Feedback Carburetor Tests W/codes GMC Cab & Chassis S15

DESCRIPTION

Note. Most Computer Command Control (CCC) problems are the result of mechanical breakdowns, poor electrical connections, or damaged vacuum hoses. Before considering the CCC system as a possible cause of problems, check ignition high tension wires, fuel supply, electrical connections, and vacuum hoses. Failure to do so may result in lost diagnostic time.

The Computer Command Control (CCC) system controls engine operation and lowers exhaust emissions while maintaining good fuel economy and driveability. The CCC system is designed to maintain a 14.7:1 air/fuel ratio under all engine operating conditions. When the ideal air/fuel ratio is maintained, the catalytic converter can control oxides of nitrogen (NOx), hydrocarbon (HC), and carbon monoxide (CO) emissions.

The CCC system consists of the following suBsystems: Fuel Control, Data Sensors, Electronic Control Module (ECM), Electronic Spark Timing, Electronic Spark Control, AIR Management, Exhaust Gas Recirculation, Evaporative Emission Control, Torque Converter Clutch (TCC), Diagnostic System.

BASIC DIAGNOSTIC PROCEDURE

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

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

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

Scheme 96

1. Turn ignition switch on but do not start engine. "SERVICE ENGINE SOON" lamp should glow. Locate Assembly Line Communication Link (ALCL) connector attached to ECM wiring harness under instrument panel near steering column (under driver's seat on "G" model vans). Insert spade lug terminal across "TEST" terminal and "GROUND" terminal. (Scheme 96) CAUTION: Inserting spade lug in terminals of ALCL connector grounds "TEST" terminal lead. Do not ground ALCL connector until after ignition is on or engine is started. (Scheme 96): ALCL Connector Terminal Locations
2. "SERVICE ENGINE SOON" light should flash code "12" ("FLASH", pause, "FLASH", "FLASH"). Code "12" will be repeated 3 times. If any trouble codes are stored in the ECM memory, they will be displayed in the same manner.
3. Trouble codes will be displayed from lowest to highest numbered codes (3 times each) and be repeated as long as the "TEST" terminal of the ALCL connector is grounded.
4. To exit diagnostic mode, turn ignition switch off and remove spade lug terminal from ALCL connector.

CLEARING TROUBLE CODES

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

READING TROUBLE CODES

The ECM stores component failure information for the CCC system under a related trouble code which can be recalled for diagnosis and repair. When recalled, these codes will be displayed by flashes of the "SERVICE ENGINE SOON" light. Trouble codes are displayed starting with the lowest numbered code. Only codes that represent a definite malfunction will be shown.

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

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

TROUBLE CODE IDENTIFICATION

ECM TROUBLE CODE IDENTIFICATION

TROUBLE CODE DETERMINATION (HARD OR INTERMITTENT)

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

An "intermittent" code is one which does not reset itself and is not present while diagnosing vehicle. Intermittent codes are frequently caused by loose connections. A "hard" code will repeat itself during 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 Performance Check
4. "Engine Cranks But Won't Run" charts.

Diagnostic Circuit Check

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

Diagnostic Symptom Check

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

System Performance Check

1. Always set parking brake and block drive wheels when performing this check. This check verifies that CCC system is functioning correctly. It should always be made after any repair on CCC system.
2. On some engines, the oxygen sensor will cool off after only a short period of time while engine is idling. This will cause engine to go into open loop. To restore closed loop mode, run engine at part throttle for several minutes, accelerating from idle to part throttle several times.

DIAGNOSTIC TOOLS

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

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

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

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

Diagnostic circuit check determines if: 1) the "SERVICE ENGINE SOON" light works, 2) the ECM is operating and can recognize a fault, and 3) any codes are stored. It also checks to see if stored codes indicate an intermittent problem. This is the starting point for any diagnosis. If no codes are indicated, go to the SYSTEM PERFORMANCE CHECK. If no additional checks are called out from the SYSTEM PERFORMANCE CHECK, go to the DRIVEABILITY SYMPTOMS.

