Abnormal Operation

In the event the self diagnostic system is not functioning normally, it will likely exhibit one of the following symptoms.

1) Check engine light fails to come on at power up (key on, engine off, T circuit open).

2) Check engine light will not flash code when T circuit grounded (T jumpered to E1), check engine lamp stays on or stays off.

These conditions must be corrected before further diagnosis can be performed! The following charts will help to direct you to perform a "Diagnostic Circuit Inspection."

Check Engine Light Fails to Come On At Bulb Check

Possible Cause

Action to Take/Items To Inspect

Faulty fuse or wiring in BATT or +B circuit(s) to ECU

• Repair cause and replace fuse.

• Repair open in BATT circuit.

• Repair EFI Main Relay to ECU +B terminal.

Faulty fuse or wiring in IGi feed to check engine lamp

• Repair cause of burned fuse and replace fuse.

• Repair open in wiring to check engine lamp.

Burned out check engine bulb

• Replace bulb.

Faulty wiring in W circuit between fuse and ECU (including ECU connection at terminal W)

• Isolate circuit fault using" multimeter starting at ECU terminal W.

- With ignition off, disconnect ECU connector, turn ignition on; there should be battery voltage at ECU harness terminal W.

- Grounding harness terminal W should turn on check engine light if circuit is good.

Faulty Ei ground circuit

• Isolate fault using multimeter and repair as necessary.

Grounded Vcc circuit in wiring or sensor

• With ECU connected and ignition on, measure voltage at Vcc terminal.

- A grounded Vcc will pull this voltage low.

- Start disconnecting sensors fed by Vcc one at a time until the voltage goes up to 5 volts; this identifies the offending sensor.

• If all sensors are disconnected and voltage is still low at Vcc, turn off the ignition and disconnect the ECU.

- Use ohmmeter to measure harness for continuity to ground on the Vcc circuit.

- If no fault is found in sensors or harness, the ECU is at fault.

Faulty ECU

• Make sure that connections at the ECU are good (use terminal contact gauge on harness to confirm).

• Check +B, W, and Ei circuit connections. If these connections are good, replace ECU.

Check Engine Light Does Not Flash Codes (Stays On or Off)

Possible Cause

Action to Take/Items To Inspect

Faulty wiring in T circuit (including ECU connection at terminal T)

• With T circuit open and ignition switch in "on" position, there should be 12 volts at the T terminal of the check connector.

- If voltage is zero, re-check voltage at ECU under same condition.

- If voltage is still zero, turn ignition switch off and disconnect ECU.

• Check connection at terminal T in harness connector using a terminal contact gauge.

- If connection checks O.K., replace ECU.

Grounded W circuit between check engine bulb and ECU

• Turn off ignition switch, disconnect ECU connector.

- Turn ignition on, if check engine light comes on, W circuit is grounded.

- Disconnect connector at combination meter and isolate unwanted ground.

Faulty Ei ground circuit

• Isolate fault using multimeter and repair as necessary.

Faulty ECU

• Make sure that connections at the ECU are good (use terminal contact gauge on harness to confirm).

- Re-confirm T connection.

- If this connection is good, replace ECU.

At this stage in your diagnosis, you may have already diagnosed the problem and are ready for repair and a quality control check. If the problem has not yet been identified, you are ready for the next diagnostic step.

Troubleshooting By Symptom

When the self diagnostic system fails to indicate a problem with the electronic control system (normal code displayed), there are two possibilities left. Either there is a problem in the electronic control system which the ECU is not capable of detecting or the problem lies outside of the electronic control system entirely. In either case, the "Troubleshooting" section of the repair manual will help you locate the appropriate diagnostic routine to quickly isolate the problem cause.

The third step in a systematic diagnosis requires use of the "Troubleshooting" and cc Voltage at ECU Wiring Connectors" sections of the repair manual. Based on the symptom the vehicle exhibits, these manual sections will lead you to the diagnostic routine which will assist in solving the problem.

Voltage at ECU Connector Checks

The self diagnostic system is not capable of detecting sensor circuits which are feeding out of range information to the ECU. By using the Voltage at ECU Wiring Connectors chart, measured voltage signals at the ECU can be compared to standard voltage values listed

in the repair manual. Signals which are out of the normal range can be identified and the cause diagnosed by referring to the far right column of the chart; this will lead to the appropriate pinpoint test to perform.

In the event that all listed values fall within a normal range, the symptom charts in the repair manual should be consulted. Starting with new models introduced after '90, repair manuals include a comprehensive troubleshooting matrix that replaces the symptoms charts. Beginning with '92 repair manuals, this matrix is located at the beginning of the Emissions (EM) section of the repair manual.

Using the Symptom Charts and Troubleshooting Matrix

The most important part of troubleshooting by symptom is to identify the symptom accurately. An accurate description of the problem will ensure that the appropriate diagnostic routines will be selected. Based on the symptom chosen, a series of testing routines are available to assist in pinpointing the problem area.

These test routines address items within the electronic control system as well as areas outside the system which could cause the symptom chosen. The technician's knowledge and experience will be his guide to which tests to perform first and which tests to disregard in any particular situation.

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