The purpose of the Antilock Brake System (ABS) is to minimize wheel slip during heavy braking. The ABS performs this function by monitoring the speed of each wheel and controlling the brake fluid pressure to each wheel independently during a braking event. This allows the driver to maintain directional stability while minimizing stopping distance.

The valve assembly provides brake fluid pressure modulation for each of the individual wheel circuits, as required, during an ABS/TCS event. During an ABS event, the valve assembly can maintain or reduce brake fluid pressure that is applied by the master cylinder. The valve assembly cannot increase pressure beyond what is applied by the master cylinder (driver). The valve assembly contains a motor-driven pump, ABS inlet and outlet valves, as well as TCS valves. With exception of the ABS control module (EBTCM), the valve assembly is not serviceable and should never be disassembled.

The valve assembly is an addition to the four-circuit diagonally-split hydraulic system found on non-ABS vehicles.

The ABS control module monitors the speed of each wheel to detect wheel slip. If wheel slip is detected, the module commands the appropriate valve positions in the valve assembly to modulate brake pressure in some or all the hydraulic circuits. This action prevents wheel slip and provides optimum braking. In addition, the ABS control module performs a continuous diagnostic routine to detect malfunctions. If the module detects and electrical malfunction, it can disable ABS/TCS/DRP, illuminate the appropriate telltale and store a Diagnostic Trouble Code (DTC).

The ABS control module contains a solenoid for each ABS/TCS valve it operates and a relay for the solenoid circuit and pump motor circuit respectively. The relays and solenoids are not serviceable.

During an ABS event, the ABS control module will control the hydraulic pressure in the individual wheel circuits to prevent any wheel from slipping. The control module can decrease or hold hydraulic pressure by energizing the appropriate valve solenoid. A rapid pulsation is felt in the brake pedal and a ticking or popping noise can be heard as control module commands valve solenoids in response to wheel speed changes.

When the ABS control module detects wheel slip, it holds pressure by closing both the inlet valve from the master cylinder and the outlet valve.

If pressure hold is not enough to control slip, the ABS control module releases pressure by opening the outlet valve and allowing some pressure bleed-off into the accumulator. The accumulator stores this fluid until the motor-driven pump returns fluid to the master cylinder.

The control module opens the inlet valve to allow master cylinder pressure to the wheel circuits again when no wheel slip is detected.

The traction control system (TCS) compares front wheel speeds to rear wheel speeds to determine if drive wheel loose traction. The TCS activates when drive wheel speed exceeds speed of non-drive wheels by a calibrated value. This allows the driver to maintain acceleration and directional stability while accelerating on low traction surfaces. The TCS limits wheel slip during acceleration when one or more of the drive wheels lose traction. The brake switch must be off for TCS to operate.

During a TCS event, the PCM controls wheel slip using a combination of spark timing retard, transmission upshift (automatic only) and fuel injector reduction. The PCM will disable TCS if:

This TCS does not use brake intervention to control wheel slip.

Dynamic Rear Proportioning (DRP) is an electronic brake proportioning feature that replaces the mechanical proportioning valve in the base brake system. DRP uses existing ABS components to regulate the vehicle's rear brake pressure and provide optimum front-rear brake balance in all braking situations.