Code 12 No Reference Pulse
Code 12 indicates that the ECM is on and sees no reference pulse from the engine speed sensor. Code display is normal 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 an open or ground in engine speed sensor reference circuit.
The engine speed sensor is a camshaft driven pick-up, mounted at center rear of engine. Sensor is sourced by a 5-volt reference voltage and allows the ECM to measure engine RPM by the number of times voltage is pulsed. The engine speed sensor pulses 4 times per revolution.
NOTE:
Test numbers refer to test numbers on diagnostic chart.
- This test checks for a good 5-volt reference. Normally, the ECM should be at about 5 volts for fully charged batteries.
- This test 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 harness disconnected from sensor.
- This test checks for good sensor ground circuit No. 452, from sensor to ECM. Since test 2) indicated an open, the results of this test indicates whether the open is in the wire or at the ECM.