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Diagnostic Circuit Check



PCM Power Supply And Diagnostic Circuit Diagram:




Diagnostic Circuit Check:






NOTE:
a. Numbers below refer to circled numbers in flow chart.
b. Since this is the starting point for all diagnostic procedures or finding the cause of an emissions test failure, always begin here and always follow the flow chart as directed.
c. For trouble code diagnosis see Charts A-1 and A-2. Related Tests, Information and Procedures


1. Bulb Check - the system check starts with a bulb check. SES light should be "ON" with the ignition turned "ON" and the engine not running.

2. If there is no "SERVICE ENGINE SOON" light, Related Tests, Information and Procedures.

3. Diagnostic Mode Check - this step checks the PCM's ability to display codes through the SES light and indicates if the diagnostic mode is working. SES light should flash Code 12 three times, followed by any diagnostic trouble codes stored in memory. A MEM-CAL error may cause inability to display Code 12.

4. "Scan" Data - this determines if the computer is supplying data through the serial data line to the ALDL connector. If a MEM-CAL malfunction is present, the system may be able to display codes, but not send serial data.

5. Engine Start - even though the PCM displays codes, a system malfunction may cause a "cranks but won't start" symptom.

6. A PCM or MEM-CAL malfunction may cause the system to display invalid codes, even though the diagnostic system appears to be operating properly.


7. No Codes - Scanning the data for typical values may indicate a problem area if the values are different from typical, even though no codes are displayed. Refer to Typical Scan Data Values found under Computers and Control Systems/System Diagnosis/Scan-Tool Diagnostics. Scan Tool Data List
If all systems appear to be functioning, Testing and Inspection

8. If scan data indicates values that are outside of the typical range for a particular system or component, the diagnostic "C - Charts" provide functional checks for specific systems and components. [1][2]C Charts.
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