P42.50-A005-79
1 | Traction system hydraulic unit |
Overview
This document contains information on:
General
Function
General
The traction system hydraulic unit contains the switching components for the following vehicle dynamics control systems:
Anti-lock braking system (ABS)
Acceleration slip regulation (ASR)
Electronic Stability Program (ESP®)
Function
Traction system hydraulic unit
The functions of the following components in the hydraulic unit traction system are individually described:
High pressure and return pump
Front axle and rear axle switchover valves
Suction ball valves for front axle and rear axle
Pressure regulating valves, hold pressure
Pressure regulating valves, reduce pressure
Front axle brake pressure sensor
Front pressure sensor
Diaphragm pulsation damper
Low pressure reservoir
High pressure and return pump
The self-priming high-pressure and return flow pump integrated into the traction system hydraulic unit has the following tasks:
Pressure buildup and release for ASR and ESP® regulations
Return for ABS regulation
Front axle and rear axle switchover valves
The switchover valves block the active high-pressure circuit in each case during ASR and ESP® control interventions against the master brake cylinder. They are designed simultaneously as pressure limiting valves and open at pressures above 150 bar. The brake fluid flowing away via the switchover valves is routed back to the master brake cylinder.
Suction ball valves for front axle and rear axle
The suction ball valves are opened during an ASR or ESP® pressure buildup phase. They allow brake fluid to flow from the hydraulic fluid reservoir through the high-pressure and return flow pump into the corresponding brake calipers.
Pressure regulating valve, hold pressure
For pressure control, one 2/2-way valve is used for each wheel for the functions of pressure buildup and holding pressure in the ABS, ASR and ESP® control mode.
The pressure regulating valves are open deenergized. If the ABS function fails, the driver can build up brake pressure in the system using pedal force which affects the master brake cylinder via the brake pedal.
Pressure regulating valve, reduce pressure
For pressure control, one 2/2-way valve is used for each wheel for the functions of pressure holding and pressure reduction in the ABS, ASR and ESP® control mode.
The pressure regulating valves are closed deenergized. This occurs, for example, during an ABS braking procedure, when a wheel shows a tendency to lock.
The pressure regulating valves, reduce pressure are not actuated in closed condition.
Front axle brake pressure sensor
The front axle brake pressure sensor measures the set brake pressure (master brake cylinder pressure) as required by the driver.
Front and rear pressure sensor
The pressure sensors have the following tasks:
Record pressure in the front and rear brake circuit
Regulate brake pressure delicately during braking procedure
Detect leakage in HOLD operation
Diaphragm pulsation damper
In order to reduce the movement noise and dampen a pulsing brake pedal, diaphragm pulsation dampers are installed for each brake circuit.
Low pressure reservoir
The low pressure reservoirs take up the brake fluid during the ABS/ASR or ESP® depressurization phase and supply it to the high pressure/return flow pump.