Antilock Brakes / Traction Control Systems: Description and Operation
Anti-Lock Control
ABS With Traction Control and Electronic Stability Control (ESC)
The ABS with traction control and Electronic Stability Control (ESC) consists of the following components:
- ABS module - attached to the Hydraulic Control Unit (HCU), but can be serviced separately.
- 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 - one tone ring is located on each front wheel hub and is serviceable separate from the wheel hub.
- HCU - which contains several valves (inlet, outlet, isolation and dump) and the hydraulic pump motor. It is mounted in the LF corner of the engine compartment.
- Rear wheel speed sensors - one sensor is located on each end of the rear axle housing. The sensors are serviced separately from the axle housing.
- Rear wheel speed sensor tone rings - one tone ring is located on each axle shaft and is serviceable separate from the axle shaft.
- Restraints Control Module (RCM) - located under the rear of the front floor console and contains several sensors (yaw rate, lateral accelerometer and longitudinal accelerometer).
- Steering wheel rotation sensor - located on the RH side of the clockspring and is serviced separately from the clockspring assembly.
- Stability/traction control switch - located in the floor console finish panel and is serviced separately from the finish panel.
Electronic Brake Distribution (EBD)
The ABS module and HCU incorporate a strategy called Electronic Brake Distribution (EBD). The EBD strategy uses the HCU as an electronic proportioning valve to create a balanced braking condition between the front and rear wheels and minimize the chance of rear wheel lockup during hard braking.
Anti-Lock Braking
The ABS aids in the prevention of wheel lock-up during braking events which will allow 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).
When the ABS system activates, any of the following can occur:
- A rumble or grinding sound may be heard
- If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal
Traction Control System
The traction control system aids in the prevention of excessive wheel spin which will allow the vehicle to maintain traction during acceleration. This is accomplished by the ABS module and the HCU modulating brake fluid pressure to the affected brake caliper(s) and by the PCM modulating engine torque.
When the traction control system activates, any of the following can occur:
- A rumble or grinding sound may be heard, much like ABS
- A small deceleration or a reduction in the acceleration of the vehicle
- The stability/traction control indicator (also known as the sliding car icon) flashes
- If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal, much like ABS
Electronic Stability Control (ESC) System
The ESC system constantly monitors the direction of travel relative to the driver's intended course, except when the vehicle is traveling in reverse, and aids in keeping the vehicle traveling along that course. This is accomplished by the ABS module and the HCU modulating brake fluid pressure to the affected brake caliper(s) and by the PCM modulating engine torque.
When the ESC system activates, any of the following can occur:
- A rumble or grinding sound may be heard, much like ABS
- A small deceleration or a reduction in the acceleration of the vehicle
- The sliding car icon flashes
- If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal, much like ABS
Stability/Traction Control Switch
The traction control switch provides the driver with the ability to turn the traction control system off, as well as the ESC system. It also allows the driver to select different modes of the ESC system, such as Sport mode. When the switch is pressed, the sliding car icon in the Instrument Panel Cluster (IPC) will illuminate steadily, indicating that the traction control system is off. The different modes of the ESC system can only be selected by starting with the stability/traction control system on. For example, Sport mode cannot be enabled when the system has been turned off. Sport mode can be turned on by pressing the switch twice within one-half second with the brakes applied and 0% throttle. Traction control and ESC can be deactivated by pressing and holding the traction control switch for at least 5 seconds with the brake applied and the vehicle at a standstill. Pressing the switch once from any of the above-mentioned modes will re-enable normal ESC and traction control operation. The conventional ABS cannot be disabled through the use of the stability/traction control switch.