Powertrain Controls - ECM/PCM

Circuit Description:
Code 12 means the ECM is "ON" and sees no reference pulse from the engine speed sensor. This is normal code with the key "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 an open or ground in the engine speed sensor reference circuit.
The engine speed sensor is a camshaft driven pick-up, mounted at the center rear of the engine.
It is sourced by 5 volt reference and allows the ECM to measure engine rpm by the number of times the voltage is pulsed. The engine speed sensor pulses 4 times per revolution.

Test Description:
Numbers below refer to the circled numbers on the diagnostic chart.
1. Check for a good 5 volt reference. Normally, the ECM should he at about 5 volts for fully charged batteries.
2. Checks for proper ECM voltage to the engine speed sensor. If the circuit to the ECM is complete, normal voltage will be about 5 volts with the harness disconnected from the sensor.
3. Checks for good sensor ground circuit (CKT 452) from sensor to ECM. Since Step 2 indicated an open, the results of this step indicates whether the open is in the wire or at the ECM.