Chrysler PT Cruiser. Manual - part 435

Excessive tone wheel runout can cause erratic

wheel speed sensor signals. For tone wheel runout
specifications, (Refer to 5 - BRAKES - ABS - SPECI-
FICATIONS). If tone wheel runout is excessive,
determine if it is caused by a defect in the driveshaft
assembly or hub and bearing. Replace as necessary.

Tone wheels are pressed onto their mounting sur-

faces and should not rotate independently from the
mounting surface. Replace the front driveshaft or
rear hub and bearing as necessary.

TRACTION CONTROL SWITCH

REMOVAL

NOTE: The Traction Control Switch (Fig. 13) is
located within the accessory switch bezel on the
instrument panel.

(1) Remove the accessory switch bezel and then

remove the traction control switch from it. (Refer to
23

BODY/INSTRUMENT

PANEL/ACCESSORY

SWITCH BEZEL - REMOVAL)

INSTALLATION

(1) Install the traction control switch in the acces-

sory switch bezel, then install the bezel on the
instrument panel. (Refer to 23 - BODY/INSTRU-
MENT

PANEL/ACCESSORY

SWITCH

BEZEL

INSTALLATION)

(2) Road test the vehicle to ensure proper opera-

tion of the switch and ABS.

HYDRAULIC CONTROL UNIT
(HCU)

DESCRIPTION

DESCRIPTION - MK20E

On early production vehicles equipped with the

MK20e ABS, the hydraulic control unit (HCU) is
mounted to the CAB as part of the ICU. The HCU
controls the flow of brake fluid to the brakes using a
series of valves and accumulators. A pump/motor is
mounted on the HCU to supply build pressure to the
brakes during an ABS stop.

VALVES AND SOLENOIDS

The valve block contains four inlet valves and four

outlet solenoid valves. The inlet valves are spring-
loaded in the open position and the outlet valves are
spring-loaded in the closed position during normal
braking. The fluid is allowed to flow from the master
cylinder to the wheel brakes.

During an ABS stop, these valves cycle to maintain

the proper slip ratio for each wheel. The inlet valve
closes preventing further pressure increase and the
outlet valve opens to provide a path from the wheel
brake to the HCU accumulators and pump/motor.
This releases (decays) pressure from the wheel brake,
thus releasing the wheel from excessive slippage.
Once the wheel is no longer slipping, the outlet valve
is closed and the inlet valve is opened to reapply
(build) pressure.

If the ABS includes the traction control feature,

there are four other valves in the HCU. Two traction
control (TC) valves, mounted in the HCU valve block,
are normally in the open position and close only
when the traction control is applied. There are also
two shuttle valves which control pressure return to
the master cylinder under ABS and traction control
conditions.

These TC valves are used to isolate the rear (non-

driving) wheels of the vehicle from the hydraulic
pressure that the HCU pump/motor is sending to the
front (driving) wheels when traction control is being
applied. The rear brakes need to be isolated from the
master cylinder when traction control is being
applied so the rear wheels do not drag.

Fig. 13 Traction Control Switch Location

1 - TRACTION CONTROL SWITCH

5 - 92

BRAKES - ABS

TONE WHEEL (Continued)

BRAKE FLUID ACCUMULATORS

There are two fluid accumulators in the HCU: one

for the primary hydraulic circuit, and one for the sec-
ondary hydraulic circuit. Each hydraulic circuit uses
a 3 cc accumulator.

The fluid accumulators temporarily store brake

fluid that is removed from the wheel brakes during
an ABS cycle. This stored fluid is used by the pump/
motor to provide build pressure for the brake hydrau-
lic system. When the antilock stop is complete, the
accumulators are drained by the pump/motor.

There are two noise dampening chambers in the

HCU on this vehicle equipped with traction control.

PUMP/MOTOR

There are two pump assemblies in the HCU: one

for the primary hydraulic circuit, and one for the sec-
ondary hydraulic circuit. Both pumps are driven by a
common electric motor. This DC-type motor is inte-
gral to the HCU and is controlled by the CAB.

