Antilock Braking System (ABS)

DTC 21 & 25 Front Wheel Speed Sensor Circuit Test:

Front Wheel Speed Sensor Circuit Test:

CIRCUIT OPERATION
As each wheel turns, the wheel speed sensor creates an AC voltage with a frequency proportional to wheel speed. This magnetically induced voltage is caused by a toothed sensor ring (mounted on the drive axle) passing the wheel speed sensor's stationary pickup coil.
The EBCM/EBTCM uses the wheel speed sensor signal to calculate vehicle reference speeds and individual wheel speed, acceleration and slip values. These values are used to determine when anti lock (or when equipped) traction control is required.

WHEEL SPEED SENSOR CABLES
In order to prevent electromagnetic interference from disturbing the wheel speed sensor signal, the cables from the EBCM/EBTCM to each wheel speed sensor are twisted in pairs a minimum of 6 to 9 turns per foot.
When servicing the sensor cables it is important to maintain the original cable's twists (6 to 9 turns per foot).

FAILURE CONDITIONS
The EBCM/EBTCM performs two basic checks on the wheel speed sensors: sensor continuity and sensor output. When the ignition is first turned to "RUN," the EBCM/EBTCM performs the wheel speed sensor continuity check.

Sensor Continuity Check
Any condition which would result in lack of continuity in the front wheel speed sensor circuit could result in a DTC 21 or 25 being set. These conditions include an open, a short to ground, or a short between the two wheel speed sensor circuits in the wiring harness between the speed sensor and EBCM/EBTCM. Also an open or short to ground across the wheel speed sensor coil could trigger this code.