Kelsey-Hayes Antilock Brake Systems

The EBC 325 ABS system uses one wheel speed sensor on each front wheel, and one speed sensor mounted in the rear axle for the rear wheels.

The RWAL II uses only one speed sensor mounted in the rear axle for the rear wheels.

The sensor measures the wheel speed by monitoring a rotating tone wheel. The signal generated by the sensor and tone wheel is transmitted to the CAB.

Each sensor has:

- a metal tone wheel with teeth on its outside diameter
- a magnetic/coil pick-up (speed sensor) that is mounted to a fixed component
- an air gap between the tone wheel and the speed sensor assembly

As the teeth of the tone wheel move through the magnetic field of the sensor, an AC voltage is generated. This signal frequency increases or decreases proportionally to the speed of the wheel. The CAB monitors this signal to check for a sudden change in single or multiple wheel decelerations. If the deceleration of one or more wheels is not within a predetermined amount, the antilock module takes control.

Diagnostically, the coil of wheel speed sensors have different amounts of resistance based upon the sensor type. When measured across the connector two terminals, the resistance should be:
AN, DN, BE and BR:
4x4 front sensors 945-1155 ohms
AB, AN and DN:
4x2 front sensors 2500-2955 ohms
BE and BR:
4x2 front sensors 1000-1250 ohms
AB, AN, DN, BE and BR:
All rear sensors 1600-2300 ohms

NOTE: For all resistance ranges add 30% to the value for extreme heat, subtract 30% for extreme cold.)

The rear wheel speed sensor used on all vehicles have the same resistance value, but there are two different sensors. Therefore, it is important to replace the rear wheel speed sensor with the proper part.

On a EBC 325 system each front wheel speed is monitored through the speed sensor mounted at the wheel end of the hub. On a EBC 325 and RWAL II system the rear wheel speed is monitored through the speed sensor mounted in the rear axle assembly.

The CAB will disable antilock control, illuminate the ABS warning light circuit via COD, and store trouble codes if it detects a problem with any or all of the wheel speed sensors:

- incorrect circuit resistance when checked with no vehicle movement
- incorrect sensor output during vehicle movement
- erratic sensor output during vehicle movement