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. See Fig 1 and Fig 2 . After performing necessary tests as described in the diagnostic circuit check, if no codes are indicated and driveability problems still exist, see TESTS W/O CODES article in this section, and SCAN TESTER USAGE under SELF DIAGNOSTIC SYSTEM in the appropriate G - TESTS W/ CODES article in this section.

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 SERVICE ENGINE SOON light to flash a Code 12, followed by any codes stored in ECM memory. See Figure. Light going from bright to dim is not considered a code. If light dims or remains on and does not flash Code 12, see A2, SERVICE ENGINE SOON LIGHT ON STEADY OR WON'T FLASH CODE 12.
   NOTE: On some models, as long as a 30-second pause may occur between code flashes. This is normal and does not indicate necessary component replacement.
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.