Jeep Wrangler TJ. Manual - part 256

FRONT WHEEL SPEED
SENSOR

DESCRIPTION

A speed sensor is used at each wheel. The front

sensors are mounted to the steering knuckles. The
rear sensors are mounted to the rear brake backing
plate.

OPERATION

The sensors convert wheel speed into a small AC

electrical signal. This signal is transmitted to the
CAB. The CAB convert the AC signal into a digital
signal for each wheel. This voltage is generated by
magnetic induction when a tone wheel passes by the
stationary magnetic of the wheel speed sensor.

A gear type tone ring serves as the trigger mecha-

nism for each sensor. The tone rings are mounted at
the outboard ends of the front and rear axle shafts.

Different sensors are used at the front and rear

wheels (Fig. 1). The front/rear sensors have the same
electrical values but are not interchangeable. The
sensors have a resistance between 900 and 1300
ohms.

FRONT SENSOR AIR GAP

Front sensor air gap is fixed and not adjustable.

Only rear sensor air gap is adjustable.

Although front air gap is not adjustable, it can be

checked if diagnosis indicates this is necessary. Front
air gap should be 0.40 to 1.3 mm (0.0157 to 0.051
in.). If gap is incorrect, the sensor is either loose, or
damaged.

REAR SENSOR AIR GAP

A rear sensor air gap adjustment is only needed

when reinstalling an original sensor. Replacement

sensors have an air gap spacer attached to the sensor
pickup face. The spacer establishes correct air gap
when pressed against the tone ring during installa-
tion. As the tone ring rotates, it peels the spacer off
the sensor to create the required air gap. Rear sensor
air gap is 0.28-1.5 mm (0.011-0.059 in.).

Sensor air gap measurement, or adjustment proce-

dures are provided in this section. Refer to the front,
or rear sensor removal and installation procedures as
required.

REMOVAL

(1) Raise vehicle and turn wheel outward to access

the sensor.

(2) Disconnect sensor wire connector at harness

plug.

(3) Remove sensor wire from mounting retainers.
(4) Clean sensor and surrounding area with shop

towel before removal.

(5) Remove

bolt

attaching

sensor

steering

knuckle and remove sensor (Fig. 2).

INSTALLATION

(1) If original sensor will be installed, wipe all

traces of old spacer material off sensor pickup face.
Use a dry shop towel for this purpose.

(2) Apply Mopar Lock N’ Seal or Loctite

t 242 on

bolt that secures sensor in steering knuckle. Use new
sensor bolt if original bolt is worn or damaged.

Fig. 1 Typical Wheel Speed Sensors

1 - FRONT SENSOR
2 - REAR SENSOR
3 - PICKUP FACE

Fig. 2 Front Wheel Speed Sensor

1 - WHEEL SPEED SENSOR PIGTAIL
2 - STEERING KNUCKLE
3 - TONE WHEEL
4 - FRONT WHEEL SPEED SENSOR

BRAKES - ABS

5 - 41

(3) Position sensor on steering knuckle. Seat sen-

sor locating tab in hole in knuckle and install sensor
attaching bolt finger tight.

(4) Tighten sensor attaching bolt to 4-6 N·m (34-50

in. lbs.).

(5) If original sensor has been installed, check sen-

sor air gap. Air gap should be 0.40 to 1.3 mm (0.0157
to 0.051 in.). If gap is incorrect, sensor is either loose,
or damaged.

(6) Route sensor wire and install into mounting

retainers.

(7) Connect sensor wire to harness.

G-SWITCH

DESCRIPTION

The G-switch is located in front of the console/

shifter mounted to a bracket on the floor pan. The
switch has directional arrow and must be mounted
with the arrow pointing towards the front of the
vehicle.

OPERATION

The switch (Fig. 3) , provides an additional vehicle

deceleration reference during 4-wheel drive opera-
tion. The switch is monitored by the CAB at all
times. The switch reference signal is utilized by the
CAB when all wheels are decelerating at the same
speed.

REMOVAL

(1) From the drivers side lift carpet back in front

of the console/shifter.

(2) Disconnect harness for switch.

(3) Remove mounting bolts and remove switch

(Fig. 4).

INSTALLATION

CAUTION:

The

mercury

switch

(inside

the

G-switch), will not function properly if the switch is
installed incorrectly. Verify that the switch locating
arrow is pointing to the front of the vehicle (Fig. 3).

(1) Position switch on mounting bracket.
(2) Install mounting bolts and tighten to 4-5 N·m

(35-45 in. lbs.)

(3) Connect harness to switch.
(4) Place carpet back into position.

REAR WHEEL SPEED SENSOR

DESCRIPTION

A speed sensor is used at each wheel. The front

sensors are mounted to the steering knuckles. The
rear sensors are mounted to the rear brake backing
plate.

OPERATION

The sensors convert wheel speed into a small AC

A gear type tone ring serves as the trigger mecha-

nism for each sensor. The tone rings are mounted at
the outboard ends of the front and rear axle shafts.

Different sensors are used at the front and rear

wheels (Fig. 1). The front/rear sensors have the same
electrical values but are not interchangeable. The

Fig. 3 G-Switch

1 - SWITCH PART NUMBER
2 - ARROW INDICATES FRONT OF SWITCH FOR PROPER
MOUNTING

Fig. 4 G-Switch

1 - ACCELERATION SWITCH
2 - MOUNTING BRACKET

5 - 42

BRAKES - ABS

FRONT WHEEL SPEED SENSOR (Continued)

sensors have a resistance between 900 and 1300
ohms.

REMOVAL

(1) Disconnect sensors at rear harness connectors.
(2) Remove wheel and tire assembly.
(3) Remove brake drum.
(4) Remove clips securing sensor wires to brake

lines, rear axle and, brake hose.

