Antilock Braking

When a wheel locking condition exists, the Pressure Modulator Valve (PMV) assembly, controlled by the Electronic Brake and Traction Control Module (EBCM/EBTCM), isolates each individual wheel hydraulic circuit and begins modulating fluid pressure to each wheel using three ABS modes: pressure hold, pressure reduce, and pressure increase. The EBCM/EBTCM allows the PMV assembly to individually adjust each wheel's hydraulic pressure according to wheel speed input signals.

The PMV assembly has two solenoid valves per individual wheel, a normally open inlet valve and a normally closed outlet valve. These valves work in pairs to achieve the three ABS modes. The ABS system isolates, monitors and controls all wheels simultaneously. The following descriptions describe the hydraulic control of a single wheel during ABS braking.


Pressure Increase
The pressure increase position is the operating valve position for normal (non-ABS) braking. The EBCM/EBTCM does not energize either valve, leaving the inlet valve open and the outlet valve closed.


Pressure Hold
The pressure hold position occurs when the EBCM/EBTCM first notices an impending wheel lockup condition. This signifies an over abundance of hydraulic brake pressure at the locking wheel. The EBCM/EBTCM closes the corresponding inlet valve to avoid any additional hydraulic pressure increases at the wheel.


Pressure Reduce
During a wheel lockup condition, the EBCM/EBTCM closes the inlet valve while opening the outlet valve. This decreases hydraulic pressure at a specific wheel without decreasing overall system pressure. The reduced hydraulic pressure allows greater wheel spin, eliminating the wheel lock-up condition.