Code 12, No Rpm Signal-Vehicle Standing
Code 12 means ECM is on and sees no RPM signal from the alternator. This is a normal code with ignition "ON" and engine not running. Code 12 is not stored and will only flash when fault is present. With engine running, Code 12 could mean either an open or ground in tach signal circuit. Code 41 will appear if engine is running with no RPM signal and vehicle is moving. Code 41 will store, even if the problem clears.
NOTE:
The following step numbers refer to the numbers in the accompanying flow chart(s).
- This step checks for voltage output at alternator "R" terminal which is source of RPM signal. Check for voltage increase as engine speed is increased.
- This step determines if a RPM signal is being sent to ECM. The voltage reading should be approximately the same as that noted in step 1). A voltage increase indicates signal is being generated by the alternator and fault is a bad connection at the ECM, or faulty ECM. To check ECM connection, the terminal must be removed from connector.
- With an open circuit, there is still a small amount of voltage at the ECM. It will not increase when throttle is opened.