Antilock Braking

ANTILOCK BRAKING
When a wheel locking condition exists, the pressure modulator valve (PMV) assembly, controlled by the 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 valves per individual hydraulic circuit. 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 control of a single wheel hydraulic circuit 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, which signifies an abnormal hydraulic pressure increase. The EBCM/EBTCM closes the corresponding inlet valve to avoid any additional hydraulic pressure Increase (thus avoiding an ABS braking condition).

Pressure Reduce
During a wheel lockup condition, the EBCM/EBTCM closes the Inlet valve while pulsing open 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 lockup condition.

TRACTION CONTROL
The traction control system (TCS) is designed to maintain traction and reduce wheel slip at the front drive wheels on slippery surfaces during acceleration. The system, designed to operate at speeds under 40 km/h (25 mph), reduces wheel slip by braking the wheel that is loosing traction.

The electronic brake and traction control module (EBTCM) monitors both front wheel speeds through the wheel speed sensors. If at any time during acceleration the EBTCM detects front wheel slip and the vehicle speed is under 40 km/h (25 mph), the EBTCM enters traction control mode. It then energizes the isolation valves in the pressure modulator valve (PMV) assembly. The isolation valves isolate the front brake hydraulic circuits from the master cylinder and the rear brake hydraulic circuits, Once the front brake hydraulic circuits are isolated, pressure can be applied to the front wheels without affecting any other brake hydraulic circuit. The EBTCM turns on the PMV pump motor to apply pressure, and begins cycling the PMV assembly's inlet and outlet valves.

The inlet and outlet valve cycling aid in obtaining maximum road surface traction in the same manner as the antilock braking mode. The difference between traction control mode and antilock braking mode is that brake fluid pressure is increased to lessen wheel spin (traction control mode), rather than reduced to allow greater wheel spin (antilock braking mode).

There are two pressure relief valves In the PMV assembly. This is because the PMV pump motor supplies more brake fluid volume than required during traction control modes. These pressure relief valves allow excess brake fluid volume to be returned to the master cylinder reservoir.

If at any time during traction control mode the brakes are manually applied, the PMV pressure switch and the brake switch signal the EBTCM to disable traction control and allow manual braking. Manual braking also overrides the traction control system through use of the isolation valves which act as one way check valves. This allows manual brake fluid pressure to be applied to the brake hydraulic system.