For ABS to work, the car must know how fast each wheel is turning. The part that makes that measurement possible is the tone ring. Here is what an ABS tone ring is.
What it does
The tone ring (or reluctor ring) is a toothed or magnetic ring that spins with the wheel. The wheel-speed sensor reads the passing teeth to calculate wheel speed; see wheel-speed sensor and how ABS works. It is the moving half of the sensor system.
Where it lives
The tone ring is often built into the hub bearing assembly or CV joint, or pressed onto the axle - depending on the vehicle; see hub assembly. That is why a hub with an integrated ring is a common replacement part.
Why a bad ring trips the ABS light
If the tone ring is cracked, rusty, or has debris on the teeth, the sensor reads an erratic or missing signal, and the ABS disables itself with a warning light; see ABS warning light. A damaged ring is a common cause of an intermittent ABS fault.
What to do
Inspect the ring for damage or rust when chasing an ABS code; replacing the hub assembly renews an integrated ring. Keep the sensor and ring clean. Keep the base brakes strong with quality pads and hardware.