2.8l Feedback Tests W/codes — Chevrolet Cab & Chassis C20 (1985-1986)

DIAGNOSTIC CIRCUIT CHECK

Diagnostic Circuit Check determines if: 1) the "CHECK ENGINE" 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.

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

Check operation of the "CHECK ENGINE" light. Key in "ON" position, engine not running, light should be on steady.
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 A-6.
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.
If the light is on, fault is still present. Go to the applicable trouble code chart.
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 120

Scheme 120: Diagnostic Circuit Check

SYSTEM PERFORMANCE CHECK

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

This test checks carburetor ability to change air/fuel mixture. Disconnecting M/C solenoid makes carburetor run full rich, reconnecting it with dwell lead grounded makes carburetor run full lean. RPM normally drops 400-1000 RPM (300 RPM minimum) as solenoid is reconnected. If plugging the PCV, purge, or bowl vent hose causes RPM to drop more than 300 RPM, that hose leads to the source of problem.
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 term. "14" to ground, open wire to ECM term. "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 term. "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.
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 121

Scheme 121: System Performance Check

CHART A1 - DWELL FIXED UNDER 10°

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

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.
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.
This test checks for normal coolant sensor circuit condition. Voltage on a normalized hot engine should be under 2.5 volts.
This step checks for an open in the ground circuit to ECM term. "14" and grounded oxygen sensor circuit. Terminal "2" voltage should be under 1.0 volt at idle. A high voltage could be caused by an open in the circuit at term. "22." Normally this will cause Codes 21 and 34 but won't set them on some engines.

Chart A1, Dwell Fixed Under 10°. Scheme 122

Scheme 122: Chart A1, Dwell Fixed Under 10°

CHART A2 - DWELL FIXED BETWEEN 10°-50°

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

Run engine 1 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 term. "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.
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.
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 123

Scheme 123: Chart A2, Dwell Fixed Between 10°-50°

CHART A3 - DWELL FIXED OVER 50°

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

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; 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.
If plugging the PCV or bowl vent vacuum hose causes the dwell to decrease, that hose leads to the source of the problem.
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.
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 124

Scheme 124: Chart A3, Dwell Fixed Over 50°

CHART A5 - "CHECK ENGINE" LIGHT INOPERATIVE

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

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

Chart A5, "Check Engine" Light Inoperative. Scheme 125

Scheme 125: Chart A5, "Check Engine" Light Inoperative

CHART A6 - NO CODE 12/"CHECK ENGINE" LIGHT ALWAYS ON

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

This step checks for short to battery voltage in wire to terminal "C" or faulty lamp driver. Normal reading is 9 to 11 volts.
This step checks to see if problem is related to the ECM or lamp driver. Grounding term. "C" should turn lamp off.
Grounding term. "G" at ECM and finding light on indicates an open in the wire to term."C" of lamp driver. Grounding term. "G" should turn lamp off.
Checks for open in wire from ECM to test terminal in ALCL connector. Lamp should flash Code 12 when terminal "5" is grounded.
This checks for proper voltage supply to ECM, both should read over 9 volts. Term. "C" is ignition and term. "R" is constant battery for long term memory.
This test checks for bad ground in ECM. Terms. "A" and "U" are connected together in the ECM.
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, No Code 12 or "Check Engine" Light Always On. Scheme 126

Scheme 126: Chart A6, No Code 12 or "Check Engine" Light Always On

CODE 12 - NO DISTRIBUTOR REFERENCE PULSES

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

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.

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. Terminal must be removed from the connector and carefully inspected.
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.
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 term. "10" to module is not opened or grounded, source of no signal is the module.

Code 12, No Distributor Reference Pulses. Scheme 127

Scheme 127: Code 12, No Distributor Reference Pulses

CODE 13 - OXYGEN SENSOR CIRCUIT

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

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

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

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

This test checks to see if problem still exists. Fixed dwell indicates fault.
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."
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.
Checks for an open in the ECM-oxygen sensor ground circuit. Normal voltage is below 1 volt if the circuit is complete. The worse the connection is, the higher the voltage will read.
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 128

Scheme 128: Code 13, Oxygen Sensor Circuit

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 term. "3," for a time longer than specified.

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

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.
Checks for ground between ECM and coolant sensor. Test light to battery positive will be "OFF" in an ungrounded circuit.

Code 14, Coolant Sensor Shorted. Scheme 129

Scheme 129: Code 14, Coolant Sensor Shorted

CODE 15 - COOLANT SENSOR OPEN

Code 15 means the ECM has seen the resistance of the Coolant Sensor circuit too high. This could be due to high resistance (cold engine temperature) or high voltage at ECM term. "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.

