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 serviced separately 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 and can be serviced separately.
- 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 serviced separately from the axle shaft.
- Restraints Control Module (RCM) - located under the rear of the front floor console and contains the yaw rate and lateral accelerometer sensors.
- Power Steering Control Module (PSCM) located in the steering gear, sends steering angle and rotation information to the ABS module.
- Stability/traction control switch - is part of the hazard flasher switch plate.
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, allowing 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 allowing 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 (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 stability/traction control indicator (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 stability/traction control switch provides the driver with the ability to turn the traction control and ESC system off. It also allows the driver to select different modes of the ESC system, such as Sport mode (if equipped). When the switch is pressed, the stability/traction control (sliding car icon) OFF in the Instrument Panel Cluster (IPC) illuminates 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. 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 re-enables normal ESC and traction control system operation. The conventional ABS cannot be disabled through the use of the stability/traction control switch. At the start of every ignition cycle, normal ESC is present regardless of the mode selected on the previous ignition cycle.