ABS Speed Sensor MaintenanceA clean sensor is a happy sensor
One of the most common omissions in maintaining a modern Anti-lock Brake System (ABS) is checking out the speed sensors. These sensors are often located at each wheel hub/rotor on four-wheel disc systems. Alternatively, on rear-drum applications, you'll usually find them mounted on the differential housing. Whatever their location, they supply information regarding wheel speed to the ABS computer. Under braking, if wheel speeds are not equal, indicating wheel lockup, the computer modulates the anti-lock feature until the speeds are equal again.
The sensors function on the principle of magnetic induction. In this case, the sensor consists of a magnet mounted in close proximity to a notched "reference ring" that spins at wheel speed causing a "sine wave" (a low-voltage rhythmic pulse) to be generated by the magnet. The computer reads this signal as wheel speed. There are basically two types of sensor/ring designs: "exposed" and "concealed." In the exposed design, the sensor and ring are, not surprisingly, exposed to the elements, most notably metallic brake dust. That's important because, as explained, the sensors are magnets that attract metallic media, and the reference rings have ridges or slots, which can become clogged with this same road grime.
Of course, at some point this process will affect the operation of the system and cause the ABS Malfunction Indicator Lamp (MIL) to illuminate, indicating that the anti-lock feature is not armed-not desirable and not as safe either. In contrast, on the concealed design, buildup of road grime is not really an issue, so it will probably not need to be cleaned and serviced nearly as regularly as the exposed design. Even though the concealed type is subject to less contamination, it wouldn't hurt to inspect it when servicing wheel bearings or during differential fluid service (depending on the location of the sensor). Any accumulation of debris is serious, as it would likely indicate a deterioration of some normally lubricated component.
How do you determine which type of sensor is on your vehicle? If possible, remove the brake rotor and inspect the backside (the one closest to the center of the vehicle), and the backing plate/splash shield assembly (attached to the steering knuckle). If the sensor is mounted somewhere on the backing plate/splash shield, and the reference ring is visibly outside of the wheel bearing grease seal, you have the exposed type.
On FWD/4WD captive-rotor models, you must remove the sensor retaining bolt from the backside of the backing plate/splash shield. Then remove the sensor from it's mounting bore, and inspect for media presence on the sensor tip, and with a flashlight through the mounting bore to view the reference ring. Exposed sensor/ring types should be serviced at every brake pad replacement at the minimum, or every 30,000 miles. The concealed type would not need regular inspection, except when servicing an adjacent component, as noted above.
For cleaning, once you've removed the rotor (or sensor), you're already half of the way there. Simply wipe off the grime with a degreaser-spritzed shop towel. Do not, under any circumstances, attempt to blow the debris off with compressed air! Static voltage can be generated, and then conducted to the ABS computer, which can damage it. Of course, you could disconnect the sensor to prevent this from happening, but why complicate things?
To clean the reference ring, use a small non-metallic brush and a squirt of degreaser. Make sure to go all of the way around the ring, and be thorough about it. If the rotor has been removed from the vehicle, its okay to carefully blow-dry the ring with compressed air. The ring does not need to be hospital-clean, just free of major debris and metal media, especially in the "troughs."
On captive rotor systems, cleaning the sensor is the same, but you'll have to work through its mounting hole to clean the reference ring, which will take some patience and, of course, a brush small enough to fit easily through the hole. It's also okay to use carefully applied compressed air to the ring as you turn the rotor/hub assembly. Now you can finish the rest of your vehicle's brake work, with the comfort of knowing that you've attended to a very important-and often neglected-aspect of your braking system.
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