Diagnostic Circuit Check: Notes

There are different/unique types of control modules, and various systems are used to control their operation depending on engine and/or vehicle type. For easier referencing, engines and models are identified or divided into system groups. See SYSTEM GROUPING CHART .

Diagnostic circuit check determines following:

• Whether Malfunction Indicator Light (MIL) works.
• Whether ECM is operating and can recognize a fault.
• Whether any codes are stored.

After performing basic diagnostic procedures listed under PRELIMINARY INSPECTION & ADJUSTMENTS; A-7, BASIC FUEL SYSTEM CHECKS and C-4, BASIC IGNITION SYSTEM CHECKS, use self-diagnostic system for determining computer-related problems. See Figure.

If codes are NOT indicated and driveability problems still exist after performing necessary tests in diagnostic circuit check, see TESTS W/O CODES - GASOLINE article and SCAN TESTER USAGE under SELF-DIAGNOSTIC SYSTEM in appropriate G -TESTS W/ CODES article in this section below.

• G - TESTS W/CODES - 4.3L
• G - TESTS W/CODES - 5.0L
• G - TESTS W/CODES - 5.7L
• G - TESTS W/CODES - 7.4L

1. Check MIL operation. Turn ignition on with scan tester not connected, DLC terminal not grounded and engine not running. MIL should be on steadily. If light glows and stays on steadily, go to next step. If light does not glow, go to A-1, MALFUNCTION INDICATOR LIGHT (MIL) INOPERATIVE. If light flashes, go to step 3).
2. Ground DLC test terminal "B". MIL should flash Code 12, followed by any codes stored in ECM memory. If light goes from bright to dim or if light remains on and does not flash Code 12, see A-2, MALFUNCTION INDICATOR LIGHT (MIL) ALWAYS ON OR WON'T FLASH CODE 12.
3. Check for short to ground in diagnostic test terminal wire between DLC terminal "B" and ECM terminal. See Fig 1-Fig 3 . If circuit is okay, replace ECM.

Fig 1: Fuel Injection Diagnostic Circuit Schematic (Group 2)

Courtesy of GENERAL MOTORS CORP.