The pump/motor provides the extra amount of

brake fluid needed during antilock braking. Brake
fluid is released to the accumulators when the outlet
valve is opened during an antilock stop. The pump
mechanism consists of two opposing pistons operated
by an eccentric camshaft. In operation, these pistons
are used to purge fluid from the accumulators back
into the master cylinder circuits. When the antilock
stop is complete, the pump/motor drains the accumu-
lators.

The pump motor is also used to build pressure

when the system goes into traction control mode.

The CAB may turn on the pump/motor when an

antilock stop is detected. The pump/motor continues
to run during the antilock stop and is turned off after
the stop is complete. Under some conditions, the
pump/motor runs to drain the accumulators during
the next drive-off.

The pump/motor is not a serviceable item; if it

requires replacement, the HCU must be replaced.

For more information, (Refer to 5 - BRAKES/HY-

DRAULIC/MECHANICAL/HCU (HYDRAULIC CON-
TROL

UNIT)

OPERATION

HYDRAULIC

CIRCUITS AND VALVES).

DESCRIPTION - MK25E

On Normal Launch production vehicles (which are

equipped with the MK25e ABS), the hydraulic control
unit (HCU) is mounted to the ABM as part of the
ICU. The HCU controls the flow of brake fluid to the
brakes using a series of valves and accumulators. A
pump/motor is mounted on the HCU to supply build
pressure to the brakes during an ABS stop.

VALVES AND SOLENOIDS

The valve block contains four inlet valves and four

During an ABS stop, these valves cycle to maintain

If the ABS includes the traction control feature,

These TC valves are used to isolate the rear (non-

BRAKES - ABS

5 - 93

HYDRAULIC CONTROL UNIT (HCU) (Continued)

BRAKE FLUID ACCUMULATORS

There are two fluid accumulators in the HCU: one

for the primary hydraulic circuit, and one for the sec-
ondary hydraulic circuit. Each hydraulic circuit uses
a 3 cc accumulator.

The fluid accumulators temporarily store brake

There are two noise dampening chambers in the

HCU on this vehicle equipped with traction control.

PUMP/MOTOR

There are two pump assemblies in the HCU: one

The pump/motor provides the extra amount of

The pump motor is also used to build pressure

when the system goes into traction control mode.

The ABM may turn on the pump/motor when an

The pump/motor is not a serviceable item; if it

requires replacement, the HCU must be replaced.

For more information, (Refer to 5 - BRAKES/HY-

DRAULIC/MECHANICAL/HCU (HYDRAULIC CON-
TROL

UNIT)

OPERATION

HYDRAULIC

CIRCUITS AND VALVES).

OPERATION - HYDRAULIC CIRCUITS AND
VALVES

NOTE: The following applies for both MK20e and
MK25e ABS.

The hydraulic fluid control valves within the HCU

control the flow of pressurized brake fluid to the
wheel brakes during the different modes of ABS
braking and traction control. The following para-
graphs explain how this works. For purposes of
explanation only, the following diagrams show
only one hydraulic circuit, the Right Front
wheel braking circuit.

NORMAL BRAKING HYDRAULIC CIRCUIT AND
SOLENOID VALVE FUNCTION (ABS WITHOUT
TRACTION CONTROL)

The hydraulic diagram shows the vehicle in the

normal braking mode of the base brake hydraulic
system (Fig. 14). The diagram shows no wheel spin
or slip occurring relative to the speed of the vehicle.
The driver is applying the brake pedal which builds
pressure in the brake hydraulic system to engage the
brakes and stop the vehicle.

5 - 94

BRAKES - ABS

HYDRAULIC CONTROL UNIT (HCU) (Continued)

Fig. 14 Hydraulic Circuit - Normal Braking Mode (Without Traction Control)

1 - OUTLET VALVE
2 - PUMP PISTON
3 - PUMP/MOTOR
4 - LOW PRESSURE ACCUMULATOR
5 - NORMALLY CLOSED VALVE (OFF)

6 - TO WHEEL BRAKE
7 - NORMALLY OPEN VALVE (OFF)
8 - MASTER CYLINDER PRESSURE
9 - FROM MASTER CYLINDER
10 - NOISE DAMPER CHAMBER

BRAKES - ABS

5 - 95

HYDRAULIC CONTROL UNIT (HCU) (Continued)

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