Knock Sensor: Testing and Inspection

Chart C-5 - Electronic Spark Control (ESC) Check:

Wiring Diagram for Chart C-5 - Electronic Spark Control (ESC) Check:

CHART C-5 - ELECTRONIC SPARK CONTROL (ESC) CHECK

Circuit Description:


If the timing is retarded at idle, it may be due to ESC operating. ESC should not operate unless a knock is present.


Test Description: Step numbers refer to step numbers on diagnostic chart.

1. This is the ESC functional check. The ECM must be monitoring mph for ESC to function, so check if code 24 is stored. With engine running above 850 RPM and drive wheels at least 20 mph, simulate an engine knock by tapping the engine block. This should normally cause an RPM drop (decrease timing), If it doesn't drop, either the timing is not retarding or is retarded all the time.

2. This should cause full retard by dropping the voltage at ECM term. "L." Retarded timing should cause an RPM drop.

3. Normally, voltage should be .08 volts AC or higher for a good knock sensor circuit.

4. "Service Engine Soon" light should be "ON" and a Code 43 set because ESC system would be retarded too long. If no light comes on, the ECM is not retarding the spark because of a voltage on CKT. 457 to terminal "L" or the ECM is faulty.


5. Checks to see if knock sensor is reason for retard signal. If engine knock is not present, and timing increases when knock sensor is disconnected, fault is an over sensitive knock sensor. Timing should not normally increase.


6. Checks to see if retard signal is due to "noise" on signal wire or faulty controller. If timing increases when wire is disconnected from controller, fault is due to knock sensor signal wire running too close to an ignition or charging system wire. Reroute wire to correct.