Expedition 2WD V8-5.4L SOHC VIN L (1997)
Hydraulic Control Assembly - Antilock Brakes: Description and Operation
OPERATION
When the brakes are applied, brake fluid is forced from the brake master cylinder outlet ports to the Hydraulic Control Unit (HCU) inlet ports.
The fluid pressure is transmitted through three normally open solenoid valves inside the HCU, through the outlet ports of HCU to the rear brakes.
One circuit of the brake master cylinder feeds the front brakes, while the other circuit feeds the rear brakes. If the Electro-Hydraulic Control Unit (
EHCU) senses that a wheel is about to lock, based on wheel speed sensor data, the solenoid valve will pulse closed preventing more fluid from
entering that circuit. The HCU then reads the sensor signal from the affected wheel again. If the wheel is still decelerating, the normally closed
solenoid valve for that circuit is opened. Any remaining pressure between the normally open valve and the brake is relieved into the HCU
accumulator. Once the affected wheel returns to vehicle speed, the HCU returns the solenoid valves to their normal condition allowing fluid flow
to the affected brake.
Brake Pulsation
During the 4WABS operation, the driver may feel a pulsation in the brake pedal, accompanied by a slight up and down movement in the pedal
height. In addition, a mechanical noise from the engine compartment may be heard. The pedal effort and pedal feel during normal braking are
similar to that of a conventional power brake system.
