Antilock Brakes / Traction Control Systems: Description and Operation

Anti-Lock Control

Three-Channel Anti-Lock Braking

The ABS 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 - 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 in the engine compartment on the LH shock tower.
- Rear axle speed sensor - the sensor is located in the rear differential housing and can be serviced separately.
- Rear axle speed sensor tone ring - the tone ring is located on the rear differential case. It can be serviced separately from the differential, but the ring gear must be removed in order to do so.
- Brake pressure transducer (if equipped), which is mounted on the underside of the master cylinder toward the rear of the vehicle.

ABS aids in the prevention of wheel lock-up during braking events, which allows 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)

Engine Only Traction Control (EOTC) System

The Engine Only Traction Control (EOTC) system aids in the prevention of excessive wheel spin, which allows the vehicle to maintain traction during acceleration. This is accomplished by using the PCM to modulate engine torque. Unlike other traction control systems, the EOTC system does not use ABS to control traction.

When the EOTC system activates, the driver may notice the following operational symptoms:

- A small deceleration or a reduction in the acceleration of the vehicle (PCM modulating engine torque)
- The traction control indicator (also known as the sliding-car icon) flashes (indicating a traction event is taking place)

Traction Control Switch

The traction control switch can be used by the driver to disable and enable the EOTC system. When the system is disabled, the traction control indicator ("sliding-car icon OFF") in the IPC illuminates, the message TRACTION CONTROL OFF is shown in the message center (if equipped). The ABS cannot be disabled through the use of the traction control switch.

Traction Control Indicators

There are 2 traction control indicators for the EOTC system, both located in the Instrument Panel Cluster (IPC). They are referred to as the sliding-car icon and the sliding car icon OFF icon. The sliding-car icon is used to alert the driver that a traction event is taking place (flashes twice per second), or to alert the driver of potential concerns in the EOTC system (illuminates solidly). The sliding car icon OFF icon illuminates to indicate that the system has been disabled. When the system is disabled, the message center displays TRACTION CONTROL OFF or TRACTION CONTROL ON (if equipped with message center).

Brake Pressure Transducer

The brake pressure transducer (if equipped) is mounted on the bottom of the master cylinder near the rear and sends a variable voltage to the ABS module depending on the pressure applied to the brake pedal. The ABS module receives this information directly from the pressure transducer.

Four-Channel Anti-Lock Braking

The ABS consists of the following components:

- ABS module - attached to the 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 - contains several valves (inlet, outlet, isolation and dump) and the hydraulic pump motor. The HCU is mounted in the engine compartment on the LH shock tower.
- Rear wheel speed sensors - the sensors are located in the rear wheel backing plates/axle housing and can be serviced separately.
- Rear wheel speed sensor tone rings - the tone ring is located on the hub/bearing assembly

ABS aids in the prevention of wheel lock-up during braking events, which allows 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.

Traction Control System with Roll Stability Control (RSC(R))

The traction control system aids in the prevention of excessive wheel spin, which allows the vehicle to maintain traction during acceleration especially on slippery or uneven road surfaces. This is accomplished by using the PCM to modulate engine torque and the ABS module to cycle the vehicle brakes. Both the PCM and ABS modules work to reduce drive wheel spin by momentarily reducing engine speed and applying vehicle brakes.

When the Traction Control System with RSC(R) activates, the driver may notice the following operational symptoms:

- A small deceleration or a reduction in the acceleration of the vehicle (PCM modulating engine torque).
- The traction control indicator (also known as the sliding-car icon) flashes (indicating a traction event is taking place).
- Application of the vehicle brakes and ABS pump noise.
- If the driver's foot is on the brake pedal, a vibration may be felt in the brake pedal.

Traction Control Switch

The traction control switch can be used by the driver to disable and enable the traction control system. When the system is disabled, the traction control indicator ("sliding-car icon") in the IPC (or message center, if equipped) illuminates. The message center displays the message TRACTION CONTROL OFF. Press and release the traction control switch again and the message should read TRACTION CONTROL ON. The ABS cannot be disabled through the use of the traction control switch. Pressing the traction control switch for more than 5 seconds and then releasing it disables the Electronic Stability Control (ESC) and RSC(R). The message that reads ADVANCE TRAC OFF should be displayed in the message center. Pressing the traction control switch again and releasing it enables the ESC and RSC(R). systems. The message center (if equipped) displays the message ADVANCE TRAC ON in the message center.

Hill Descent Control

Hill descent control uses the ABS pump motor and HCU to provide vehicle speed control while descending a downhill grade, information from the Restraints Control Module (RCM) to determine whether or not the vehicle is on a downhill grade and wheel speed sensor information to determine vehicle speed. During a hill descent while the hill descent control system is activated, the driver sets vehicle speed through the use of the accelerator and brake pedals. Once the desired speed has been set, the system maintains that speed until: the vehicle is no longer on a downhill grade, the driver presses the switch again, vehicle speed exceeds 31 km/h (20 mph) or the system is disabled for cooling.

If the hill descent control system is overheated the message HILL CNTRL SYSTEM COOLING is displayed in the message center. Anytime the hill descent control system shuts down while active the message DRIVER RESUME CONTROL is displayed In the message center.

Hill Start Assist

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 by preventing the vehicle from rolling back in the opposite direction. The system engages automatically. The inputs to the system are:

- Road inclination.
- The brakes are applied and the vehicle is at a complete stop.
- The vehicle transmission information must indicate that the driver intends to drive up the hill.
- The parking brake is not engaged.

When the driver releases the brake pedal, hill start assist acts to hold the car stationary for up to 2 seconds giving the driver time to accelerate the vehicle. As the vehicle accelerates, hill start assist gradually reduces the brake pressure to ensure that the vehicle is neither rolling back nor driving off until there is sufficient driving torque to accelerate the vehicle forward. Hill start assist functions when the vehicle is facing downhill if the reverse gear is selected.

Trailer Sway Control

Trailer sway control is a unique function of the ABS that uses information from the RCM to determine if a trailer sway event is taking place. Trailer sway is the undesirable yaw force a trailer can apply to the towing vehicle. If the ABS module determines that a trailer sway event is taking place, it sends a message to the IPC, which flashes the sliding-car icon and the message TRAILER SWAY REDUCE SPEED is displayed in the message center. If necessary, the PCM reduces engine torque thus reducing vehicle speed. Once the trailer sway has been corrected, the IPC extinguishes the sliding-car icon.