Antilock Brakes / Traction Control Systems: Description and Operation
Anti-Lock Control
Anti-Lock Braking
The ABS consists of the following components:
- ABS module - attached to the Hydraulic Control Unit (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 - which contains several valves (inlet, outlet, isolation and dump) and the hydraulic pump motor. The HCU is mounted on the LH side of the engine compartment underneath the brake master cylinder and brake booster.
- Rear wheel speed sensors - one sensor is located at each rear wheel end, is mounted to the brake backing plate and can be serviced separately.
- Rear wheel speed sensor tone rings - one tone ring is located on each rear wheel hub and is serviced with the wheel hub.
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) 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 ( HCU pump motor and valves)
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. A slight bump sensation may be felt in the brake pedal when EBD is active.
ABS with AdvanceTrac(R)
The ABS with AdvanceTrac(R) system consists of the same components as the conventional ABS above with the following additional components:
- Stability control sensors - located under the front floor console, in front of the gear selector lever assembly, and contains several sensors (yaw rate, lateral and longitudinal accelerometers).
- Stability/traction control switch - located in the instrument panel center finish panel and is serviced separately from the finish panel.
- Steering wheel rotation sensor and sensor ring assembly - the rotation sensor and ring assembly is a sealed unit that is located on the backside of the clockspring and is serviced separately from the clockspring.
The AdvanceTrac(R) system is made up of the following 2 distinct vehicle systems; traction control and Electronic Stability Control (ESC).
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 in the same manner as anti-lock braking but also includes the use of the PCM for modulating engine torque.
Electronic Stability Control (ESC) System
The ESC system constantly monitors vehicle direction of travel relative to the driver's intended course and aids in keeping the vehicle traveling along that course. The ESC also aids in keeping the vehicle stable during sudden or abrupt maneuvers. This is accomplished in the same manner as anti-lock braking and traction control.
When either of these 2 systems activate, the driver may notice the same operational symptoms as ABS along with the following additional symptoms:
- A small deceleration or a reduction in the acceleration of the vehicle (PCM modulating engine torque)
- The stability/traction control indicator (also known as the sliding-car icon) flashes (indicating a traction or stability event is taking place)
Stability/Traction Control Switch
The traction control switch can be used by the driver to disable and enable the traction control portion of the AdvanceTrac(R) system. When the switch is pressed, the sliding car icon in the Instrument Panel Cluster (IPC) will illuminate. The conventional ABS and the ESC system cannot be disabled through the use of the traction control switch.
Stability/Traction Control Indicator
The stability/traction control indicator (also known as the sliding-car icon) is located in the instrument cluster. The sliding-car icon is used to alert the driver that the traction control portion of the AdvanceTrac(R) system has been disabled by the driver. It is also used to alert the driver that a stability/traction event is taking place (flashes twice per second) and to alert the driver of potential concerns in the AdvanceTrac(R) system (illuminates solidly).