How Antilock Brake Systems Work

Rear Wheel ABS

   Most light trucks and SUVs use a form of ABS known as Rear Wheel ABS. The Rear Wheel Anti Lock (RWAL) system reduces the occurrence of rear wheel lockup during severe braking by regulating rear hydraulic line pressure. The system monitors the speed of the rear wheels during braking. The Electronic Brake Control Module (EBCM) processes these values to produce command controls to prevent the rear wheels from locking.

   This system uses three basic components to control hydraulic pressure to the rear brakes. These components are:

• Electronic Brake Control Module
• Anti-Lock Pressure Valve
• Vehicle Speed Sensor

ELECTRONIC BRAKE CONTROL MODULE:
The EBCM mounted on a bracket next to the master cylinder, contains a microprocessor and software for system operation.

ANTI-LOCK PRESSURE VALVE:
The Anti-Lock Pressure Valve (APV) is mounted to the combination valve under the master cylinder, has an isolation valve to maintain or increase hydraulic pressure and a dump valve to reduce hydraulic pressure.

VEHICLE SPEED SENSOR:
The Vehicle Speed Sensor (VSS) located on the left rear of the transmission on two-wheel drive trucks and on the transfer case of four-wheel drive vehicles, produces an AC voltage signal that varies in frequency according to the output shaft speed. On some vehicles the VSS is located in the rear differential.

BASE BRAKING MODE:
During normal braking, the EBCM receives a signal from the stop lamp switch and begins to monitor the vehicle speed line. The isolation valve is open and the dump valve is seated. This allows fluid under pressure to pass through the APV and travel to the rear brake channel. The reset switch does not move because hydraulic pressure is equal on both sides.

ANTILOCK BRAKING MODE:
During a brake application the EBCM compares vehicle speed to the program built into it. When it senses a rear wheel lock-up condition, it operates the anti lock pressure valve to keep the rear wheels from locking up. To do this the EBCM uses a three-step cycle:

• Pressure Maintain
• Pressure Decrease
• Pressure Increase

Rear Wheel ABS

PRESSURE MAINTAIN:
During pressure maintain the EBCM energizes the isolation solenoid to stop the flow of fluid from the master cylinder to the rear brakes. The reset switch moves when the difference between the master cylinder line pressure and the rear brake channel pressure becomes great enough. If this happens, it grounds the EBCM logic circuit.

PRESSURE DECREASE:
During pressure decrease the EBCM keeps the isolation solenoid energized and energizes the dump solenoid. The dump valve moves off its seat and fluid under pressure moves into the accumulator. This action reduces rear pipe pressure preventing rear lock-up. The reset switch grounds to tell the EBCM that pressure decrease has taken place.

PRESSURE INCREASE:
During pressure increase the EBCM de-energizes the dump and isolation solenoids. The dump valve reseats and holds the stored fluid in the accumulator. The isolation valve 9pens and allows the fluid from the master cylinder to flow past it and increase pressure to the rear brakes. The reset switch moves back to its original position by spring force. This action signals the EBCM that pressure decrease has ended and driver applied pressure resumes.

SYSTEM SELF-TEST:
When the ignition switch is turned "ON," the EBCM performs a system self-test. It checks its internal and external circuit and performs a function test by cycling the isolation and dump valves. The EBCM then begins its normal operation if no malfunctions are detected.

Brake pedal pulsation and occasional rear tire "chirping" are normal during RWAL operation. The road surface and severity of the braking maneuver determine how much these will occur. Since these systems only control the rear wheels, it is still possible to lock the front wheels during certain severe braking conditions.

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