A knock sensor is installed on all fuel-injected vehicles. It senses excess vibration, caused by detonation, in the engine. A lean air-to-fuel mixture -- when there is too much air in the mixture -- causes the engine to detonate. The knock sensor sends a signal to the on-board computer, which in turn adjusts the ignition timing to add more fuel to the mixture. If the computer cannot correct the mixture adequately, the knock sensor could be malfunctioning.
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Things you need
- Power graphing meter, such as the Snap-On Vantage
- 3/8-inch extension
Hook the red lead on the power graphing meter to the red wire (which is usually the only wire) on the knock sensor. Hook the black lead on the power graphing meter to a good ground on the engine.
Start the vehicle.
Knock on the block near the knock sensor using the 3/8-inch extension. The graphing pattern should show short spikes when there is no knocking and taller spikes when knocking occurs. If the spike pattern does not change in this way, the knock sensor is suspect.
Check the wiring on the knock sensor. The computer provides a 5-volt reference to the sensor. The sensor will drop the voltage to 2.5 volts. If the wire does not start out with 5 volts, the wiring or the computer is suspect.
Tips and warnings
- If you do not have a power graphing meter, you can perform this test with a voltmeter; however, the test is more accurate with the power graphing meter.