DTC 12

Fig. 012 - WIRING DIAGRAM for Code 12 No RPM Signal. VIN N, VIN T:

CODE 12

NO RPM SIGNAL-VEHICLE STANDING

4.3L VIN "T"

5.7L VIN "N"

Code 12 means the ECM is "ON" and sees no RPM signal from generator. This is a normal code with the ignition "ON" and engine not running. Code 12 is not stored and will only flash when the fault is present. With the engine running, Code 12 could mean either an open or ground in the tach signal circuit. Code 41 will appear if the engine is running with no RPM signal and vehicle is moving. Code 41 will store, even if the problem clears.

Fig. 011 - Chart Code 12 No RPM Signal.:

1. This step checks for voltage output at the generator "R" terminal which is the source of the RPM sginal. Check for voltage increase as engine speed is increased.
2. This step determines if a RPM signal is being sent to the ECM. Voltage reading should be approximately the same as was noted in step #1.

A voltage increase indicates the signal is being generated by the generator and fault is a bad connection at the ECM, or faulty ECM. To check the connection at the ECM, the terminal must be removed from the connector.

3. With an open circuit, there is still a small amount of voltage at the ECM. It will not increase when the throttle is opened.