When a wheel speed sensor (WSS) fails or there's a problem in the sensor's wiring circuit, it usually disables the ABS system and causes the ABS warning light to come on. Loss of a wheel speed signal is a serious problem because the ABS module needs accurate input from all its sensors to determine whether or not a wheel is locking up. Without this vital information, the ABS system can't do its thing.
Wheel speed sensors produce an alternating current (AC) output voltage that varies in frequency and amplitude with wheel speed. The faster the wheel turns, the greater the frequency and amplitude of the sensor's output signal. The strength of the signal can be affected by resistance in the sensor, resistance in the wiring and connectors, metallic debris on the end of the sensor, and the air gap between the sensor and tone ring mounted on the axle, hub, brake rotor, drum or CV joint.
A narrow air gap is usually necessary to induce a strong signal in the sensor's magnetic windings. Air gaps typically range from .016 in. to as much as .050 in. (0.40 to 1.3 mm) depending on the application. If the ABS warning light is on and you find a code for a wheel speed sensor (and the sensor is adjustable), the problem may be nothing more than too wide an air gap. Use a brass or nonmagnetic feeler gauge set the gap to the factory recommended specs.
Variations in the air gap can also cause fluctuations in the sensor's output signal. These may be caused by bad wheel bearings or missing, broken or chipped teeth on a tone ring. Even minor damage to the tone ring that's nearly impossible to see can sometimes cause a problem. One equipment supplier makes a test bench with a magnetic pickup and oscilloscope to check tone rings on remanufactured FWD axle shafts. The setup simulates the signal produced in a wheel speed sensor. Tests have shown that a variation in height of only about .010 inch on a single tooth can cause a noticeable fluctuation in the sensor's output signal!
One way to check a suspicious wheel speed sensor is to measure its output voltage. One way to do this is to plug a breakout box into the ABS module's wiring harness and attach the test leads from a digital volt ohm meter (DVOM) to the appropriate pins for the WSS circuit.
A good wheel speed sensor will generally produce an alternating current (AC) voltage reading of 50 to 700 MV when the wheel is spun by hand at about one revolution per second. Refer to a shop manual for the sensor's exact voltage specifications.
A low voltage reading or no reading calls for a direct measurement of the resistance in the WSS circuit (with the key off). This should be done through the breakout box to check the entire circuit. A good wheel speed sensor and circuit will typically have a resistance of 800 to 1400 ohms (specs vary, so refer to a manual for the exact numbers).
If the sensor circuit has too much resistance, reads open or is shorted (little or no resistance), measure the resistance across the sensor itself. If the sensor itself reads within specs, the problem is in the wiring or connectors. If not, then you have identified a bad sensor that needs to be replaced.
This isn't something a do-it-yourselfer can do, but a dynamic check of a sensor's output with an oscilloscope is one of the best ways to analyze the performance of a wheel speed sensor. The waveform on the scope can reveal problems that might not be detectable by other means. A damaged tooth on a tone ring, for example, may not produce a noticeable change in the sensor's output voltage if you are reading the output with a DVOM or an analog voltmeter. But it may distort the waveform enough to upset the operation of the ABS system and set a fault code.
The scope connection can either be made through the breakout box or hooked directly to the wheel speed sensor. A "good" scope pattern should show a sine wave alternating current pattern that changes both in frequency and amplitude with wheel speed. Spinning the wheel faster should cause both frequency and amplitude to increase.
If the scope pattern is flattened (diminished amplitude) or is erratic, it usually indicates a weak signal caused by an excessively wide air gap between the tip of the sensor and its ring, or a buildup of metallic debris on the end of the sensor. A weak signal can also be caused by internal resistance in the sensor or its wiring circuit, or loose or corroded wiring connectors.
Damaged or missing teeth on the sensor ring will show up as flat spots or gaps in the sine wave pattern. A bent axle or hub will produce an undulating pattern that changes as the strength of the sensor signal changes with every revolution.
Something else that can be detected with a scope is mismatched parts. If the brake rotor, CV joint or axle has been recently replaced, and the new part did not have the correct number of teeth on the sensor ring, it will cause the sensor to read fast or slow compared to the others and set a fault code. Comparing the sensor patterns side-to-side will reveal this kind of problem.
On some late model vehicles (such as Jeep, Chrysler and others), a different type of wheel speed sensor is used. Active wheel speed sensors (also called Magnetic Resistance Element or MRE sensors) use a Hall effect sensor to generate a digital speed signal with a square wave pattern. This type of sensor can generate a more accurate wheel speed signal at low speeds (down to 1 mph) than a magnetic WSS. It can also tell if the wheel is rotating forwards of backwards.
With this type of sensor, the body control module or ABS module provides a reference voltage to power the sensor. Inside the sensor is a pair of pickups that detect changes in the magnetic field of a magnetic ring on the wheel bearing. The ring has permanent magnets that alternate north and south poles. When the wheel rotates, the changing magnetic field produces a small electric current in the sensor pole pieces. The internal electronics in the sensor then converts the voltages into a digital output signal that goes back to the control module. The voltage, amperage and frequency of the return signal is proportional to wheel speed.
Some of these sensors have three wires while others have only two. On the three-wire sensors, one wire is for power, one for ground and one for the return signal. On the two-wire sensors, one is the power and the other is ground. The return signal travels back to the module over the power supply wire. On Jeep and Chrysler applications with two-wire sensors, the return signal is a 7 to 14 milliamp square wave signal. You should be able to see the signal by SLOWLY rotating the wheel while backprobing the return circuit with a multimeter set to read milliamps or a digital scope. No signal means a bad sensor or wiring fault.
NOTE: Active wheel speed sensors cannot be accurately tested by measuring resistance across the sensor's terminals. You have to look for a good return signal from the sensor while it is receiving power from the control module or a 12-volt battery, and while the wheel is turning.
If you have a scan tool that can read ABS codes (not just engine/transmission codes), you can also check for wheel speed sensor codes on your vehicle. A scan tool that can also display the speed readings from each sensor while driving can also help you pinpoint which sensor is bad. All the sensors should read the same while driving straight ahead. If one is showing a different speed (faster or slower than the others), there is a problem in that sensor circuit (either a bad sensor, a faulty magnetic sensor ring, or a wiring fault). The sensor may be misreading wheel speed if the magnetic ring on the wheel bearing is cracked or damaged.