Antilock Brakes / Traction Control Systems: Description and Operation
GENERAL DESCRIPTIONANTILOCK BRAKES SYSTEM (ABS)
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 an braking event. This allows the driver to maintain directional stability while minimizing stopping distance.
VALVE ASSEMBLY
The valve assembly provides brake fluid pressure modulation for each of the individual wheel circuits, as required, during an ABS 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. The valve assembly contains a motor-driven pump, ABS inlet and outlet valves. With exception of the Electronic Brake And Traction 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 vehicles with ABS.
ELECTRONIC BRAKE AND TRACTION CONTROL MODULE (EBTCM)
The EBTCM 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 of the hydraulic circuits. This action prevents wheel slip and provides optimum braking. In addition, the EBTCM performs a continuous diagnostic routine to detect malfunctions. If the module detects an electrical malfunction, it can disable ABS/TCS/DRP, illuminate the appropriate telltale and store a Diagnostic Trouble Code (DTC).
The EBTCM contains it solenoid for each ABS valve it operates and a relay for the solenoid circuit and pump motor circuit respectively. The relays and solenoids are not serviceable.
ABS OPERATION
During an ABS event, the EBTCM will control the hydraulic pressure in the individual wheel circuits to prevent any wheel from slipping. The control module can decrease or hold 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 EBTCM 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 wheel slip, the EBTCM 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.
TRACTION CONTROL SYSTEM (TCS) (FRONT WHEEL DRIVE ONLY)
The Traction Control System (TCS) compares front wheel speeds to rear wheel speeds to determine if drive wheels lose 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 traction event, the EBTCM sends a requested torque value to the ECM/TCM over the serial data link. The ECM/TCM initiates an engine torque reduction routine to slow down the drive wheels. This routine consists of ignition timing reduction, fuel injector cut-off and transmission shift control. The ECM/TCM also sends a torque delivered value to the EBTCM over the serial data link.
If the EBTCM detects a malfunction, it will disable TCS and will command the TCS LED Off. The driver can also disable TCS, if desired, by pressing the TCS switch.
DYNAMIC REAR PROPORTIONING (DRP)
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. Non-ABS vehicles retain the mechanical proportioning valves. A failure in the DRP will turn the red brake telltale On.