Tone Ring

A tone ring, also known as an ABS ring or reluctor ring, is a circular metal ring with evenly spaced teeth or notches. It is pressed onto a rotating component of the drivetrain, typically the CV axle shaft, drive axle, or the wheel hub assembly. The tone ring works in tandem with the anti-lock braking system (ABS) wheel speed sensor.

As the wheel rotates, the tone ring spins. The ABS sensor, which is positioned just millimeters away from the ring's teeth, uses electromagnetism to monitor the passing teeth. Each tooth that passes the sensor generates an electrical pulse, creating a frequency that the ABS control module uses to calculate the exact rotational speed of the wheel.

Tone rings operate in a harsh environment under the vehicle, making them susceptible to rust, dirt buildup, and cracking. If a tone ring cracks or loses a tooth due to road debris, the ABS sensor will receive an erratic speed signal. This causes the ABS computer to believe a wheel is slipping or locked up when it isn't, leading to unwanted ABS activation during normal braking or disabling the ABS system entirely.

Symptoms of a bad tone ring include an illuminated ABS warning light, traction control malfunctions, or a pulsating brake pedal at slow speeds. Depending on the vehicle, a damaged tone ring can sometimes be replaced individually, but on many modern vehicles, it is integrated into the CV axle or wheel hub assembly, requiring replacement of the complete unit.