Antilock Brakes / Traction Control Systems: Description and Operation

Fig. 1 Anti-Lock Brake System (ABS):

The anti-lock brake system, Fig. 1, consists of a pump motor assembly, fluid accumulator, pressure switch, fluid reservoir with integral filter, hydraulic booster/master cylinder, wheel speed sensors, Electronic Brake Control Module (EBCM) and a valve block assembly.

ELECTRONIC BRAKE CONTROL MODULE (EBCM)

The EBCM monitors speed of each wheel to determine if any wheel is beginning to lock. If a locking condition is sensed, brake pressures are automatically adjusted to provide for maximum stopping without wheel lock.

TIMER FLASHER MODULE

On some models, the pump circuit includes a timer flasher module, located in the instrument panel behind driver side close out panel, which monitors pump running time. If pump runs for more than three minutes, the timer flasher module will cause BRAKE warning lamp to flash.

HYDRAULIC UNIT

Fig. 2 Anti-lock Brake Hydraulic Unit:

Main components of hydraulic unit are hydraulic booster master cylinder, valve block assembly, pump motor assembly, pressure switch, accumulator and fluid level sensor, Fig. 2.
When brake pedal is depressed, hydraulic booster/master cylinder operates front brakes in normal manner and also provides modulated accumulator pressure to rear brakes.
The valve block assembly is a series of solenoid controlled valves which cycle very quickly to apply and release hydraulic pressure to each wheel.
The pump and motor assembly supplies high pressure brake fluid to accumulator. This high pressure fluid is used for power assist and also to apply rear brakes.
The pressure switch monitors accumulator pressure. When pressure drops below the lower limit, pressure switch activates pump motor relay which turns on pump. Once pressure reaches upper limit, pressure switch deactivates the relay and shuts off pump. If pressure leak or pump failure occur, pressure switch will signal EBCM to disable anti-lock function. The pressure switch will also light BRAKE lamp located in instrument panel.
The accumulator is pressure storage device which can hold brake fluid under very high pressures. The accumulator has an internal diaphragm with nitrogen trapped on one side. As pump fills accumulator, diaphragm moves and compresses trapped nitrogen.
The fluid level sensor is located in fluid reservoir. This sensor has two functions. It can signal the EBCM that there is a low fluid condition. The EBCM will then disable the Anti-Lock function. The sensor will also light the BRAKE lamp located in instrument panel.

ANTI-LOCK WARNING LAMP

Vehicles equipped with the Anti-Lock system have an amber ANTI-LOCK warning lamp in the instrument panel. The ANTI-LOCK warning lamp will illuminate if a malfunction in the system is detected by the electronic controller. In case of an electronic malfunction, the controller will turn the ANTI-LOCK warning lamp on, then disable the anti-lock braking function. If ANTI-LOCK and BRAKE warning lamps come on at the same time, this would indicate a problem in the hydraulic brake system.
The ANTI-LOCK lamp will illuminate during engine starting and will usually stay on approximately three seconds after ignition switch is returned to the Run position. If the car has not been started in several hours, the ANTI-LOCK lamp may stay on for as long as thirty seconds. The ABS pump is recharging the hydraulic accumulator to normal operating pressures. The lamp will stay on until charging is complete. If lamp stays on longer than thirty seconds after engine is started or comes on while driving, the brake system should be inspected for a malfunction.

BRAKE WARNING LAMP

The Anti-Lock brake system uses a three circuit design so that some braking capacity is still available if hydraulic pressure is lost in one circuit. The BRAKE warning lamp is located at the lefthand side of instrument cluster and is designed to alert driver of conditions that would result in reduced braking ability.
The BRAKE warning lamp should turn on under several conditions. The BRAKE lamp will illuminate during engine cranking, when parking brake is not fully released, low brake fluid level, or low accumulator pressure. If BRAKE warning lamp stays on any longer than thirty seconds after starting engine or comes on during driving, there may be a malfunction in brake hydraulic system.