Necessary

1. Coolant Temperature
2. TPS
3. VSS
4. Codes - If 24 Is Present, See Code Chart 24
5. Also, Perform Mechanical Checks, Such As Linkage, Oil Level, Etc., Before Using This Chart

TCC Elect. Diagnosis (4.3/5.0/5.7L Under 8500 GVW) (1 Of 2). Scheme 145

TCC Elect. Diagnosis (4.3/5.0/5.7L Under 8500 GVW) (2 Of 2). Scheme 146

TCC Elect. Diagnosis Schematic Diagram (4.3/5.0/5.7L Under 8500 GVW). Scheme 147

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 148

THM 400 Downshift Control Electrical Diagnosis. Scheme 149

THM 400 Downshift Control Electrical Diagnosis Schematic. Scheme 150

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.