Hydraulic Control Assembly - Antilock Brakes: Description and Operation

DESCRIPTION
The modulator valve assembly, located in the rear compartment behind the driver's seat, contains a recirculation pump with separate circuits for the front and rear brakes. During the ABS pressure reduction function the modulator valve transfers fluid from the brake calipers back to the master cylinder. During the ASR/TCS brake intervention function, the valve transfers fluid from the master cylinder reservoir to the rear brake calipers.

The TCS and ASR systems utilize three way valves added to the rear brakes. These valves control each rear brake individually and use an additional rear brake pipe. The load valve is used to isolate the master cylinder prime pipe from the pump during brake application and ABS control, the valve is spring loaded to the open position. The two way pilot valve is electronically operated to isolate the master cylinder from the pump during ASR/TCS operation. When the pilot valve is closed, the pump can direct fluid to the rear brake circuit, with excess fluid passing through the pressure limiting valve. The master cylinder positive center valve is designed to open when the secondary piston returns to the rest position. This happens when the center valve pintle contacts a cross shaft at the rest position, ensure pressure relief if excess fluid is pumped into the master cylinder.