Charge Plunger Unit

Location
Behind lining on left front fender.

Task





The pressurizing piston unit is a hydraulic connecting component in the brake circuits between the tandem master brake cylinder and the ESP hydraulic unit.

Design





There are two separate pressurizing pistons (b) which are held in their normal position by the springs (d) in the cylinder housing (a) of the pressurizing piston unit.
When the pressurizing piston is in this position, the central valves (c) of both circuits located at the end face are opened.

The ESP brake pressure sensor (B34) is attached to the pressure chamber for the front axle circuit.





Function
A Braking Process
During braking, the brake pressure from the tandem master brake cylinder is led via the line inputs (HZ1 and HZ2) and the central valves (c) of the pressurizing pistons (b). During the pressure build-up, the two pressurizing pistons (b) and the central valves (c) remain in the rest position. The pressure building up for the front and rear axle circuit is transmitted into the hydraulic unit and to the wheel brakes via the line outputs (FA and RA).

B ESP/ASR Control Process
A charging pressure of 3-15 bar is led to both charging pistons (b) via the pipe inlet (VLP) during ESP/ASR control.
This moves both charging pistons outwards and the two central valves (c) close. This interrupts the pipe connections (HZ1 and HZ2) to the master brake cylinder. The charging pressure originating from the charging piston is available to the high-pressure/return pumps in both control circuits via the pipe connections (FA and RA) in the hydraulic unit.
The charging pressure of 3-15 baris determined by a pressure limiting valve in the charging pump. A restrictor in the return flow (BA) ensures the reduction in volume of the brake fluid at the end of an ASR or ESP control.