A-2 ~ Data Link Connector (DLC) Diagnosis and MIL "ON"

Chart A-2 Wiring Diagram:

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

CIRCUIT DESCRIPTION
There should always be a steady "CHECK ENGINE" light, when the ignition is "ON" and engine stopped. Ignition voltage is supplied directly to the light bulb. The Electronic Control Module (ECM) will control the light and turn it "ON" by providing a ground path through CKT A5 to the ECM.
With the diagnostic terminal grounded, the light should flash DTC 12, followed by any DTC(s) stored in memory.
A steady light suggests a short to ground in the light control CKT A5 or an open in diagnostic CKT A9


TEST DESCRIPTION
Numbers below refer to circled numbers on the diagnostic chart.
1. If there is a problem with the ECM that causes a "Scan" tool to not read Serial data, then the ECM should not flash a Code 12. If Code 12 does flash, be sure that the "Scan" tool is working properly on another vehicle. If the "Scan" is functioning properly and CKT A8 is OK, the MEM CAL or ECM may be at fault for the NO ALDL symptom.
2. If the light goes "OFF," when the ECM connector is disconnected, then CKT A5 is not shorted to ground.
3. This step will check for an open diagnostic CKT A9.
4. At this point, the "CHECK ENGINE" light wiring is OK. The problem is a faulty ECM or PROM. If Code 12 does not flash, the ECM should be replaced using the original PROM. Replace the PROM only after trying an ECM, as a defective PROM is an unlikely cause of the problem.