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Chrysler Town, Dodge Caravan. Manual - part 34

SPECIFICATIONS - REAR DRIVELINE MODULE

TORQUE SPECIFICATIONS

DESCRIPTION

N·m

Ft. Lbs.

In. Lbs.

Bolt, Driveline Module-to-Body

—

Bolt, Halfshaft-to-Ouput Flange

—

Bolt, Overrunning Clutch Housing-to-Differential

—

Bolt, Torque Arm-to-Differential Assembly

—

Bolt, Torque Arm Mount-to-Body

—

Nut, Input Flange

135

100

—

Plug, Differential Drain/Fill

—

Plug, Overrunning Clutch Housing Drain/Fill

—

Vent, Differential/Overrunning Clutch Housing

—

110

SPECIAL TOOLS

BI-DIRECTIONAL
OVERRUNNING CLUTCH

DESCRIPTION

The bi-directional overrunning clutch (BOC) (Fig.

28) works as a mechanical disconnect between the
front and rear axles, preventing torque from being
transferred from the rear axle to the front. The BOC
is a simply an overrunning clutch which works in
both clockwise and counter-clockwise rotations. This
means that when the output (the rear axle) is rotat-
ing faster in one direction than the input (front axle),
there is no torque transmission. But when the input
speed is equal to the output speed, the unit becomes
locked. The BOC provides significant benefits regard-
ing braking stability, handling, and driveline durabil-
ity. Disconnecting the front and the rear driveline
during braking helps to maintain the braking stabil-
ity of an AWD vehicle. In an ABS/braking event, the
locking of the rear wheels must be avoided for stabil-
ity reasons. Therefore brake systems are designed to
lock the front wheels first. Any torque transfer from
the rear axle to the front axle disturbs the ABS/brak-
ing system and causes potential instabilities on a
slippery surface. The BOC de-couples the rear driv-
eline as soon the rear wheels begin to spin faster
than the front wheels (front wheels locked) in order
to provide increased braking stability. Furthermore
the BOC also reduces the likelihood of throttle off
over-steer during cornering. In a throttle off maneu-
ver, the BOC once again de-couples the rear driveline
forcing all the engine brake torque to the front
wheels. This eliminates the chance of lateral slip on
the rear axle and increases it on the front. The vehi-
cle will therefore tend to understeer, a situation
which is considered easier to manage in most circum-
stances. During this maneuver, and during the ABS
braking event, the BOC does not transmit torque

Tool 6958

Tool 8493

Tool 8802

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REAR DRIVELINE MODULE

REAR DRIVELINE MODULE (Continued)

through to the rear wheels. The rear driveline mod-
ule, with the BOC, will perform the same as a front
wheel drive vehicle during these events. The gear
ratio offset between the front and rear differentials
force the BOC into the overrunning mode most of the

time. This allows BOC to significantly reduce the
rolling resistance of the vehicle, which improves fuel
consumption, allows the downsizing of the driveline
components, and prevents the PTU and propshaft
joints from overheating.

Fig. 28 Bi-directional Overrunning Clutch and Viscous Coupler

1 - POWERFLOW - BOC OVERUNNING

6 - VISCOUS COUPLER

2 - POWERFLOW - BOC LOCKED

7 - BOC ROLLER CAGE

3 - BOC GROUND TAB

8 - BOC INPUT SHAFT

4 - FRICTION BRAKE SHOES

9 - INPUT FLANGE

5 - BOC ROLLERS

REAR DRIVELINE MODULE

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BI-DIRECTIONAL OVERRUNNING CLUTCH (Continued)

OPERATION

In order to achieve all-wheel drive operation in

reverse, the overrunning clutch locking functional
direction must be reversible. The bi-directional over-
running clutch (BOC) changes the operational mode
direction depending on the propeller shaft direction.
The propeller shaft rotates in the clockwise (when
viewed from the front) direction when the vehicle is
moving forward, which indexes the BOC to the for-
ward overrunning position. When the vehicle is in
reverse, the propeller shaft will rotate counter-clock-
wise and index the BOC to the reverse overrunning
position.

The BOC acts as a mechanical stator. It is active

(transmitting torque), or it is not active and in over-
running mode (not transmitting torque). This “all or
nothing” approach to torque transfer would cause a
sudden application of all available power to the rear
wheels, which is not desirable. Therefore it is run in
series with a viscous coupler to smooth, dampen, and
limit the transmission of torque to the rear axle and
to prevent a step style torque input to the rear axle.

