Technical Forum (Read Only) > Computers, Fuel and Emissions

No ALDL Data or Won't Flash Code 12 "Service Engine Soon" Light On CHART A-2 TBI



Circuit Description :
There should always be a steady "Service Engine Soon" light when the ignition is "ON" and engine stopped. Battery ignition voltage is supplied to the light bulb. The ECM will turn the light on by grounding CKT 419.

With the diagnostic terminal grounded, the light should flash a Code 12, followed by any trouble code(s) stored in memory.

A steady light suggests a short to ground in the light control CKT 419, or an open in diagnostic CKT 451.

Test Description : Numbers below refer to circled numbers on the diagnostic chart.
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 461 is OK, the PROM/Mem-Cal or ECM may be at fault for the NO ALDL symptom.
If the light goes "OFF" when the ECM connector is disconnected, then CKT 419 is not shorted to ground.
This step will check for an open diagnostic CKT 451.
At this point the "Service Engine Soon" light wiring is OK. The problem is a faulty ECM or PROM/Mem-Cal. If Code 12 does not flash, the ECM should be replaced using the original PROM/Mem-Cal. Replace the PROM/Mem-Cal only after trying an ECM, as a defective PROM/Mem-Cal is an unlikely cause of the problem.


[0] Message Index

Go to full version