Knock sensor - How It Works
General points
The knock sensor is on the outside of the engine block. It is used to record knocking sounds in the engine during all operating states in order to avoid engine damage.
Function
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 ignition 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
■ Interrupted cables
■ Cable short-circuit
■ Mechanical damage
■ Faulty attachment
■ Corrosion
Troubleshooting
■ Read out the fault code
■ Check correct fit and torque of the sensor
■ Check electrical connections of the sensor cables, the connector and the sensor for correct connection, breaks and corrosion.
■ Check the ignition timing (older vehicles)
Testing with the 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. value: <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
.
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. Installation note Refer to manufacturer’s torque setting during installation. Do not use spring washers or any other washers.
The knock sensor is on the outside of the engine block. It is used to record knocking sounds in the engine during all operating states in order to avoid engine damage.
Function
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
■ Interrupted cables
■ Cable short-circuit
■ Mechanical damage
■ Faulty attachment
■ Corrosion
Troubleshooting
■ Read out the fault code
■ Check correct fit and torque of the sensor
■ Check electrical connections of the sensor cables, the connector and the sensor for correct connection, breaks and corrosion.
■ Check the ignition timing (older vehicles)
Testing with the multimeter
1. Connect the ohmmeter between the knock sensor connector and the removed control unit connector. Ref. value: <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
.
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. Installation note Refer to manufacturer’s torque setting during installation. Do not use spring washers or any other washers.
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