Hydraulic Control Assembly - Antilock Brakes: Description and Operation
PURPOSEContained in the Hydraulic Control Unit (HCU) are one pair of hydraulic valves for each brake caliper. Each valve is operated by a solenoid that is in turn controlled by the ABS Control Unit. One valve of each pair controls the input of fluid to the caliper and can isolate that caliper from the master cylinder. The other valve controls the output of fluid from the caliper.
OPERATION
When a wheel is in a locking condition, the ABS Control Unit will close that wheel's isolation valve, reventing the master cylinder from providing any additional fluid into that wheel's caliper. The ABS Control Unit will then open the dump valve to bleed fluid out of the caliper and thus allow that wheel to rotate. The rate at which these valves are cycled is rapid and will continue until the ABS Control Unit determines that the wheels are under control.