Rover Holic

Rover 820 Lucas Ignition

Knock Sensor

• Purpose: To pick-up high frequency vibrations generated by pinking and pass the signal to ignition ECU which retards the spark timing.
• Description: Piezo-Electric vibration transducer can be though of a a sensitive microphone exactly tuned to frequency band of noise generated by pinking.
• Location: On the 16 Valve 820 is located in the centre of the rear side of the cylinder block, can be very difficult to get access to. Single cam carb equipped 820 models may have it located on the front of the block as on some Montego models.
• Output Signal: Series of high frequency low (circa 0.7 V) Voltage peaks, will not show up on normal digital or analogue multimeter.

Connections & Pin Out.

Two signal wires which connect to pins (1) and (3) of the Lucas AB17 Ignition ECU on the engine compartment  bulkhead and a grounded screen to  prevent radio frequency (RF) interference with the output.  On  820 16 valve engines the connection from the sensor to the ignition loom can be difficult to gain access to from above because of the inlet manifold, throttle body and starter however the output signal wires can be easily reached at the Ignition ECU where they are located near the top of the multiplug.  Wire piercing adapters (sold by Tandy) on the test equipment leads greatly simplify making the connection.

Testing The Knock Sensor

Test (1) Reaction of ECU to Knock Sensor Signal

• Warm the engine up fully until the electric cooling fan cuts in and then cuts out again RPM should be stable at the normal 820 idle speed about 950 RPM, the disconnect the idle speed  control valve stepper motor plug.
• Tap the front of the cylinder block for a few seconds near the centre with a metallic object such as socket extension.
• While tapping the block listen for a change in engine note. If you can't hear a change in engine note try connecting a dwell meter or timing light or low range handheld tachometer and repeat the test looking for a change of dwell or retarding of ignition timing or change in idle speed, all  of these indicate the sensor is working and the ECU is responding to the signal.
• Reconnect stepper motor and switch off engine.

If you can't get a pass in the above test check the connections to the sensor also the condition of the wiring and that the sensor is tight in the threaded mounting boss on the block.

Test (2) Checking Sensor Output

For this test an oscilloscope is required although I have had results using a home made  low voltage (0 to 1.0 volt) tester I  use for measuring Lambda sensor output.

• Set the oscilloscope voltage input range on the Y scale to read volt full scale deflection.
• Tap into the two output wires from the sensor, on the 820 this may be easiest  at the point where they enter the plug for the ignition ECU wire piercing adapters on the scope probes.
• With the engine off tap the cylinder block with a socket extension and look for output voltage peaks of about 0.7 Volts.

A pass on Test (2) and Fail on Test (1) indicates an ECU or connection problem, a fail on both Tests (1) & (2) would indicate a sensor failure. Spurious trigering of the retard function of the ECU may be due to RF interence from the HT leads  so remember to check condition and grounding of the shielding on the sensor connecting cable.

Test (3) Sensor Resistance

Because of the very high impedance (30 million  Ohms) of the sensor this test is generally inconclusive, however if an oscilloscope is not available it may be of some diagnostic value if a suitable digital multimeter is available.

• Isolate sensor from Ignition  ECU.
• Check sensor resistance using multimeter.
• Reconnect on completion of test.