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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  

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