1. Check operation of the "SERVICE ENGINE SOON" light. With key on and engine not running, light should be on steady.
2. Grounding test terminal will flash a Code 12 and any stored trouble codes. The light must go on and off to indicate a code. The light going from "Bright" to "Dim" is not considered a code. See CHART A6.
3. This step will determine if any codes, other than Code 12, are still present or were intermittent and are no longer stored. Clear memory. Run vehicle for 2 minutes. See if trouble code(s) reset.
4. If the light is on, fault is still present. Go to the applicable trouble code chart.
5. If the light is off the fault is either intermittent, or it is a code that cannot be set with vehicle stationary. For codes that cannot be set during the DIAGNOSTIC CIRCUIT CHECK, the applicable trouble code chart will determine if those codes are intermittent.

Diagnostic Circuit Check. Scheme 97

1. This test checks carburetor ability to change air/fuel mixture. Disconnecting M/C solenoid makes carburetor run full rich, and reconnecting it with dwell lead grounded makes carburetor run full lean. RPM normally drops 400-1000 RPM (300 RPM minimum) as solenoid is reconnected. 1A) If plugging the PCV, purge, or bowl vent hose causes RPM to drop more than 300 RPM, that hose leads to the source of the problem. If RPM increases as M/C solenoid is connected, it indicates system is running extremely rich. This can sometimes be caused by incorrect valve timing.
2. This test checks for proper control of idle circuit. 2A) This indicates a full rich command to the carburetor, caused by: lean engine condition, grounded oxygen sensor wire or bad sensor, open wire from ECM terminal "14" to ground, open wire to ECM terminal "22.", or open coolant sensor switch. 2B) This indicates an open loop condition that can be caused by: an open oxygen sensor circuit or bad sensor, an open coolant sensor circuit, or an open wire from ECM terminal "14" to ground. 2C) This indicates a full lean command from a rich engine condition caused by: M/C solenoid wires reversed, leaking bowl vent valve, excessive fuel in vapor canister, fuel in crankcase, faulty carburetor calibration or carburetor. 2D) Indicates closed loop operation, normal dwell reading is between 10°-50° but varying.
3. Checks for proper control of main metering system. RPM must be at least 3000 to get into the main metering system operation. 3A) A missing "O" ring between the switching valve solenoid and the valve, or a defective valve, may cause air to leak to the exhaust ports at higher RPM only.

System Performance Check. Scheme 98

CHART A1: DWELL FIXED UNDER 10°

1. This test determines if problem is CCC or engine related. Dwell should start increasing as soon as engine is choked and increase until it is over 50°. If dwell responds, problem is lean engine. 1A) Checks for cause of lean condition that resulted in full rich command.
2. This test checks for ECM response to input to oxygen sensor circuit. The voltmeter is used to put a voltage on the oxygen sensor circuit to simulate a rich condition. Dwell should increase (a lean command) if ECM and harness are good.
3. This test checks for normal coolant sensor circuit condition. Voltage on a normalized hot engine should be under 2.5 volts.
4. This step checks for an open in the ground circuit to ECM terminal "14" and grounded oxygen sensor circuit. Terminal "2" voltage should be under one volt at idle. A high voltage could be caused by an open in the circuit at terminal "22." Normally this will cause Codes 21 and 34 but won't set them on some engines.

Chart A1: Dwell Fixed Under 10°. Scheme 99

CHART A2: DWELL FIXED BETWEEN 10°-50°

1. Run engine one minute to warm oxygen sensor. Grounding oxygen sensor input checks ECM response to a "lean" signal. Normal response is dwell decreasing to full rich command. 1A) On some ECM's, an open circuit to terminal "14" can cause open loop. 1B) Checks output of oxygen sensor with full rich command from ECM caused by grounded oxygen sensor input. Normal response is voltage at oxygen sensor over .8 volt.
2. This step grounds oxygen sensor circuit at ECM to check for opens in wiring to ECM terminals "9" and "14." Normal response to "lean" signal is dwell decrease.
3. This step checks for voltage to the coolant sensor. Normal reading on a warm engine is less than 2.5 volts. An open circuit would cause a reading of approximately 5 volts.

