What is the ABS Speed Sensor all about?
Most modern vehicles have anti-lock brake systems that use sensors to detect wheel rotation rates, reporting to a computer that is able to modulate brake pressure if one or more wheels should lock during a braking operation. Usually the sensor is a Halleffect, or so-called reluctance component, which looks at a toothed ring (called a tone ring) that is affixed to the wheel or axle. The teeth passing by the sensor creates a wave form that can be recognized and interpreted by the ABS controller. On three-channel ABS systems (typically found on older pickups), the rear axle is monitored by one sensor while the front wheels have their own individual sensors and anti-lock control. Four-channel systems monitor each wheel independently.
Keep in mind:
If one or more ABS speed sensors fail, the entire system defaults to a non-operational status, providing no anti-lock control. The diagnostic system will reveal to the mechanic which sensors (if more than one) have failed.
If a sensor fails, the ABS warning light will illuminate every time you start the car, and stay lit.
If you lose ABS intervention, you may lock up the brakes in an emergency, increasing stopping distance and losing all steering ability.
How it's done:
- The vehicle is raised and supported, then the tire is removed
- The defective ABS Sensor is replaced
- The tire is reinstalled and the vehicle is lowered
- The ABS system is rechecked for any codes
- The vehicle is road tested for proper operation
Have a mechanic inspect the vehicle's brake system and replace any wheel speed sensors that are defective. The difference between a vehicle with and without operational ABS is potentially a matter of life and death. Your brakes will still operate, but the chances of a lockup on slippery surfaces or in an emergency – without ABS – are pretty much 100 percent.
What common symptoms indicate you may need to replace the ABS Speed Sensor?
- The ABS warning light will illuminate when the car is started, and it will stay on permanently.
- If you test the brakes on gravel or ice, you will no longer feel or hear the ABS mechanisms kick in, and the brakes will lock up.
How important is this service?
Functional ABS is the difference between being able to simultaneously brake and steer past an obstacle, or locking up and hitting it. It's considered important enough for insurance companies to offer discounts to owners of cars equipped with ABS systems. With a failed speed sensor, you're driving around with sophisticated electronic control systems, relays and sensors that could save your life. If only they worked.