How to Know If a Crankshaft Position Sensor Is Bad?

by Richard Rowe

Crankshaft position sensors are essentially super-accurate tachometers, responsible not only for telling the computer how fast the engine spins, but also exactly where the crankshaft is positioned in its rotation. The computer uses this information to determine when to inject fuel or trigger the spark plugs; because of this, a bad CPS can manifest in any number of systems. Your best bet is to skip the guess-work altogether and test the sensor directly.

Step 1

Test drive the vehicle to get an idea as to what's going on with the engine. A lightly-malfunctioning CPS will cause a slight loss in power and fuel economy, as well as some intermittent stuttering under acceleration. A badly-worn CPS will cause multiple misfires, bucking and surging, and will ultimately manifest a complete refusal to start or idle.

Step 2

Go to your local Advance, Pep Boys, O'Reilly, Jiffy Lube, Napa or other large auto parts chain store and have them read your vehicle's computer codes. A bad CPS will always trigger a "Check engine" light, and any chain parts store will have a scanner available to read the codes stored in the vehicle's computer. Some stores charge for this service and others do not.

Step 3

Locate the CPS. You'll need a repair manual to get the CPS sensor's voltage anyway, so refer to the reference material to find your CPS. Follow the wiring harness connected to the sensor to a location on top of the engine where you can access it while cranking the motor. Unplug the wiring harnesses going into your ignition coils to keep the car from starting while you crank it over.

Step 4

Set your digital multimeter to read in AC volts and plug the needle-probe wires into the meter. Poke the negative (black) needle probe into the darker of the two wires coming out of the CPS; plug the red needle probe into the lighter of the two wires. Turn the multimeter on and have an assistant crank the engine as though attempting to start it.

Step 5

Observe the reading on the digital multimeter and compare it to your reference material. The reading should fluctuate by the readings specified in your reference material; typical output will range from 0.2 volts on the low reading to 1.5 volts on the high side. If the oscillations are irregular or you don't get a voltage reading, then the sensor is bad.

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