Hydraulic Control Assembly - Antilock Brakes: Description and Operation

NORMAL POSITION (Pressure build-Up)
In normal position. the solenoid valve is de-energized. The piston is pressed down toward the normal position of the return spring. The brake fluid is now able to flow freely from the master cylinder out through the solenoid valve to the wheel cylinder. This position is the normal position in braking without ABS control and the pressure build-up position in ABS control.
In ABS control, the control module can run up to ten control cycles per second.





PRESSURE HOLDING POSITION
In this position, the solenoid valve has received around half the current from the ABS control module. The piston has moved into the pressure holding position by rising slightly and the passage from the master cylinder to the wheel cylinder is consequently closed.





PRESSURE RELIEF
In this position, the solenoid valve obtains full current from the control module. The piston in the valve has risen to the pressure relief position and opens the channel from the wheel cylinder to the pressure accumulator which can quickly receive pressure from the wheel cylinder. The control module energizes the return pump and the pressure can be pumped back to the master cylinder.

Fig. 5 Rear Wheel Equalization Valve:

In addition to the solenoid valve, there is an equalization valve, Fig. 5, for the rear wheel circuit so both rear wheels can be operated with only one solenoid valve.
The equalization valve ensures both rear wheels receive the same brake pressure in ABS control. This means the vehicle has good directional stability.
In the case of ABS control of the rear wheels, the solenoid valve moves to the pressure holding position. The channel to the rear right wheel is now blocked. If the pressure from the master cylinder rises, a pressure difference develops below and above the piston (1). The piston is raised and the valve (2) closes the pressure to the rear left wheel. If the pressure to the rear right wheel drops (pressure relief), a greater pressure difference develops above and below the piston (1), and the piston is lifted further upward until the pressure for the right and left rear wheels has balanced out.

The hydraulic unit consists of the valve block with three 3/3 solenoid valves (1 valve for front right wheel, 1 for front left wheel and 1 for both rear wheels), equalization valve and control module with main and pump relays and return pump.
When the ignition key is turned to drive position and the wheels start to rotate, the return pump is always tested by the control module starting the pump for approximately .5 seconds.
The master cylinder is separated from the hydraulic unit and is connected via two pipes from the primary and secondary circuits.