Antilock Brakes / Traction Control Systems: Description and Operation
Anti-Lock Control
The ABS consists of the following components:
- ABS module- the ABS module is attached to the Hydraulic Control Unit (HCU). It can be serviced separately from the HCU (Hydraulic Control Unit) on vehicles with ABS only. The ABS module and HCU (Hydraulic Control Unit) are only serviced together on vehicles that are equipped with AdvanceTrac(R) with Roll Stability Control (RSC(R)).
- 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 (Hydraulic Control Unit)- which contains several valves (inlet, outlet, isolation and dump) and the hydraulic pump motor. The HCU (Hydraulic Control Unit) 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 spindle and can be serviced separately.
- Rear wheel speed sensor tone rings- one magnetic strip 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 (Hydraulic Control Unit) 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 (Hydraulic Control Unit) pump motor)
- If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal (HCU (Hydraulic Control Unit) pump motor and valves)
Electronic Brake Distribution (EBD)
The ABS module and HCU (Hydraulic Control Unit) incorporate a strategy called Electronic Brake Distribution (EBD). The EBD (Electronic Brake Distribution) strategy uses the HCU (Hydraulic Control Unit) 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 (Electronic Brake Distribution) is active.
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 (Hydraulic Control Unit) modulating brake fluid pressure to the affected brake caliper(s) and by the PCM modulating engine torque.
When the traction control system activates, the driver may notice a small deceleration or a reduction in the acceleration of the vehicle. This is the PCM modulating the engine torque and/or the HCU (Hydraulic Control Unit) modulating brake fluid pressure.
ABS with AdvanceTrac(R) and Roll Stability Control (RSC(R))
The ABS with AdvanceTrac(R) and Roll Stability Control (RSC(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, roll rate, lateral accelerometer and longitudinal accelerometer).
- 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) with RSC(R) (Roll Stability Control) system is made up of the following 3 distinct vehicle systems:
- Traction Control System- helps to avoid excessive drive-wheel spin and loss of traction.
- Electronic Stability Control (ESC)- helps to avoid skids and lateral slides.
- RSC(R) (Roll Stability Control)- helps to avoid vehicle roll-over.
Stability/Traction Control Indicator
The stability/traction control indicator (also known as the "sliding-car" icon) is located in the Instrument Cluster (IC). The icon is 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).
Traction Control System
The traction control system aids in the prevention of excessive drive-wheel spin which will allow the vehicle to maintain traction during acceleration. This is accomplished in the same manner as anti-lock control but also includes the use of the PCM for modulating engine torque.
ESC (Electronic Stability Control) and RSC(R) (Roll Stability Control) Systems
Both systems constantly monitor vehicle direction of travel relative to the driver's intended course and aid in keeping the vehicle traveling along that course. The RSC(R) (Roll Stability Control) system also monitors the vehicle roll motion and 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 any of the above 3 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 due to the PCM modulating engine torque and the HCU (Hydraulic Control Unit) modulating brake pressure
- The sliding-car icon flashes (indicating a traction or stability event is taking place)
Stability/Traction Control Switch
The vehicle is not equipped with a switch to deactivate any of the AdvanceTrac(R) systems.