Anti-lock brake systems. Traction control. Electronic stability control. Great safety enhancements, but none of them could exist without wheel speed sensors. Even variable assist power steering and variable ride height systems rely in part on information from the wheel speed sensors.
There are two basic types of wheel speed sensors, the magnetic sensors and the Hall Effect sensors which have become more popular in the past few years.
Since vehicles use CAN (controller area network) multiplexing, the wheel speed sensor information is often integrated with the vehicle speed sensor information to control various systems.
Along with an accelerometer input, the wheel speed information is essential to stability control. Even some vehicles without anti-lock brake systems (ABS) will use wheel speed sensors as the vehicle speed input for the engine control module and transmission control module.
The traditional wheel speed sensor consists of a tone wheel (sometimes called a trigger wheel) plus a magnetic pickup. Inside the sensor itself is a permanent magnet surrounded by a coil of copper wire.
The magnetic field changes as the teeth of the tone wheel pass, creating an alternating voltage (AC) signal the changes in both frequency and strength based on speed. It is a typical sine wave. The sensor has two wires: ground and signal.
This type of wheel speed sensor is sometimes called a passive sensor because there is no power going to it.
A magnetic wheel speed sensor does not start working until the vehicle is travelling about 5 mph or faster.
The tone wheel may be mounted on the axle, the brake rotor, the drum or the constant velocity joint.
The sensor is stationary. The gap between the two is critical and, although some sensors may be adjusted, most can’t. You need a non-ferrous (brass) feeler gauge to measure the gap. If it is out of spec, the tone wheel or wheel speed sensor usually need replacement. A worn, cracked or warped tone ring won’t produce a usable signal.
Even a small chip in the tone wheel can cause problems so you should not use a pry bar or hammer to remove a half-shaft from a steering knuckle.
Despite the proper gap, signals from the sensors will be incorrect if the wheel bearings are worn or loose. The ABS module needs both signal amplitude and frequency to determine if brake application is necessary. Even a small difference can affect ABS operation, particularly at slower speeds, usually below 10 mph.
This is when false activation of the ABS system usually occurs. Since the system interprets the inputs from each sensor individually, any difference may be interpreted as wheel slip and that will trigger the ABS to kick in, despite the fact that the wheel is not slipping. False activation can also be caused by erratic, inconsistent signals such as those due to worn wheel bearing movement.