Initial Inspection and Diagnostic Overview

Since the ECM can have a failure which may effect only one circuit, following the diagnostic procedures in this section can reliably tell when a failure has occurred in the ECM. Also, a Diagnostic Trouble Code (DTC) 55 indicates a failure of the ECM.
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.
- The ECM or Programmable Read Only Memory (PROM) is not correct for the application. - The incorrect ECM or PROM 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 relay 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 to the computer will not damage the ECM, but will cause the circuit and controlled component to be inoperative. When the circuit fault is not present or has been repaired, the "Quad-Driver", will again operate in a normal manner due to its fault protected design. If a fault has been repaired in a circuit controlled by a "Quad- Driver", the original ECM should be reinstalled and the circuit checked for proper operation.

ECM replacement will not be necessary if the repaired circuit or component now operates correctly.
J-34636 tester or equivalent provides a fast, accurate means of checking for a shorted coil or short to battery voltage.
- The PROM may be faulty. - Although the PROM rarely fails, it operates as part of the ECM. Therefore, it could be the cause of the problem. Substitute a known good PROM.
- The replacement ECM may be faulty. - After the ECM is replaced, the systems 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.
The components or circuits and the DTCs or charts, related to them are:
- DTC 55 indicates a failure of the ECM.
- PROM-DTC51.
- Engine Coolant Temperature Sensor - Code 14- 15.
- Manifold Absolute Pressure Sensor - DTCs 33 or 34. To check the sensor with no DTC set, use CHART C-1D.
- Throttle Position Sensor - DTCs 21 or 22.
- Crank Signal - CHART C-1B.
- Oxygen Sensor - DTC 13, 44, 45.
- A/C Request signal - If the A/C request signal is not reaching the ECM, it can cause rough idle, with A/C "ON". SEE "Rough Idle" in "Symptoms".
- Vehicle Speed Sensor - DTC 24.
- Distributor - DTC 42 indicates an Ignition Control failure.
- Distributor - CHART C-4 ignition system check.

Engine Control Module (ECM)
A faulty ECM will be determined in these diagnostic charts, or by a Diagnostic Trouble Code (DTC) 55.