(5) Unseat sensor wire support plate grommet.
(6) Remove bolt attaching sensor to bracket (Fig.

5) and remove sensor.

INSTALLATION

(1) If original sensor is being installed, remove

any remaining pieces of cardboard spacer from sen-
sor pickup face. Use dry shop towel only to remove
old spacer material.

(2) Insert sensor wire through support plate hole.

Then seat sensor grommet in support plate.

(3) Apply Mopar Lock N’ Seal or Loctite

t 242 to

original sensor bolt. Use new bolt if original is worn
or damaged.

(4) Install sensor bolt finger tight only at this

time.

(5) If original rear sensor was installed, adjust

sensor air gap to 0.28-1.5 mm (0.011-0.059 in.). Use
feeler gauge to measure air gap (Fig. 6). Tighten sen-
sor bolt to 12-14 N·m (106-124 in. lbs.).

(6)

If new sensor was installed, push cardboard

spacer on sensor face against tone ring (Fig. 7). Then
tighten sensor bolt to 12-14 N·m (106-124 in. lbs.).
Correct air gap will be established as tone ring
rotates and peels spacer off sensor face.

(7) Secure the rear sensor wires to the retainer

clips. Verify that wire is clear of rotating components.

(8) Connect sensor wire to harness connector.

(9) Install brake drum and wheel and tire assem-

bly. (Refer to 22 - TIRES/WHEELS/WHEELS -
STANDARD PROCEDURE).

(10) Lower vehicle.
(11) Connect sensor wire to harness connector.

HCU (HYDRAULIC CONTROL
UNIT)

DESCRIPTION

The HCU consists of a valve body, pump motor,

and wire harness.

OPERATION

Accumulators in the valve body store extra fluid

released to the system for ABS mode operation. The
pump provides the fluid volume needed and is oper-
ated by a DC type motor. The motor is controlled by
the CAB.

Fig. 5 Wheel Speed Sensor

1 - TONE WHEEL
2 - WHEEL SPEED SENSOR

Fig. 6 Setting Air Gap On Original Rear Sensor

1 - WHEEL SPEED SENSOR
2 - BRASS FEELER GAUGE
3 - TONE RING

Fig. 7 New Rear Sensor

1 - REAR SENSOR
2 - AIR GAP SPACER ATTACHED TO SENSOR FACE

BRAKES - ABS

5 - 43

REAR WHEEL SPEED SENSOR (Continued)

The

valves

modulate

brake

pressure

during

antilock braking and are controlled by the CAB.

The HCU provides three channel pressure control

to the front and rear brakes. One channel controls
the rear wheel brakes in tandem. The two remaining
channels control the front wheel brakes individually.

During antilock braking, the solenoid valves are

opened and closed as needed. The valves are not
static. They are cycled rapidly and continuously to
modulate pressure and control wheel slip and decel-
eration.

During normal braking, the HCU solenoid valves

and pump are not activated. The master cylinder and
power booster operate the same as a vehicle without
an ABS brake system.

During antilock braking, solenoid valve pressure

modulation occurs in three stages, pressure increase,
pressure hold, and pressure decrease. The valves are
all contained in the valve body portion of the HCU.

PRESSURE DECREASE

The outlet valve is opened and the inlet valve is

closed during the pressure decrease cycle.

A pressure decrease cycle is initiated when speed

sensor signals indicate high wheel slip at one or
more wheels. At this point, the CAB closes the inlet
then opens the outlet valve, which also opens the
return circuit to the accumulators. Fluid pressure is
allowed to bleed off (decrease) as needed to prevent
wheel lock.

Once the period of high wheel slip has ended, the

CAB closes the outlet valve and begins a pressure
increase or hold cycle as needed.

PRESSURE HOLD

Both solenoid valves are closed in the pressure

hold cycle. Fluid apply pressure in the control chan-
nel is maintained at a constant rate. The CAB main-
tains the hold cycle until sensor inputs indicate a
pressure change is necessary.

PRESSURE INCREASE

The inlet valve is open and the outlet valve is

closed during the pressure increase cycle. The pres-
sure increase cycle is used to counteract unequal
wheel speeds. This cycle controls re-application of
fluid apply pressure due to changing road surfaces or
wheel speed.

REMOVAL

(1) Install prop rod on the brake pedal to keep

pressure on the brake system.

(2) Remove negative battery cable from the bat-

tery.

(3) Pull up on the CAB harness connector release

(Fig. 8)and remove connector.

(4) Remove brake lines from the HCU.
(5) Remove HCU/CAB mounting nuts and bolt

(Fig. 9) and remove HCU/CAB.

INSTALLATION

NOTE: If the CAB is being replaced with a new CAB
is must be reprogrammed with the use of a DRB III.

(1) Install HCU/CAB on the mounting studs.
(2) Install mounting nuts and bolt. Tighten to 11.5

N·m (102 in. lbs.).

(3) Install brake lines to the HCU and tighten to

19 N·m (170 in. lbs.).

(4) Install wiring harness connector to the CAB and

push down on the release to secure the connector.

(5) Install negative battery cable to the battery.
(6) Bleed ABS brake system (Refer to 5 - BRAKES

- STANDARD PROCEDURE).

Fig. 8 CAB Harness Connector Release

1 - CAB
2 - CAB HARNESS RELEASE

Fig. 9 HCU/CAB Mounting

1 - HCU
2 - CAB
3 - HCU/CAB BRACKET
4 - MOTOR

5 - 44

BRAKES - ABS

HCU (HYDRAULIC CONTROL UNIT) (Continued)

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