Diagnostic Circuit Check (Except PFI CADILLAC): Notes

The Diagnostic Circuit Check determines:

• If SERVICE ENGINE SOON light works.
• If ECM is operating and can recognize a fault.
• If any codes are stored.

After performing procedures in PRELIMINARY INSPECTION & ADJUSTMENTS, BASIC FUEL SYSTEM CHECKS and BASIC IGNITION SYSTEM CHECKS, this is the starting point for utilizing the self-diagnostic system for determining computer-related problems. After performing necessary tests as described in the diagnostic circuit check, if no codes are indicated and driveability problems still exist, refer to the TESTS W/O CODES article. Also see SCAN TESTER USAGE in TESTS W/CODES .

1. Check operation of "SERVICE ENGINE SOON" light. Turn ignition on with engine off. "SERVICE ENGINE SOON" light should be on steady. If light illuminates and stays on steady, go to next step. If light does not illuminate, go to A1 - "SERVICE ENGINE SOON" LIGHT INOPERATIVE. If light flashes, go to step 3).
2. Grounding the ALDL "test" terminal "B" at this time should cause the "SERVICE ENGINE SOON" light to flash a Code 12, followed by any codes stored in ECM memory. See Fig 1. If light goes from bright to dim, this is not considered a code. If light dims or remains on, and does not flash Code 12, see A2 - "SERVICE ENG SOON" ALWAYS ON/WON'T FLASH CODE 12.
3. If light begins to flash as soon as ignition is turned on, check for a short to ground on the diagnostic test terminal wire between ALDL terminal "B" and ECM terminal No. 5. If circuit is okay, replace ECM.