Anti-locking Brake System speed sensors are designed to provide information to the on-board vehicle computer. The speed sensor detects the speed of the vehicle and can sense whether or not the brakes are locking. The on-board computer will then provide a signal to the brake system to begin pumping the brakes. The speed sensor is located underneath the wheel arch. The sensors need to be tested each time the vehicle is serviced. You can also test them yourself by following the right steps.
Lift the back of the car up with a car jack, then the car on to jack stands. Position wheel blocks around the tires to the front the car.
Take the rear tires off using a ratchet. You will need to do this to test the speed sensors.
Find the place where the engine's electrical cables come into contact with the sensors' cables.
Take the sensor out of the sensor housing using a quarter-inch ratchet. The sensor looks like a small black box.
Check the sensor and see if it is dusty. Use brake cleaner to remove the excess dirt. Wipe it with a rag and repeat if it is very dirty. Dirty speed sensors can send the wrong information to the on-board computer.
Detach the engine cabling from the sensor cabling. Put the voltage of the meter to "Ohms" and attach the positive and negative terminals of the meter to the sensor. Check the reading on the meter. One of the things you should look for is the signal that states "infinity." This means you will need to get a new sensor.
Change the setting to "Volts" on the meter. If the reading is over 5, then need to get a new sensor. If it just reading 5 or below 5, continue to clean and reinstall the sensor. Bring the car back down and remove the jack stand. Re-install the tires.