Hcu (Hydraulic Control Unit); Description - Chrysler 2005 Crossfire SRT6 Service Manual

ZH BRAKES - ABS SERVICE INFORMATION 5 - 349

HCU (HYDRAULIC CONTROL UNIT)

DESCRIPTION

The CAB (Mark 20 or Mark 25 ) is a microprocessor-
based device which monitors the Antilock Brake Sys-
tem (ABS), and the Electronic Stability Program (ESP)
during normal braking and driving functions. The CAB
controls brake pressure when the vehicle is in an ABS
stop, or if the vehicle is in a spin. The CAB is mounted
to the Hydraulic Control Unit (HCU) (3) as part of the
Integrated Control Unit (ICU). The CAB uses a 47-way
electrical connector on the vehicle wiring harness. The
power source for the CAB is through the 50 amp fuse
located in the Engine Fuse Block (2).
OPERATION
All switching components of the ABS system are combined in the ABS hydraulic unit. A 2/2-way valve for build-up/
hold pressure or hold/reduce pressure functions is used on each wheel for control. At the rear, the same valves are
used for both wheels. The return pump integrated in the hydraulic unit is used to return the brake fluid during the
pressure reduction phase.
An increase in pressure compared to the pressure controlled by the master brake cylinder is not possible during the
ABS control phase. A muffler is fitted for each brake circuit to reduce delivery noise. During the ABS pressure reduc-
tion phase, the brake fluid flows back to the return pumps via the low-pressure accumulators.
NORMAL BRAKING FUNCTION
When the driver is applying the brake pedal during a no wheel spin or slip situation, the vehicle builds pressure in
the brake hydraulic system to engage the brakes and stop the vehicle. The hydraulic shuttle valve internal to the
ICU closes with every brake pedal application so pressure is not created at the inlet to the pump/motor and the
vehicle stops normally.
ABS BRAKING FUNCTION
During a stop where one wheel is slipping because the driver is attempting to stop the vehicle at a faster rate than
is allowed by the surface on which the tires are riding, the hydraulic shuttle valve closes upon brake application so
that the pump/motor cannot siphon brake fluid from the master cylinder. The normally open and normally closed
valves modulate (build/decay) the brake hydraulic pressure as required. The pump/motor is switched on so that the
brake fluid from the low pressure accumulators is returned to the master cylinder circuits. The brake fluid is routed
to either the master cylinder or the wheel brake, depending on the position of the normally open valve.
ABS TRACTION CONTROL FUNCTION
The vehicle in the traction control TC (ASR) mode. If the drive wheel is spinning and brake pressure is required to
reduce its speed. The normally open TC (ASR) valve is energized to isolate the brake fluid being pumped from the
master cylinder and to isolate the driven wheel. The normally open TC (ASR) valve bypasses the pump output back
to the master cylinder at a fixed pressure setting. The normally open and normally closed valves modulate (build/
decay) the brake pressure as required to slow the spinning wheel.