Car knock sensor — function and troubleshooting

The knock sensor monitors the combustion process in the engine. Its signal helps the engine control to prevent knocking combustion and therefore protect the motor/engine control. On this page we will provide you with various information, including how a defective knock sensor becomes noticeable, the causes that may lead to its failure, and how it can be tested in the workshop.

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 suitably qualified personnel only.

Car knock sensor function

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.

Check the knock sensor

TROUBLESHOOTING:

Read out the stored fault code ✓
Check the sensor for correct fit and tightening torque ✓
Check the electrical connections of the sensor wiring, the plug, and the sensor for correct connection, breaks, and corrosion ✓
Check the ignition point (older vehicles) ✓

Testing with the multimeter

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

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 knock sensor: Setpoint — Setpoint: <1 Ohm

Check the pin against ground using an ohmmeter

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.

Attention:

A connection pin can act as shielding and thus have continuity to ground.

Check with the oscilloscope when the engine is warm

Connect the test probes of the oscilloscope

Connect the test probes of the oscilloscope between the control unit pin for the knock sensor and ground.

Briefly open the throttle valve

The oscillogram must produce a signal with considerably enlarged amplitude.

Signal not clear

If the signal is not clear, tap gently against the engine block near the sensor.

Knocking is not detected

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.

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