Chart A-2, No ALDL Data Or Won't Flash Code 12/Check Eng Light On Steady

There should always be a steady CHECK ENGINE light when the ignition is on, and engine is not running. Battery voltage is supplied directly to light bulb. ECM activates CHECK ENGINE light by grounding Purple wire at ECM.

With ALDL diagnostic connector grounded, CHECK ENGINE light should flash Code 12, followed by any fault code(s) stored in memory. A steady light indicates short to ground in the Purple wire, or an open in the Orange/Yellow wire.

1. If a problem in ECM is causing Scan tester not to read serial data (ALDL connector terminal No. 2), then ECM should not flash Code 12. If Code 12 is flashed, ensure Scan tester is working properly on another vehicle. If Scan tester and Orange/Black wire are okay, ECM may be faulty.
2. If CHECK ENGINE light goes off when ECM connector is disconnected, then Purple wire is not shorted to ground.
3. This step checks for an open diagnostic Orange/Yellow wire.
4. CHECK ENGINE light wiring is okay. Problem is a faulty ECM. If Code 12 is not flashed, replace ECM.

Fig 1: Chart A-2 Flow Chart, No ALDL Data Or Won't Flash Code 12 Check Engine Light On Steady

Courtesy of GENERAL MOTORS CORP.

Fig 2: Chart A-2 Schematic, No ALDL Data Or Won't Flash Code 12 Check Engine Light On Steady

Courtesy of GENERAL MOTORS CORP.