Wheel Speed Sensors

WHEEL SPEED SENSORS
The EBC 125 (RWAL) system uses only one speed sensor mounted in the rear axle for the rear wheels.

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 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 antimagnetic 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, and 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 CAB takes control for Antilock action through the HCU.

Diagnostically, the coils of the wheel speed sensors have different amounts of resistance based upon the location. When measured across the connector two terminals, the resistance should be:

Front sensor:
90 °C (194 °F) 2259 - 2761 ohms
25 °C (77 °F) 1800 - 2200 ohms
-40 °C (-40 °F) 1332 - 1628 ohms

Rear sensor:
90 °C (194 °F) 2900 - 3500 ohms
25 °C (77 °F) 1600 - 2300 ohms
-40 °C (-40 °F) 1000 - 1400 ohms

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

On the EBC 325 (ABS) system each front wheel speed is monitored through the speed sensor mounted at the wheel end of the hub. On the EBC 125 (RWAL) and EBC 325 (ABS) systems 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 indicator circuit via the PCI BUS, and store diagnostic 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