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

If problem still exists, "CHECK ENGINE" light will come on and Code 15 will be set.
This test checks if fault is coolant sensor or lack of voltage to sensor. Normal reading is 5 volts across coolant sensor connector.
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 terms. "3" to "7."
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 130

Scheme 130: Code 15, Coolant Sensor Open

CODE 21 - TPS CIRCUIT HIGH

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 terms. "21" and "2" within the ECM, an open in the TPS circuit will place about 5 volts (high TPS signal) at term. "2" of ECM.

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

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 megohms resistance meter must be used. A lower resistance voltmeter would read virtually zero at term. "B."
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.
This test simulates closed throttle. Dwell should increase if the ECM is good.
This tests the resistance of the TPS switch. Normal reading is less than 20,000 ohms.

Code 21, TPS Circuit High. Scheme 131

Scheme 131: Code 21, TPS Circuit High

CODE 23 - M/C SOLENOID CIRCUIT LOW

Code 23 indicates the ECM has sensed a low steady voltage at ECM term. "18." Normal voltage at term. "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, smoky exhaust or poor driveability.

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

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.
Checks for battery voltage on the Pink ignition source wire. Test light should light between the ignition source and ground.
This test checks for an open in the solenoid to ECM circuit. Normal circuit will read about battery voltage at Term. "18" of the ECM.
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 term. "18" or a faulty ECM. NOTE: A test light must be used in this step. A voltmeter may give an inaccurate indication.
This test checks for ground in wire to ECM term. "18." If wire is grounded, light will stay "ON."

Code 23, M/C Solenoid Circuit Low. Scheme 132

Scheme 132: Code 23, M/C Solenoid Circuit Low

CODE 34 - DIFFERENTIAL PRESSURE (VACUUM) SENSOR

Code 34 says that the ECM has seen the following

Pressure outside a specified voltage range (seen by ECM as voltage at term. "20").
Engine RPM less than a given value.
Engine at operating temperature.
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.

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

This test checks output of sensor at idle to determine if sensor is within specification. Normal sensor will read less than 1 volt with key "ON," engine "OFF" and over 3 volts with engine idling (15 in. Hg minimum).
Normal sensor will drop below 1 volt with no vacuum.
This test checks for a ground in wire from term. "B" of vacuum sensor to ECM. Line is open if voltage is over 2 volts.
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 133

Scheme 133: Code 34, Differential Pressure (Vacuum) Sensor

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 rotten egg odor from the exhaust.

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

This test checks to see if vacuum sensor voltage changes with loss of vacuum supply. A good sensor will change voltage at terms. "A" to "B" by 1 volt or more.
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 134

Scheme 134: Code 41, No Distributor Reference Signal

CODE 42 - ELECTRONIC SPARK TIMING (EST)

Code 42 says that the ECM has seen

Open or grounded By-pass Circuit (term. "11").
Open or grounded EST Circuit (term. "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.

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

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.
This step eliminates the ECM and ECM connections from the module input. By jumpering terms. "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 term. "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.
By removing the jumper, you are opening the EST signal, and the engine should stop.
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 135

Scheme 135: Code 42, Electronic Spark Timing (EST)

CODE 44 - LEAN EXHAUST INDICATION

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

Voltage lower than specified.
Closed Loop.
Above a specified TPS value.
For a time longer than specified.

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

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.
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.
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.
This step puts a rich oxygen sensor signal (about 1 volt) into term. "9" of the ECM. Dwell should increase (lean command).

Code 44, Lean Exhaust Indication. Scheme 136

Scheme 136: Code 44, Lean Exhaust Indication

CODE 45 - RICH EXHAUST INDICATION

Code 45 indicates that the ECM has seen

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

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

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

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.
This step tests ECM response to a lean engine condition. A drop in dwell indicates that ECM and oxygen sensor are not faulty.
This step tests ECM response to a lean oxygen sensor signal (low voltage). If no dwell change with a grounded lead to oxygen sensor term. "9," fault is in ECM. Open oxygen sensor wire would have set Code 13.
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 137

Scheme 137: Code 45, Rich Exhaust indication

CODE 51 - FAULTY PROM

Code 51 sets if any of the following occur

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

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

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

Code 51 PROM. Scheme 138

Scheme 138: Code 51 PROM

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

CODE 54 - M/C SOLENOID CIRCUIT VOLTAGE 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.

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

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 18-32 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.
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.