Traction systems hydraulic unit, function - GF42.50-P-4000-04F

Schematic representation

Fig 1: Identifying Traction Systems Hydraulic Unit Function Diagram

Courtesy of MERCEDES-BENZ USA

General
- The traction system hydraulic unit (A7/3) is the electromechanical actuator in the braking torque control circuit of the ABS, ETS and ESP control systems.
- The traction system hydraulic unit (A7/3) functions differently according to the different requirements of the three control systems (ABS, ETS and ESP) for building up brake pressure.
- A well defined brake pressure can be produced at the front and rear axle brakes (5a, 5b, 6a and 6b) as a result of selectively actuating the integrated components and the combined actions of these components. This brake pressure is produced in the three control phases - pressure build-up, hold pressure and pressure reduction - which make up a control interval.
  - Solenoid valves (A7/3y6 to A7/3y23):
    The flow of the brake fluid within the traction system hydraulic unit (A7/3) is controlled by actuating the solenoid valves at different times.
    
    The individually adjustable brake pressure required is set at the wheel brakes (5a, 5b, 6a and 6b) by actuating the high pressure and return pumps (A7/3m1, A7/3p1 and A7/3p2).
  - High pressure and return pumps (A7/3m1, A7/3p1 and A7/3p2):
    The high pressure and return pumps (A7/3m1, A7/3p1 and A7/3p2) determine the direction of flow to either increase or decrease the pressure in the front and rear wheel brake circuit.
Function
The following example describes the different ways of actuating the solenoid valves (A7/3y6 to A7/3y23) and the high pressure and return pumps (A7/3m1, A7/3p1 and A7/3p2) according to the activated ABS, ETS or ESP control mode during the three control phases at the right front wheel brake (5a). The relevant solenoid valves of the front and rear axle brake circuit would be actuated in the same way at the other wheel brakes.
1. 2.1 Front axle inlet solenoid valve (A7/3y22)
  Same function as rear axle inlet solenoid valve (A7/3y23).
  - ABS
    Pressure build-up, hold pressure, pressure reduction: No actuation, remains in lock position during entire control interval.
  - ESP, ETS
    Pressure build-up: Actuated, switches to flow position.
  - ESP, ETS (with calculated brake pressure of ≤60 bar)
    Hold pressure, pressure reduction: No longer actuated, switches to lock position.
  - ETS (with calculated brake pressure of > 60 bar)
    Hold pressure, pressure reduction: Still actuated, remains in flow position.
2. 2.2 Front axle switchover solenoid valve (A7/3y18)
  Same function as rear axle switchover solenoid valve (A7/3y19).
  - ABS
    Pressure build-up, hold pressure, pressure reduction: No actuation, remains in flow position during entire control interval.
  - ETS, ESP
    Pressure build-up, hold pressure: Actuated, is in lock position and therefore uncouples the front axle brake circuit from the master brake cylinder. The integrated pressure limiting valve (approx. 170 bar) limits the high pressure.
  - ESP, ETS (with calculated brake pressure of ≤ 60 bar)
    Pressure reduction: No longer actuated, switches to flow position.
  - ETS (with calculated brake pressure of > 60 bar)
    Pressure reduction: Still actuated, remains in lock position.
3. 2.3 Left front solenoid valve, hold pressure (A7/3y6)
  Same function as:
  
  Right front solenoid valve, hold pressure (A7/3y8)
  - Left rear solenoid valve, hold pressure (A7/3y10)
  - Right rear solenoid valve, hold pressure (A7/3y12)
  - ABS
    Pressure build-up, hold pressure, pressure reduction: Not actuated and remains in flow position. The same brake pressure is therefore applied at the left front wheel brake (5b) as at the right front wheel brake (5a).
  - ESP, ETS
    Pressure build-up, hold pressure: Actuated and switches to lock position. High pressure is applied so that if necessary immediate build-up of pressure would also be possible at the left front wheel brake (5b).
  - ESP, ETS
    Pressure reduction: Still actuated and remains in lock position. The same reduced pressure is applied as at the right front wheel brake (5a).
4. 2.4 Right front solenoid valve, hold pressure (A7/3y8)
  Same function as:
  
