Rick's Free Auto Repair Advice

Tone ring

What is a tone ring?

A tone ring, also called a reluctor ring, is a notched or toothed ring that’s used in conjunction with a sensor to determine the number of revolutions or speed of revolutions.

Where are tone rings used?

A tone ring can be used in an engine to help the computer determine the exact location of the crankshaft and camshaft. In those cases, the tone ring will have a timing notch that’s used to indicate that the shaft has completed a full revolution.

tone ring, reluctor



Tone rings are also used in anti-lock brake systems (ABS) to detect wheel rotation rate. ABS systems do not need to know the exact location of the shaft, just the speed of rotation, so there are no notches on an ABS tone ring.

Picture of new ABS tone ring

Tone ring for ABS brakes. No notches or gaps

Tone rings can be internal or external

Crankshaft and camshaft tone rings can be located on the end of the shaft and exposed to external elements or contained inside the engine.

relutor ring on crankshaft

Reluctor ring built into the crankshaft and operates inside the engine


ABS tone rings can be mounted on the drive shaft, in the wheel hub or inside the wheel bearing


tone ring

Tone ring mounted on the harmonic balancer at the the front of the engine and exposed to the elements

How a tone ring and sensor works

Passive sensor

A passive sensor consists of a coil of wire wrapped around a magnet. The wiring harness has  2-conductors; one for a reference voltage and another for ground to the coil wrapped around the magnetic core. It is placed in close proximity to the tone wheel. As the steel or iron tone ring

Jeep wheel speed sensor

Wheel speed sensor

rotates past the sensor, each tooth on the tone ring disturbs the magnetic field while also generating an A/C voltage in the coil. The voltage increases as the tooth approaches the magnet and drops to zero as the tooth aligns directly with the magnet. Then, as the tooth passes the magnet, it induces a negative voltage in the magnetic coil. A passive sensor can be tested with a DVOM meter for resistance and AC voltage

Active sensors

An active sensor still detects the movement of a magnet by either blocking the magnetic field with a rotating vane style tone ring or by detecting the change in North/South poles of the magnet. These sensors don’t generate voltage like passive sensors. Instead they require power to the semiconductor chips and report either a digital or analog signal.

Posted on by Rick Muscoplat


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