- Powertrain/On-Board Diagnostic System Check

Diagnostic Chart:

ECM Circuit:

CIRCUIT DESCRIPTION
The On-Board Diagnostic System Check is an organized approach to identifying a problem created by an electronic engine control system malfunction. It must be the starting point for any driveability complaint diagnosis, because it directs the service technician to the next logical step in diagnosing the complaint. Understanding the chart and using it correctly will reduce diagnostic time and prevent the unnecessary replacement of good parts.

TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.

1. This step is a check for the proper operation of the Malfunction Indicator Lamp (MIL) (Service Engine Soon). The MIL should be "ON" steady.

2. No MIL at this point indicates that there is a problem with the MIL circuit or the Engine Control Module (ECM) control of that circuit.

3. This test checks the ability of the ECM to control the MIL. With the diagnostic terminal grounded, the MIL should flash a Diagnostic Trouble Code (DTC) 12 three times, followed by any diagnostic trouble code stored in memory.

4. Most of the diagnostic procedures use a Tech 1 scanner to aid diagnosis, therefore, serial data must be available. If a Programmable Read Only Memory (PROM) error is present, the ECM may have been able to flash DTC 12/51, but not enable serial data.

5. Although the ECM powered up, a "Cranks But Will Not Run" symptom could exist because of an ECM or system problem.

6. This step will isolate if the customer complaint is a MIL or a driveability problem with no MIL. An invalid diagnostic trouble code may be the result of a faulty scan tool.

7. Comparison of actual control system data with the typical values is a quick check to determine if any parameter is not within limits. Keep in mind that a base engine problem (i.e., advanced cam timing) may substantially alter sensor values.

8. Installation of a scan tool will provide a good ground path for the ECM and may hide a driveability complaint due to poor ECM grounds.

9. If the actual data is not within the typical values established, the System Diagnosis procedures in Computer and Control Systems will provide a functional check for a particular component or system.