How to Test a GM Knock Sensor

by Russell Wood
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The knock sensor on your GM engine is designed to sense detonation in the engine that causes a knocking noise from an improper air/fuel/spark ratio. The sensor takes that information and sends it to the computer, which in turn retards the timing on the engine to fix the problem. If the sensor goes out, then the detonation will continue and could cause problems with the engine. To find out if your knock sensor is broken, you'll need to test it, which should take 10 minutes.

Step 1

Unlock the hood and prop it open. Connect the red clamp on the timing light to the positive terminal on the battery, then connect the black clamp to the negative terminal.

Step 2

Clamp the spark plug clamp around the No. 1 spark plug wire, which on a GM V8 engine is the front right spark plug wire. Turn the vehicle on and let it run.

Step 3

Shine the timing light at the bottom of the engine block and locate the timing indicator. Note where the engine is timed mentally, noting where the mark hits the indicator.

Step 4

Strike the side of the engine with the hammer while shining the timing light on the timing indicator at the bottom of the engine block. Look to see if the timing slows down on the timing light. If it does, then the knock sensor is good. If not, it needs to be replaced.

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