ANTI-LOCK BRAKE SYSTEM
D1
CIRCUIT OPERATION
2
ABS Booster Unit (Z103)
The ABS Booster Unit (Z103) contains 2 isolation
solenoid valves and 4 pairs of solenoid control
valves which are grounded through the harness.
When the ETC is fitted, the booster unit also
contains an additional 2 solenoid valves. The pairs of
solenoid control valves each include a fluid pressure
inlet and outlet valves that control ABS braking to
one wheel.
The Anti–Lock Brake System ECU (Z108) operates
these valves by applying battery voltage to them.
The valves are designed to decrease, hold or
increase pressure to retain wheel rotation and
optimum braking.
The 2 isolation valves consist of 2 solenoid valves
that control fluid inlet and outlet. Their function is to
disconnect or isolate the master cylinder from the
servo cylinder and to connect the servo cylinder to
the reservoir return during ABS functions.
Wheel Speed Sensors (X137, X140, X158, X161)
A wheel speed sensor is located at each wheel. The
speed sensors generate an AC voltage signal as a
toothed ring rotates past the stationary sensor
pickup. The Anti–Lock Brake System ECU (Z108)
calculates the wheel speed by measuring the
frequency of the AC voltage signal generated by the
sensors.
ABS Hydraulic Pump (M102)
The hydraulic boost for the system is provided by the
ABS Hydraulic Pump (M102), which is controlled by
the ABS Pump Relay (K102) and the ABS Pressure
Switch Unit (Z104).
The Pressure Switch unit incorporates three
electro– mechanical switches. The first operates the
pump, two more illuminate the low pressure
condition and signal that ABS and ETC functions
should be curtailed. The Hydraulic Pump includes a
hydraulic accumulator and non–return valve, as well
as a pressure relief valve to protect the system.
When low pressure occurs in the brake system, a
switch in the pressure switch unit closes to ground
the coil of the pump relay. The pump relay now
energizes and applies battery voltage from the fuse
to the hydraulic pump through the closed relay
contacts. The hydraulic pump runs to increase
pressure in the hydraulic accumulator. When
sufficient pressure is developed in the system, the
pressure switch opens to de–energize the pump
relay and to turn off the hydraulic pump.
ABS Warning Lamp
The Anti–Lock Brake System ECU (Z108) sends
signals to the BeCM which then controls the
illumination of the ABS warning lamp. The BeCM
also controls the display of the system messages.
Solenoid Valve Relay
This is energized by the ABS ECU and sends
voltage back to the part of the same ECU which
controls the solenoid valves in the booster. When the
ABS ECU detects a fault that requires a complete
system shutdown, the relay is de–energized.