Chart A2: Dwell Fixed Between 10°-50°. Scheme 100

CHART A3: DWELL FIXED OVER 50°

1. This test determines whether problem is related to engine or electronics. Normal response is dwell decrease; this indicates that oxygen sensor, harness and ECM are okay, and that problem is a rich engine. NOTE: If engine is very rich, a large air leak may be required to lean mixture. When mixture is lean enough, engine will begin to run rough.
2. If plugging the PCV or bowl vent vacuum hose causes the dwell to decrease, that hose leads to the source of the problem.
3. This test checks ECM response to a "lean" oxygen sensor signal. Normal response to this test is low dwell. No dwell change indicates a defective ECM. This test also eliminates the possibility of an open sensor wire. An open wire would cause open loop operation and may set Code 13.
4. This test checks for excessive voltage in oxygen sensor line. If under .55 volt, wire and ECM are okay. Fault is in oxygen sensor. If over .55 volt, wire is shorted to battery voltage or ECM is faulty.

Chart A3: Dwell Fixed Over 50°. Scheme 101

CHART A5: "SERVICE ENGINE SOON" LIGHT INOPERATIVE

1. This checks for blown gauge fuse or open in "SERVICE ENGINE SOON" light circuit (including I.P. connector), printed circuit and "SERVICE ENGINE SOON" lamp. Normal response is lamp on.
2. This test checks for shorted ECM. Grounding ECM terminal "G" will turn the "SERVICE ENGINE SOON" light off. Normal response is lamp on.
3. This test checks for grounded wire from terminal "C" of lamp driver to terminal "G" of ECM, an open circuit to terminal "B" of lamp driver, a bad ground or faulty lamp driver. Normal reading is about 9 to 11 volts.
4. This test checks for open in the wire to terminal "B". Normal response is approximately battery voltage. A) This test checks for an open wire to terminal "E" from the "SERVICE ENGINE SOON" lamp. With terminal "E" grounded, lamp should normally light.
5. This test checks for a grounded wire from driver terminal "C" to ECM terminal "G." Normal response is light on.

Chart A5: "Service Engine Soon" Light Inoperative. Scheme 102

CHART A6: WON'T FLASH CODE 12/"SERV ENG SOON" ALWAYS ON

1. This step checks for short to battery voltage in wire to terminal "C" or faulty lamp driver. Normal reading is 9 to 11 volts.
2. This step checks to see if problem is related to the ECM or lamp driver. Grounding terminal "C" should turn lamp off.
3. Grounding terminal "G" at ECM and finding light on indicates an open in the wire to terminal "C" of lamp driver. Grounding terminal "G" should turn lamp off.
4. This step checks for open in wire from ECM to test terminal in ALCL connector. The lamp should flash Code 12 when terminal "5" is grounded.
5. This checks for proper voltage supply to ECM. Both should read over 9 volts. Terminal "C" is ignition and terminal "R" is constant battery for long term memory.
6. This test checks for bad ground in ECM. Terminals "A" and "U" are connected together in the ECM.
7. This step distinguishes between a faulty ECM and PROM. Normal response is for Code 51 to flash even though the PROM is not installed in the ECM. If no Code 51, ECM is faulty.

Chart A6: Won't Flash Code 12 or "Service Engine Soon" Light On At All Times. Scheme 103

CODE 12: NO DISTRIBUTOR REFERENCE PULSES

Code 12 means the ECM is on and sees no reference pulse from the distributor. This is a normal code with the ignition on and the engine not running. Code 12 is not stored and will only flash when the fault is present. With engine running, Code 12 could mean an open or ground in distributor reference circuit. Code 41 will appear with Code 12 if engine is running with no distributor reference signal. If problem clears, Code 41 will store.

1. This test checks for a poor connection at EST 4-wire connector as being the source of no reference pulse. Check for corrosion, connector terminals not fully seated, or terminal not properly attached to wire. Teminal must be removed from the connector and carefully inspected.
2. This step determines if a reference pulse is being sent to the ECM. Voltage should increase as you go from idle to part throttle. A voltage increase indicates the signal is being generated by the module and fault is a bad connection at the ECM, or faulty ECM. To check the connection at ECM, terminal must be removed from connector.
3. With an open circuit, there is still a small amount of voltage at the ECM. It will not increase when throttle is opened. If circuit from terminal "10" to module is not opened or grounded, source of no signal is the module.

