Knock Sensor: Testing and Inspection

Functional Check:

The Tech 1 has several positions to check for diagnosing ESC circuit. "Knock signal" is used to monitor the input signal form the knock sensor. This position should display "YES" to indicate when a knock is being detected. "Knock retard" is the indication of how much the ECM is retarding the spark. "ESC Sensors" is an indication of an open or grounded knock sensor circuit.

ESC System Test:

Knock Sensor Circuit:

Circuit Description
The knock sensor system is used to detect engine detonation. The ECM will retard the spark timing based on a signal being received from the knock sensor. The knock sensors produce an AC voltage which is sent to the ECM. The amount of AC voltage produced by the sensors is determined by the amount of knock. The circuitry within each knock sensor causes the ECM's 5 volts to be pulled down so that under a no knock condition, CKT 496 would measure about 1.5 volts.

ESC System Check Chart C-5:

Test Description
Numbers below refer to circled numbers on the diagnostic chart.

1. With engine idling, there should not be a knock signal present ant the ECM, because detonation is not likely under no load condition.

2. Tapping on the engines right or left exhaust manifold should simulate a knock signal to determine if the sensors are capable of detecting detonation. If no knock is detected, try tapping on engine block closer to sensors before replacing a sensor.

3. If the engine has an internal problem which is creating a knock, the knock sensor may be responding to the internal failure.

4. This test determines if the knock sensor is faulty or if the ESC portion of the MEM-CAL is faulty. If it is determined that the MEM-CAL is faulty, be sure that it is properly installed and latched into place. If not properly install, repair and retest.

Diagnostic Aids
While observing knock signal on the Tech 1, there should be an indication that knock is present when detonation can be heard. Detonation is most likely to occur under high engine load conditions.