DTC 12

Code 12 Diagnostic Chart:

Code 12 Circuit Diagram:

CIRCUIT DESCRIPTION:

The ECM receives a signal from the ignition system in the form of a pulse delivered from the tach. filter. This pulse is present when the engine is not running and, in the absence of any other codes, indicates a normal condition. This code will not store in memory and will only flash when the fault is actually happening. The actual fault that could cause Code 12 to flash would be an open or ground in the circuit between the ECM and the ignition coil.

TEST DESCRIPTION: Numbers below refer to circled numbers on the diagnostic chart.

1. Code 12 will flash if:

^ Ignition is "ON."
^ ALDL is grounded.
^ System is functioning normally. Under these conditions a Code 12 is normal and a fault should be suspected if it does not flash. A scan tool should display rpm if the tach. signal is being supplied to the ECM.

2. This determines if signal voltage is present at ECM.

3. The rest of the voltage checks determine if there is a short or open in the circuits between the ignition and the tach. filter.

DIAGNOSTIC AIDS:

Before replacing ECM be sure to check for bent pins or loose connections at the ECM connector. If Code 12 or the symptoms are intermittent, refer to DIAGNOSIS BY SYMPTOM under TESTING PROCEDURES.