Code 12: No Distributor Reference Pulses. Scheme 104

CODE 13: OXYGEN SENSOR CIRCUIT

Code 13 indicates an open in the oxygen sensor circuit with the following conditions

1. Oxygen sensor voltage is within a specified range.
2. Above a specified TPS value.
3. More than specified time after engine has warmed up.

The ECM supplies about .45 volt between terminals "9" and "14." Voltage may read as low as .32 volt when measured with a 10-megohm digital volt-ohm meter. The oxygen sensor varies the voltage within a range of about one volt (rich exhaust) to about .1 volt (lean exhaust).

1. This test checks to see if problem still exists. Fixed dwell indicates fault.
2. By grounding the oxygen sensor circuit to the ECM, a "low voltage (lean) signal" is sent to the ECM. This should result in a "full rich (low dwell) command."
3. This test checks oxygen sensor. With the rich command, the oxygen sensor should read a high voltage, over .8 volt. If the oxygen sensor functions, fault is in the connections to the sensor.
4. Checks for an open in the ECM-oxygen sensor ground circuit. Normal voltage is below one volt if the circuit is complete. The worse the connection is, the higher the voltage will read.
5. This grounds the oxygen sensor signal wire at the ECM. Dwell should go to below 10°, since this is a "low voltage signal" indicating lean exhaust. No change indicates a problem at the ECM connections, or the ECM.

Code 13: Oxygen Sensor Circuit. Scheme 105

CODE 14: COOLANT SENSOR SHORTED

Code 14 means the ECM has seen low resistance of the coolant sensor circuit as high engine temperature, or low voltage at ECM terminal "3" for a time longer than specified.

1. This test determines whether fault is in sensor or circuit. Normal circuit voltage is about 5 volts. NOTE: Coolant sensor IS NOT connected during this test.
2. This step checks for a grounded circuit between ECM and coolant sensor. Test light to B+ should be off in an ungrounded circuit. The coolant sensor is not connected during test.

Code 14: Coolant Sensor Shorted. Scheme 106

CODE 15: COOLANT SENSOR OPEN

Code 15 means the ECM has seen the resistance of the coolant sensor circuit as too high. This could be due to high resistance (cold engine temperature) or high voltage at ECM terminal "3", for too long a time. This may cause detonation on a warm engine due to excessive spark advance, or poor driveability due to inaccurate fuel control.

1. If problem still exists, "SERVICE ENGINE SOON" light will come on and Code 15 will be set.
2. This test checks if fault is coolant sensor or lack of voltage to sensor. Normal reading is 5 volts across coolant sensor connector.
3. This test determines whether the low voltage at the sensor connector is due to opens in the coolant sensor wires, or in another part of the 5-volt reference circuit. Normal voltage is about 5 volts from ECM terminals "3" to "7."
4. This test checks resistance of the coolant sensor. If the resistance is within the chart specifications, coolant sensor is not faulty. Check for corrosion at the connector or low coolant level.

Code 15: Coolant Sensor Open. Scheme 107

CODE 21: TPS OPEN OR MISADJUSTED

Code 21 means that the ECM has seen a high TPS voltage for more than about 10 seconds, below a specified RPM or below a specified engine load. Due to the pull-up resistor between terminals "21" and "2" within the ECM, an open in the TPS circuit will place about 5 volts (high TPS signal) at terminal "2" of ECM.

1. This test checks the circuits from the TPS connector back to the ECM. Both wires should read about 5 volts due to the pull-up resistor in the ECM. NOTE: A 10-megohm resistance meter must be used. A lower resistance voltmeter would read virtually zero at terminal "B."
2. This test checks if low voltage at TPS connector is an open in the circuit or a faulty ECM. A normal reading at the ECM is about 5 volts.
3. This test simulates closed throttle. Dwell should increase if the ECM is good.
4. This tests the resistance of the TPS switch. Normal reading is less than 20,000 ohms.

