Hella Ireland advises on the functions of the Knock sensor, which is found on outside of the engine block and is used to record knocking sounds in the engine during all operating states in order to avoid engine damage.
The knock sensor “monitors” the structure-borne vibrations on the engine block and transforms these into electrical voltage signals. These are filtered and evaluated in the control unit. The knock signal is assigned to the respective cylinder. If knocking occurs, the ignitnion signal for the respective cylinder is retarded as far as necessary until knocking combustion ceases.
Effects of failure:
A sensor can become noticeable in different ways through the fault recognition of the control unit and the resulting emergency running strategy.
Frequent fault symptoms are:
Engine warning light comes on
Fault code is stored
Reduced engine performance
Increased fuel consumption
There can be a number of reasons for sensor failure:Internal short circuits
Cable short circuit
Read out the fault code
Check correct fit and torque of the sensor
Check elecrtrocla connections of the senro cable, the connector and the sensor for correct connection , breaks and corrosion.
Check the ignition timing (older vehicles)Testing with a multimeter
Check the wiring to the control unit by checking every single wire to the control unit connector for transmission and connection to ground.
1. Connect the ohmmeter between the knock sensor connector and the removed control unit connector. Ref: valve <1 ohm (Fig.1) (circuit diagram for the pin allocation of the control unit is necessary)
2. Use the ohmmeter to test the respective pin at the wiring harness connector and removed control unit connector to ground. Ref: value: at least 30 MOhm.
Note: A connection pin can serve as a shield and show a transmission to ground.
Testing using the oscilloscope with the engine hot
1. Connect the test probes of the oscilloscope between the control unit pin for the knock sensor and ground.
2. Briefly open the throttle valve. The oscillogram must show a signal with a considerably increased amplitude (Fig. 2)
3. If the signal is not absolutely clear, knock lightly against the engine block near the sensor.
4. If the signal is still not detected this is an indication of a faulty sensor or circuit.
Refer to manufacturer’s torque setting during installation. Do not use spring washer or any other washers.
(Images: Fig 2 – Knock Sensor Optimum image, Live image knock sensor OK, Live image knock sensor with fault)