ABS System Description
HYDRAULIC UNIT (H/U)Solenoid Valves
Solenoid Valve:
Normal Braking
During normal (non anti-lock) braking, the solenoid valves are without current and closed due to spring force.
Brake fluid travels through the center of the normally open isolation valve around the normally closed dump valve and on to the brake pistons.
Pressure Isolation (Pressure Maintain)
The electro-hydraulic control unit is activated when the brakes are applied which sends a signal to the coil integrated module to prepare for a possible anti-lock stop. If the information from the wheel speed sensors indicates excessive wheel deceleration (imminent lockup), the first step in the anti-lock sequence is to isolate the brake pressure being applied by the brake cedar.
The microprocessor in the coil integrated module sends a voltage to the coil to energize and close the isolation valve. This prevents any additional fluid pressure applied by the brake pedal from reaching the wheel. With the isolation valves closed, further unnecessary increase in the brake pressure is therefore prevented.
Pressure Reduction
Once the brake pressure is isolated, it must be reduced to allow the wheels to unlock. This is accomplished by dumping a portion of the brake fluid pressure into a low pressure accumulator.
The microprocessor activates the normally closed dump valve to open, allowing fluid from the wheels to be dumped into the accumulator. This is done with very short activation pulses opening and closing the dump valve passageway. Brake pressure is reduced at the wheel and allows the wheel to begin rotating again. The fluid from the brake piston is stored in the accumulator against spring pressure and a portion of this fluid also primes the pump.
The dump valves are operated independently to control the deceleration of the wheel. At this point, the brake pedal is isolated from the base brake system, the hydraulic control unit pumps are primed and the attenuators are ready to pump fluid.
Pressure Increase (Re-Apply)
The re-apply sequence is initiated to achieve optimum braking. The isolation valve is momentarily opened to allow master cylinder and pump pressure to reach the brakes. This controlled pressure rise continues until the wheel is at optimum brake output or until the brake pressure is brought up to the master cylinder output pressure.
If more pressure is required, more fluid is drawn from the master cylinder and applied to the brakes. The driver may feel slight pedal pulsations, or pedal drop, this is normal and expected.
As fluid is re-applied to the brakes, the wheel speed will reduce. If the wheels approach imminent lockup again, the module will isolate, dump and re-apply again. This cycle occurs in millisecond intervals, allowing several cycles to occur each second. It is a much faster and more controlled way of "pumping the pedal".
Brake Release
At the end of the anti-lock stop, when the brake pedal is released, the pump will remain running for a short time to help drain any fluid from the accumulators. As this fluid returns into the system, the spring forces the piston back to its original position.
The isolation valve opens and fluid may return to the master cylinder. Conventional braking is then resumed.