Car knock sensor function
Here you will find useful basic information and important tips relating to knock sensors in vehicles.
Important safety note
The following technical information and practical tips have been compiled by HELLA in order to provide professional support to vehicle workshops in their day-to-day work. The information provided on this website is intended for use by suitably qualified personnel only.
The knock sensor is located on the outside of the engine block. It is intended to record knocking noise in all engine operating states in order to prevent engine damage.
The knock sensor "listens out for" the structure-borne vibrations from the engine block and converts these into electrical voltage signals. The signals are filtered and evaluated in the control unit. The knocking signal is assigned to respective cylinder. If knocking occurs, the ignition signal for the respective cylinder is adjusted in the "late" direction until knocking combustion no longer occurs.
A faulty sensor can manifest itself in different ways through fault detection by the control unit and the resulting emergency program strategy.
Frequent fault symptoms are:
Failure can be caused by different reasons:
Check the wiring to the control unit by checking continuity and short circuit to frame for every wire to the control unit plug.
Connect the ohmmeter between the knock sensor connector and the removed control unit connector. Setpoint: <1 Ohm (circuit diagram required for the pin assignment of the control unit).
Check the respective pin at the wiring harness connector against ground using an ohmmeter and with the control unit connector removed. Setpoint: at least 30 MOhm.
A connection pin can act as shielding and thus have continuity to ground.
Connect the test probes of the oscilloscope between the control unit pin for the knock sensor and ground.
The oscillogram must produce a signal with considerably enlarged amplitude.
If the signal is not clear, tap gently against the engine block near the sensor.
If the knocking is not detected, this indicates a defective sensor or circuit.
Observe the tightening torque during assembly. Do not use spring washers or washers.