Four-Channel Anti-Lock Braking: Notes

The ABS consists of the following components:

• ABS module - attached to the HCU, but can be serviced separately from the HCU.
• Front wheel speed sensors - one sensor is located in each front wheel knuckle. The sensors are serviced separately from the knuckle.
• Front wheel speed sensor tone rings - the tone ring is a magnetic strip that is part of the front wheel bearing and is serviced with the bearing.
• HCU - contains several valves (inlet, outlet, isolation and dump) and the hydraulic pump motor. The HCU is mounted in the engine compartment on the LH shock tower.
• Rear wheel speed sensors - the sensors are located in the rear wheel backing plates/axle housing and can be serviced separately.
• Rear wheel speed sensor tone rings - the tone ring is located on the hub/bearing assembly.
• Brake booster vacuum sensor - located on the brake booster and can be serviced separately.
• Brake vacuum pump (diesel engine only) - a gear-driven pump mounted to the front of the engine behind the cooling fan drive assembly and is serviced as an assembly.

ABS aids in the prevention of wheel lock-up during braking events, which allows the driver to maintain steering control and stop in the shortest distance possible under most conditions. This is accomplished by the ABS module and HCU modulating brake fluid pressure to the affected brake caliper(s) and wheel cylinder(s).

When the ABS activates, the driver may notice the following operational symptoms:

• A rumble or grinding sound may be heard (HCU pump motor).
• If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal.