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BRAKES - ABS
HCU (HYDRAULIC CONTROL UNIT) (Continued)
ABS BRAKING HYDRAULIC CIRCUIT, SOLENOID
VALVE, AND SHUTTLE VALVE FUNCTION (ABS
WITH TRACTION CONTROL)
The hydraulic diagram (Fig. 18) shows the vehicle
in the ABS braking mode. The diagram shows 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 posi-
tion of the normally open valve.
Fig. 18 ABS With Traction Control - ABS Braking Hydraulic Circuit
1 - OUTLET VALVE
2 - PUMP PISTON
3 - PUMP MOTOR
4 - SUCTION VALVE
5 - LOW PRESSRUE ACCUMULATOR
6 - NORMALLY CLOSED VALVE (MODULATING)
7 - TO RIGHT FRONT WHEEL
8 - NORMALLY OPEN VALVE (MODULATING)
9 - NORMALLY OPEN ASR VALVE (OFF)
10 - FROM MASTER CYLINDER
11 - HYDRAULIC SHUTTLE VALVE
12 - NOISE DAMPER CHAMBER
JR
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