ECM/PCM Diagnosis

DIAGNOSIS

To read the codes, use a Scan" tool or ground the diagnostic terminal with the engine not running and the ignition "ON." The "Service Engine Soon" light will flash Code 12 three times and then flash each code stored in memory three times.

All codes stored in memory would have been read when Code 12 was flashed again. No new codes can be stored when in the diagnostics mode (diagnostics lead grounded).

This eliminates confusion while the system is being worked on.

To clear the codes from memory.
- Ignition "OFF."
- Disconnect ECM fusible link for 30 seconds.

Since the ECM can have a failure which may affect only one circuit, following the diagnostic procedures in this section will determine which circuit has a problem and where it is.

If a diagnostic chart indicates that the ECM connections or ECM is the cause of a problem and the ECM is replaced, but does not correct the problem, one of the following may be the reason:
- There is a problem with the ECM terminal connections. The diagnostic chart will say ECM connections or ECM. The terminals may have to be removed from the connector in order to check them properly.
- Mem-Cal is not correct for the application. Incorrect components may cause a malfunction and may or may not set a code.
- The problem is intermittent. This means that the problem is not present at the time the system is being checked. In this case, refer to the "Symptoms" portion of the manual and make a careful physical inspection of all portions of the system involved.
- Shorted solenoid, relay coil. or harness. Solenoids and relays are turned "ON" and "OFF" by the ECM using internal electronic switches called "drivers." Each driver is part of a group of four called "quad-drivers."

A shorted solenoid,, relay coil, or harness will not damage the GMP4 ECM but will cause the component to be inoperative.

J 34636 or BT 8405 testers, or equivalent, provide a fast, accurate means of checking for a shorted coil or a short to battery voltage.

- The Mem-Cal may be faulty. Although these rarely fail, it operates as part of the ECM. Therefore, it could be the cause of the problem. Substitute a known good Mem-Cal.
- The replacement ECM may be faulty. Meter the ECM is replaced, the system should be rechecked for proper operation. If the diagnostic chart again indicates the ECM is the problem, substitute a known good ECM. Although this is a rare condition, it could happen.

MEM-CAL

An incorrect or faulty Mem-Cal, which is part of the ECM, may set a Code 51.