Briefly describe the stages of ABS solenoid operation.

Question

What will be an ideal response?

Answer 1

1. Pressure buildup, also called increase. This is normal braking. The computer does not energize either the inlet (block) or outlet (dump) solenoid valve, and pressure from the master cylinder flows normally to the brakes. This mode is called buildup, or increase, because when the driver pushes harder on the brake pedal, this will still increase hydraulic pressure to the wheels.

2. Pressure hold. The computer detects a wheel slowing rapidly during a stop. It closes the inlet valve, shutting off flow from the master cylinder and blocking further pressure increase at the wheel. The dump valve remains closed and system pressure remains constant at this point.

3. Pressure reduce. The computer reduces pressure to a wheel by opening the dump valve at the outlet while the inlet valve remains closed. Pressure escapes to a low-pressure area in the system. This can result in a drop in pedal height. When pedal travel reaches about 40%, the electric pump increases pressure. The computer energizes a relay, turning on the pump until the pedal height rises enough to close the switch. The pump is capable of more pressure than the ABS needs. Relief valves bleed off excess fluid pressure to the master cylinder reservoir.

4. Pressure reapply. During an ABS stop, the accumulator and pump can assist in reapplying the brakes as needed. You can sometimes feel the pedal height increase slightly due to the action of the pump on the master cylinder piston.