Code 21: TPS Open Or Misadjusted. Scheme 108

CODE 23: M/C SOLENOID CIRCUIT LOW TO ECM

Code 23 indicates that the ECM has sensed a low steady voltage at ECM terminal "18." Normal voltage at terminal "18" is rising and falling as the solenoid is turned on and off. This code could be caused by a ground on the ECM side of the M/C solenoid or an open in the M/C solenoid circuit. A grounded circuit will cause a full lean condition and very poor driveability. An open circuit will cause a full rich condition and poor economy, odor, smokey exhaust or poor driveability.

1. This test checks for a complete circuit from the battery to the M/C solenoid dwell lead. Normal reading should be battery voltage. Battery voltage means there might be an open circuit between dwell connector and ground. No voltage could be either an open between the connector and battery or a ground on the ECM side of the M/C solenoid.
2. This test checks for B+ on Pink ignition source wire. Test light should light between ignition source and ground. 2A) This test checks for an open in the solenoid to ECM circuit. Normal circuit will read about battery voltage at terminal "18" of the ECM.
3. This test determines whether fault is in the M/C solenoid, a ground in the circuit to the ECM or the ECM. A light will indicate a ground in circuit to terminal "18" or a faulty ECM. NOTE: A test light must be used in this step. A voltmeter may give an inaccurate indication.
4. This test checks for ground in wire to ECM terminal "18." If wire is grounded, light will stay on.

Code 23: M/C Solenoid Circuit Low To ECM. Scheme 109

CODE 34: DIFFERENTIAL PRESSURE (VACUUM) SENSOR

Code 34 says that the ECM has seen the following

1. Pressure outside a specified voltage range (seen by ECM as voltage at terminal "20").
2. Engine RPM less than a given value.
3. Engine at operating temperature.
4. All the above for a time greater than specified.

The vacuum sensor measures the difference in pressure between atmosphere and manifold. The vacuum sensor supplies high voltage at high vacuum. High voltage increases spark advance.

1. This test checks output of sensor at idle to determine if sensor is within specification. Normal sensor will read less than one volt with key on, engine off and over 3 volts with engine idling (15 in. Hg minimum).
2. Normal sensor will drop below one volt with no vacuum.
3. This test checks for a ground in wire from terminal "B" of vacuum sensor to ECM. Line is open if voltage is over 2 volts.
4. This test checks to see if the fault is in the sensor, the ECM wiring, or the ECM. If the voltage goes over 2 volts with the sensor disconnected, the sensor or sensor connections are faulty.

Code 34, Differential Pressure (Vacuum) Sensor. Scheme 110

CODE 41: NO DISTRIBUTOR REFERENCE SIGNAL

Code 41 says that there are no distributor references pulses to the ECM at a specified engine vacuum. This code could set with the key on, engine "Not Running" if the vacuum sensor was indicating "Engine Running" voltage with the key just on. With a constant open or ground in the reference signal circuit, Code 12 would be set along with 41. Use Chart 12 if 12 and 41 are set. Code 41 alone indicates the problem is intermittent. When the distributor reference line signal is lost, the engine runs full rich and with retarded (base) spark timing. The result is poor performance, poor fuel economy, and possibly a rotten egg odor from the exhaust.

1. This test checks to see if vacuum sensor voltage changes with loss of vacuum supply. A good sensor will change voltage at terminals "A" to "B" by one volt or more. 1A) Since voltage change was less than one volt, problem is in vacuum system. ECM has seen engine running vacuum equivalant with no distributor reference signal, with key on and engine not running.
2. This test checks for cause of an intermittent open or ground in the distributor circuit. This includes the Hall Switch, if so equipped. Fault could also be a vacuum sensor that is intermittently stuck, at the same voltage output as an engine "running," when the key is only on. This condition will produce no reference signal. Terminals must be removed from connector to properly check them. The distributor pick-up coil should also be checked.

Code 41: No Distributor Reference Signal. Scheme 111

CODE 42: ELECTRONIC SPARK TIMING (EST)

Code 42 indicates ECM has seen

1. Open or grounded By-Pass Circuit (terminal "11").
2. Open or grounded EST Circuit (terminal "12").

With a grounded EST Circuit, the engine may not run. A grounded EST may sometimes not set a code unless cranked 10 seconds or longer with circuit grounded.

