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.
Knock Sensor connections
Vehicle Wiring Colour Code
|
Ignition ECU Pin Number
|
Light Green /
Slate Grey trace
|
1
|
Slate Grey
|
3
|
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.
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