Knock Sensor Test using scope

[scott.john.mockry@bangate1.tek.com]

Mike, Piotr

If you have an oscilloscope and want to test the knock sensor, disconnect the sensor 
at the firewall connector and connect your scope to the two signal terminals and set 
the scope to 500us/div and 50mV or 100mV per division. Tap on the block near the 
sensor and you should see a damped sine wave produced. If you have a digital storage
 scope use the single shot trigger mode to capture the event. Set the trigger level to 
be around 25-50mV.

The service bulletins mention that the newer knock sensors come with "gold" plated
terminals. If you replace the sensor you are also supposed to replace the 3 terminals
in the connector with gold plated ones (cheap around $1 each).

My 89 200TQ had a trouble code show up for one of the knock sensors which was caused
by an intermittent connection inside the Knock sensor. The voltage output would break up
(discontinuites) when I tested it with a scope and tapped on the block. I replaced the sensor
(came with gold plated terminals) along with the connector terminals and have not had
any problems since. The ECU will back off the timing on the cylinders monitored by the defective
knock sensor and this can reduce your performance/gas mileage and cause some hesitations
and rough running. Someone removed the "check engine light" bulb on my 89 200TQ and 
I wasn't even aware that I had a defective knock sensor until a friend with a VAG1552
scan tool found the problem.

Hope this helps
Scott M.