How to Test a Wheel Speed Sensorby Chris Stevenson
Most vehicles today have anti-lock brakes. The wheel speed sensor works in conjunction with the anti-lock brake system by interpreting the rotational speed of the tire through a magnetic signal. The wheel speed sensor can tell if a tire stops rotating or locks up, and it sends that signal to the anti-lock brake system so it can relieve pressure and allow the wheel to turn. Wheel speed sensors mount on each wheel to record individual wheel speeds, and activate a signal after the car is traveling at approximately 3 to 5 mph. Since the signal sends an electronic pulse, you can test the wheel speed sensor with a multimeter.
Park the vehicle and turn off the engine with the transmission in "Park" or neutral. Set the emergency brake.
Locate your vehicle's main fuse block. Refer to your owner's manual for its location. Look in the engine compartment, driver's side kick panel or in the glove box. Remove the fuse block lid and locate the ABS fuse. Make sure the filament inside the fuse appears intact; replace if necessary.
Loosen the lugs on all four wheels with a tire iron -- do not remove the lug nuts. Lift the front of the vehicle with a floor jack and place two jack stands under the frame near each wheel. Lift and support the rear of the vehicle in the same way. Finish removing all the lug nuts with the tire iron, then set the wheels aside.
Release the emergency brake and set the gearshift in neutral. Sit down underneath the front wheel well and look for a wire coming from the wheel speed sensor mounted on the rotor, CV joint or wheel hub. It will look like a small plastic box. The wire will lead up through the fender well. Disconnect the wire at the jack by pulling it apart with your fingers. Look at the two-pin connector.
Set a multimeter to measure resistance (ohms). Place each probe of the multimeter on each pin inside the connector. Connect it to the end of the wire that comes from the sensor. Note the ohm reading on the gauge. Have an assistant manually rotate the wheel hub as fast as he can while you hold the probes in place. See if the ohm number changes with the spin of the wheel. Any change in ohms indicates a good connection to the sensor. No change indicates a broken or shorted wheel sensor wire.
Switch the multimeter setting to the volts scale, 10 volts maximum. Insert a flying lead between the two wire connections. Flying leads have extensions that plug into the female side and the opposite male side, with some bare metal exposed, so you can probe both wire sides with the jack connected. Place one lead from the multimeter onto one flying lead, and the other probe to the other flying lead.
Have your assistant turn the ignition key to the "On" position. Look at the voltage reading on the gauge. Normal voltage will be between +5 or +12 volts, depending upon the ABS specifications. Refer to your owner's manual for the exact number. With the key still on, have your assistant rotate the wheel hub again. If you see a voltage change, the wheel speed sensor functions properly. If the voltage does not change you have defective sensor.
Check the rest of the wheel speed sensors on each wheel with this same procedure. Any difference in the ohm or voltage readings on a wheel will indicate either a wire break, short or a defective sensor. Be certain to reconnect all the wheel speed sensor jacks when finished. Mount the wheels on the vehicle and use a tire iron to partially tighten all the lug nuts. Use the floor jack to lift the vehicle and remove the jack stands. Use a torque wrench to tighten all the lug nuts to manufacturer's specifications.
- All of the wheels on your vehicle must be the same size diameter and profile to get consistent and accurate readings from your wheel speed sensor. If the tires are mismatched, each reading for each tire might differ and confuse the ABS signal.
Items you will need
- Owner's repair manual
- Floor jack
- Jack stands
- Tire iron
- Digital Multimeter
- Flying leads
- Torque wrench
- Red cars image by Catabu from Fotolia.com