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) and cannot be serviced separately from the HCU.
- HCU - which contains several valves (inlet, outlet, isolation and dump) and the hydraulic pump motor. The HCU is mounted on the RH side of the engine compartment and is serviced with the ABS module.
- Front wheel speed sensors - one sensor is located in each front wheel knuckle. The sensors are serviced separately from the knuckle.
- 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.
- Wheel bearing magnetic strip - the wheel bearing magnetic strip is part of the wheel bearing and is serviced with the bearing.

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) 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 Advance Trac(R)

The ABS with Advance Trac(R) system consists of the same components as the conventional ABS above with the following additional components:

- Stability control sensors - located inside the Restraints Control Module (RCM) under the front floor console, and contains several sensors (yaw rate, lateral and longitudinal accelerometers).
- Steering wheel rotation sensor - is part of the Electronic Power Assist Steering (EPAS) column assembly and is not available separately for service.

The Advance Trac(R) system is comprised of traction control and Electronic Stability Control (ESC). ABS and the traction control portion of the Advance Trac(R) system will operate with the transmission in reverse gear, but ESC will be disabled.

Traction Control System

The traction control system aids in the prevention of excessive wheel spin which allows the vehicle to maintain traction during acceleration. This is accomplished in the same manner as anti-lock braking (brake traction control) but also includes using the PCM for modulating engine torque (engine traction control). The traction control system can be disabled by the driver through the message center. The system will remain disabled until the driver enables it through the message center or until the ignition is cycled to OFF and back to RUN.

Electronic Stability Control (ESC) System

The ESC system constantly monitors vehicle direction of travel relative to the 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. ESC cannot be disabled by the driver.

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 Indicator

The stability/traction indicator (also known as the sliding-car icon) is located in the Instrument Panel Cluster (IPC). It alerts the driver to a stability/traction event taking place (flashes twice per second) and to alert the driver of potential concerns in the AdvanceTrac(R) system (illuminates solidly).

Stability/Traction OFF Indicator

The stability/traction OFF indicator (also known as the sliding-car OFF icon) is located in the IPC. Illumination of the indicator confirms to the driver that the brake traction control portion of the Advance Trac(R) system has been disabled when the TRACTION CNTRL setting in the message center is set to OFF.

Hill Start Assist (Automatic transmission only)

Hill start assist helps the driver when accelerating from a stop on an incline or side of a hill. Hill start assist supports the driver during drive-off situations on upward gradients. When the driver releases the vehicle brake, hill start assist prevents the vehicle from rolling back in the opposite direction.