1. This checks operation of EST. Grounding the "test" terminal causes timing to go to a fixed value which is normally different from that obtained with EST operating. Therefore, the timing should change. Usually the change can be heard in engine RPM. If so, the timing change does not have to be checked.
2. This step eliminates the ECM and ECM connections from the module input. By jumpering terminals "A" and "B," the distributor reference signal is fed directly into the EST line of the module. By putting voltage through the test light on terminal "C" of the harness, the module is switched to the EST mode and the vehicle should run. If the engine stops, there is no EST signal reaching the module due to open or poor connections, or the module is faulty.
3. By removing the jumper, the EST signal opens and engine should stop.
4. The engine ran when the module was jumpered. The problem is not in the distributor (if the correct HEI module is installed). The wrong HEI module can set a Code 42.

Code 42: Electronic Spark Timing (EST). Scheme 112

CODE 43: ESC VOLTAGE LOW TO ECM (4.3L)

Code 43 indicates Electronic Spark Control (ESC) retard signal has been seen by ECM for too long. When voltage at Terminal "L" at the ECM is low, spark is retarded. Normal voltage in non-retard mode is about 7.5 volts or more.

1. Normal voltage would be over 7.5 volts. If 7.5 volts is present at terminal "L", the reason for a Code 43 is a poor connection to ECM or faulty ECM.
2. Over 6 volts indicates an overly sensitive knock sensor or controller, or noise in engine that fools the knock sensor.
3. Checks for grounded ECM.
4. This test checks for an open wire between the ESC and ECM. More than 6 volts at terminal "C" of ESC indicates an open to terminal "L" of ECM.
5. Checks for proper 12V ignition source to ESC terminal "B".
6. Checks to see if spark retard is due to engine knock or a faulty knock sensor. If spark advances when knock sensor is disconnected, fault is result of engine "noise" or sensor.
7. Checks to see if spark retard is due to a faulty ESC controller or "noise" on ESCto-knock sensor wire. If spark advances when terminal "E" is removed from connector, check for improper routing of knock sensor signal wire.

Code 43: ESC Voltage Low to ECM (4.3L). Scheme 113

CODE 44: LEAN EXHAUST INDICATION

Code 44 indicates that the ECM has seen oxygen sensor voltage under the following conditions

1. Voltage lower than specified.
2. Closed Loop.
3. Above a specified TPS value.
4. For a time longer than specified.

1. A fixed dwell of under 10° indicates the problem is still present. A fixed dwell under 10° at idle, with dwell varying at 3000 RPM, usually indicates an intake leak. Check this area prior to replacing oxygen sensor.
2. This test checks if the ECM is able to respond to a rich condition caused by choking the engine. If it does, the problem is a lean engine condition, NOT ELECTRICAL.
3. If dwell increases to over 50° with heavy choking, the fault is an air leak. If air is going to exhaust ports, disconnect the solenoid(s) for the air control valve. If air still goes to the ports, air valve is faulty.
4. This step puts a rich oxygen sensor signal of about one volt, into terminal "9" of the ECM. Dwell should increase (lean command).

Code 44: Lean Exhaust Indication. Scheme 114

CODE 45: RICH EXHAUST INDICATION

Code 45 indicates that the ECM has seen

1. High oxygen sensor voltage.
2. More than specified time (about 2 minutes).
3. Above a specified TPS value.
4. Closed loop.

A high voltage can be caused by a rich exhaust or an oxygen sensor contaminated with silicone.

1. Dwell under 50° indicates that engine should be checked for cause of intermittent rich condition (purge or bowl vent valves leaking, fuel in crankcase, fuel in evaporative canister, or sticking mixture control solenoid metering rods).
2. This step tests ECM response to a lean engine condition. A drop in dwell indicates that ECM and oxygen sensor are not faulty. Look for source of constant rich condition. See step 1) examples.
3. This step tests ECM response to a lean oxygen sensor signal (low voltage). If no dwell change with a grounded lead to oxygen sensor terminal "9," fault is in ECM. Open oxygen sensor wire would have set Code 13.
4. This step checks voltage from the ECM at the oxygen sensor harness. Normal voltage at this point is the ECM bias voltage for no oxygen sensor signal, approximately .45 volt. If voltage is high, the wire to the ECM could be shorted to battery voltage, or ECM is faulty.

