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 in the engine compartment on the LH frame rail.
- 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.
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)
Engine Only Traction Control System
The Engine Only Traction Control (EOTC) system aids in the prevention of excessive wheel spin which will allow 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") in the Instrument Cluster (IC) (or message center, if equipped) will illuminate. 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. One indicator is in the IC (or message center, if equipped) and is also known as the sliding-car icon. The other indicator is part of the switch and is an LED style indicator. The sliding-car icon is used to alert the driver that a traction event is taking place (flashes twice per second), to alert the driver of potential concerns in the EOTC system (illuminates solidly) and to indicate that the system has been disabled. The switch indicator is also used to confirm that the EOTC system has been disabled.