LR3/Disco 3
The HCU has three operating modes: Normal braking/EBD, ABS braking and active braking.
Normal Braking/EBD Mode
Initially, all of the solenoid operated valves are de-energized. Operating the brake pedal produces a corresponding
increase or decrease of pressure in the brakes, through the open pilot valves and inlet valves. If the ABS module
determines that EBD is necessary, it energizes the inlet valves for the brakes of the trailing axle, to isolate the brakes from
any further increase in hydraulic pressure.
ABS Braking Mode
If the ABS module determines that ABS braking is necessary, it energizes the inlet and outlet valves of the related brake
and starts the return pump. The inlet valve closes to isolate the brake from pressurized fluid; the outlet valve opens to
release pressure from the brake into the accumulator and the return pump circuit; the reduced pressure allows the wheel
to accelerate. The ABS module then operates the inlet and outlet valves to modulate the pressure in the brake to apply
the maximum braking effort without locking the wheel. Control of the valves for each wheel takes place individually.
Active Braking Mode
The active braking mode is used to generate and control hydraulic pressure to the brakes for functions other than ABS
braking, e.g. DSC, EBA, ETC, HDC and dynamic application of the parking brake.
For active braking, the ABS module energizes the pilot valves and priming valves, starts the return pump and energizes
all of the inlet valves. Brake fluid, drawn from the reservoir through the master cylinder and priming valve, is pressurized
by the return pump and supplied to the inlet valves. The ABS module then operates the inlet valves and outlet valves, as
required, to modulate the pressure in the individual brakes. Some noise may be generated during active braking.
ABS MODULE
The ABS module controls the brake functions using the HCU to modulate hydraulic pressure to the individual wheel
brakes.
The ABS module is attached to the HCU, in the plenum box on the driver side of the engine compartment. A 46 pin
connector provides the electrical interface between the ABS module and the vehicle wiring.
1
-
Master cylinder
2
-
Reservoir
3
-
Brake booster
4
-
Brake pedal
5
-
HCU
6
-
Pressure sensor
7
-
Priming valve
8
-
Pilot valve
9
-
Return pump motor
10
-
Return pump
11
-
Inlet valve
12
-
Relief valve
13
-
Accumulator
14
-
Outlet valve
15
-
Left front brake
16
-
Right front brake
17
-
Right rear brake
18
-
Left rear brake