Code 45: Rich Exhaust Indication. Scheme 115

CODE 51: PROM

Code 51 sets if any of the following occur

1. Faulty PROM unit.
2. PROM unit improperly installed (may not set a code if installed backward).
3. Some PROM pins not making contact (i.e. bent).

Always check to see that the PROM pins are not bent and inserted properly into ECM. Make sure the PROM is installed in the proper direction as shown in the chart. Check that all pins are fully inserted in the socket. If okay, replace PROM and recheck. If problem not corrected. Replace ECM.

Code 51: PROM. Scheme 116

Small notch of carrier should be aligned with small notch in socket. Press on PROM carrier until it is firmly seated into the socket. Do not press on PROM; only the carrier.

CODE 54: M/C SOLENOID CIRCUIT HIGH

Code 54 will be set if there is constant high voltage at ECM terminal "18." A short circuit to 12 volts will cause M/C solenoid to remain in the full rich position, resulting in potential ECM damage, excessive fuel consumption and excessive exhaust order.

1. This test checks the M/C solenoid resistance to determine if the fault is in the solenoid or ECM harness/ECM. Normal reading for a solenoid is 20-32 ohms. NOTE: After replacing a faulty M/C solenoid, a system performance test is necessary to be certain the M/C solenoid was the only faulty part. Solenoid may have caused the ECM to fail. This will reset code.
2. This test checks if reason for high voltage to terminal "18" is a faulty ECM or a short to 12 volts on that wire. If the test light to ground lights at the M/C solenoid test lead with both ends of harness disconnected, there is a short to 12 volts in the wire.

Code 54: M/C Solenoid Circuit High. Scheme 117

CHART C1 - ECM REPLACEMENT CHECK CHART

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

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

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

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

ECM QDR IDENTIFICATION

Scheme 118

Chart C1E, Differential Pressure (VAC) Sensor Check. Scheme 119

Chart C2F: TPS Enrichment Check. Scheme 120

Chart C2T-1: Idle Speed Control Check "Throttle Kicker" - 4.3L With A/C Only. Scheme 121

Chart C2T-2: Idle Speed Control Check "Throttle Kicker" - 4.3L With A/C Only (Cont.). Scheme 122

Chart C2W: Idle Speed Control Check "Throttle Kicker" - 4.3L Without A/C Only. Scheme 123

Chart C3: Solenoid Valve Check. Scheme 124

Chart C4A: Ignition System Check With EST. Scheme 125

CHART C4D: EST PERFORMANCE CHECK

1. Grounding the test terminal causes system to go a fixed spark advance which should be different from that obtained with EST operation. Engine is running at fast idle to get more spark advance. Usually change is enough for RPM change to be heard. If so, it is not necessary to check timing.
2. Checks to see if fault is in vacuum system.

Chart C4D: EST Performance Check. Scheme 126

Chart C4E:, Ignition System Check Without EST. Scheme 127

Chart C5, Electronic Spark Control (ESC) Check (4.3L). Scheme 128

Chart C6C, Air Management Chk Elect Air Control (EAC) Valve. Scheme 129

Chart C7C, EGR System Check Without ECM. Scheme 130

Chart C7D, EGR Bleed Solenoid Check 5.0L/5.7L Fed. w/ A/T. Scheme 131

Chart C7E, Pulse Width Modulate (PWM) EGR System Check. Scheme 132

Chart C8B, TCC Electrical Diagnosis. Scheme 133

Chart C9C, EFE System Check-With TVS. Scheme 134

ECM Terminal ID & Voltage Values (4.3L Carbureted Engine). Scheme 135

ECM Terminal ID & Voltage Values (5.0L & 5.7L Engines). Scheme 136

Wiring Diagram (4.3L Carbureted, 5.0L & 5.7L Engines). Scheme 137