  Left front solenoid valve, hold pressure (A7/3y6)
  - Left rear solenoid valve, hold pressure (A7/3y10)
  - Right rear solenoid valve, hold pressure (A7/3y12)
  - ABS, ETS, ESP
    Pressure build-up: Not actuated, remains in flow position and therefore allows pressure build-up in the right front wheel brake (5a).
  - ABS, ETS (with calculated brake pressure of > 60 bar)
    Hold pressure, pressure reduction: Actuated and switches to lock position.
  - ESP, ETS (with calculated brake pressure of ≤60 bar)
    Hold pressure, pressure reduction: Not actuated, remains in flow position, the check valves in the brake circuit prevent pressure reduction.
5. 2.5 Front axle high pressure / return pump (A7/3p1)
  Same function as rear axle high pressure / return pump (A7/3p2).
  - ABS
    Pressure build-up, hold pressure: Not actuated, no flow.
    
    Pressure reduction: Actuated, pumps brake fluid against the existing brake pressure via the open front axle switchover solenoid valve (A7/3y18) back into the reservoir.
  - ESP, ETS
    Pressure build-up: Actuated, draws brake fluid out of the reservoir via the open front axle inlet solenoid valve (A7/3y22) and increases the high pressure in the system for the front axle brake circuit.
  - ESP, ETS (with calculated brake pressure of ≤60 bar)
    Hold pressure: Still actuated, cannot draw any more brake fluid out of the reservoir when the front axle inlet solenoid valve (A7/3y22) is closed and therefore does not increase the brake pressure any further.
  - ETS (with calculated brake pressure of > 60 bar)
    Hold pressure: Actuated, continues to draw brake fluid out of the reservoir via the open front axle inlet solenoid valve (A7/3y22) and increases the pressure in the system for the front axle brake circuit.
  - ESP, ETS (with calculated brake pressure of ≤60 bar)
    Pressure reduction: Still actuated and pumps the brake fluid back to the reservoir via the open right front solenoid valve, hold pressure (A7/3y8) and the front axle switchover solenoid valve (A7/3y18), which is also open.
  - ETS (with calculated brake pressure of > 60 bar)
    Pressure reduction: Still actuated and pumps the brake fluid back to the reservoir via the open right front solenoid valve, reduce pressure (A7/3y9) and via the open front axle inlet solenoid valve (A7/3y22) as well as partially via the pressure limiting valve integrated in the switchover solenoid valve (A7/3y18) via the master brake cylinder.
6. 2.6 Right front solenoid valve, reduce pressure (A7/3y9)
  Same function as:
  - Left front solenoid valve, reduce pressure (A7/3y7)
    Left rear solenoid valve, reduce pressure (release) (A7/3y11)
  - Right rear solenoid valve, reduce pressure (A7/3y13)
  - ABS
    Pressure build-up, hold pressure: Not actuated, remains in lock position and brake pressure is applied.
    
    Pressure reduction: Actuated, switches to flow position and allows pressure reduction in right front wheel brake (5a).
  - ETS
    Pressure build-up, hold pressure: Not actuated, remains in lock position and prevents pressure reduction in the right front wheel brake (5a).
    
    Pressure reduction:
    - Calculated brake pressure of ≤60 bar: Still actuated, remains in lock position.
    - Calculated brake pressure > 60 bar: Actuated switches to flow position.
  - ESP
    Pressure build-up, hold pressure, pressure reduction
    
    No actuation, remains in lock position during entire control interval.
7. 2.7 Low-pressure reservoir (11)
  Pressure build-up, pressure reduction with ABS or ESP control
  
  The low-pressure reservoirs (11) fill with returned brake fluid and send it to the high pressure and return pumps (A7/3p1, A7/3p2).
8. 2.8 Damper (10)
  The damper (10) dampens the pumping noise of the front axle and rear axle high pressure and return pumps (A7/3p1, A7/3p2).