The ABS Control Module, or ECU, sends commands in the form of
electrical signals to the hydraulic control unit. This unit
executes the commands, using three solenoid valves connected in
series with the master cylinder and the brake circuits - one
valve for each front wheel hydraulic circuit, and one for both
of the rear wheels. In normal, non-ABS braking, brake pedal
force is transmitted to the master cylinder, then through the
solenoid valve to the brake unit at the wheel. When the signals
from the wheel speed sensor show no tendency for the wheel to
lock up, the ECU does not send any control current to the
solenoid coil. The solenoid valve is not energized, and the
hydraulic pressure from the master cylinder is supplied to the
brake unit at the wheel. When the control unit detects any lock-
up tendency, perhaps from too-rapid wheel deceleration, it sends
a command current to the solenoid coil. This causes the armature
and valve to move upward, and isolate the brake circuit from the
master cylinder. That keep