Volvo 240/740/760/780/940/960 1990-1998

Knock Sensor



The operation of the Knock Sensor (KS) is to monitor preignition or engine knocks and send the signal to the ECM. The ECM responds by adjusting ignition timing until the knocks stop. The sensor works by generating a signal produced by the frequency of the knock as recorded by the piezoelectric ceramic disc inside the KS. The disc absorbs the shock waves from the knocks and exerts a pressure on the metal diaphragm inside the KS. This compresses the crystals inside the disc and the disc generates a voltage signal proportional to the frequency of the knocks ranging from zero to 1 volt.


There is real no tests for this sensor, the sensor produces its own signal based on information gathered while the engine is running. The sensors also are usually inaccessible without major component removal. The sensors can be monitored with an appropriate scan tool using a data display or other data stream information. Follow the instructions included with the scan tool for information on accessing the data. The only test available is to test the continuity of the harness from the ECM to the sensor.


See Figure 1

The knock sensors are usually inaccessible without the removal of several components. It is best to locate the knock sensors and figure out what is required to be removed, then proceed to the appropriate section for removal of that component.

  1. Disconnect the negative battery cable.
  3. Remove any components necessary to access the sensor(s).
  5. Unplug the sensor(s).
  7. Remove the sensor-to-engine block retaining bolt.
  9. Remove the knock sensor(s).

To install:
  1. Place the sensor(s) into position and tighten the retaining bolt(s).
  3. Plug the sensor(s) connector in.
  5. Install any components removed for access.
  7. Connect the negative battery cable.

Click image to see an enlarged view

Fig. Fig. 1: Knock sensors are located on the engine block under the intake manifold