STEADY STATE, LOW TO MODERATE SPEED, NO
FRONT WHEEL SLIP, FORWARD DIRECTION

During normal driving conditions, (no wheel slip),

the inner shaft (front axle) and outer race (viscous
coupler) are running at different speeds due to the
different gear ratios between the front and rear dif-
ferentials. In this condition, the outer race is always
spinning faster (overdriving between 5-32 rpm) than
the inner shaft. When the BOC (Fig. 29) is running
under these conditions, at low vehicle speeds the
drag shoes and the cage keep the rollers up on the
left side (forward side) of the inner shaft flats. This is
what is known as “overrunning mode.” Notice that
when the clutch is in overrunning mode, the rollers
are spinning clockwise and with the outer race, thus
no torque is being transferred.

NOTE: Low speed, forward and reverse operation is
identical, just in opposite directions. (Fig. 29)
shows forward direction in reverse the rollers are
on the other side of the flats due to a reversal of
the cage force.

TRANSIENT CONDITION (BOC LOCKED), FRONT
WHEEL SLIP, FORWARD DIRECTION

When the front wheels lose traction and begin to

slip, the propeller shaft and rear axle pinion speed
difference decreases to zero. At this point the input
shaft (cam) becomes the driving member of the BOC
(Fig. 30), compressing the rollers against the outer
race. This locks the input shaft with the outer race
and transmits torque to the housing of the viscous
coupler, that in turn transmits torque to the rear
axle pinion. It should also be noted that when the
device is locked, the inner shaft and the outer race
are rotating at the same speed. The rollers are
pinched at this point and will stay locked until a
torque reversal (no front wheel slip) occurs. When
locked, the viscous coupler slips during the torque
transfer and the amount of torque transferred is
dependent on the coupling characteristic and the
amount of front wheel slip.

Fig. 29 BOC Operation at Low Speeds With No

Front Wheel Slip

1 - CAGE
2 - ROLLER
3 - INPUT SHAFT

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REAR DRIVELINE MODULE

BI-DIRECTIONAL OVERRUNNING CLUTCH (Continued)

STEADY STATE, HIGH SPEED, NO WHEEL SLIP

The roller cage positions the rollers on the input

shaft flats during low and high speed overrunning
and during initial BOC lockup. The roller cage is
rotating at input shaft (propeller shaft) speed at all
times. At low speeds, the friction shoes (Fig. 31) are
pressed against the friction ground via the garter
spring (Fig. 32), creating a drag force on the roller
cage. The drag force positions the cage, which in turn
positions the rollers to one side of the flat. The direc-
tion of this drag force (position of the roller) is depen-
dent

the

input

(propeller

shaft)

rotational

direction. Since the rollers are always in contact with
the outer race, due to centrifugal forces, the rollers
want to follow the outer race due to drag. During
overrunning operation, the outer race is rotating
faster than the input; causing the rollers to want to
traverse the flat from one side to the other. During
low speeds, the brake shoes counteract this effect. To
avoid excessive wear, the ground shoes are designed
to lift off from the friction ground due to centrifugal
forces at higher rotational speeds.

To keep the rollers in the overrunning position and

avoid undesired

9high speed lockup9, a high speed

latch (Fig. 33) positions the cage before the ground
shoes lift off. A further explanation of the high speed
effects follows as well. Utilizing only the friction
shoes approach means that at high speed the
required ground shoe drag torque would cause exces-
sive brake shoe wear or the roller will begin to
migrate to the opposite side of the flat due to the
drag force of the outer race. This would result in sys-
tem lock-up. (Fig. 34) shows the BOC as it crosses

the speed where the brake shoe force is overcome by
the roller drag on the outer race. Notice that the
roller is locking up on the opposite side of the flat
and the cage supplies no force on the rollers.

Fig. 30 BOC Operation with Front Wheel Slip

Fig. 31 Front View of BOC

1 - GARTER SPRING
2 - FRICTION BRAKE SHOES
3 - FRICTION GROUND CONNECTED TO GROUND TAB
4 - INPUT SHAFT

Fig. 32 Location of the Grounding Element

1 - DIFFERENTIAL HOUSING
2 - GROUND TAB
3 - GARTER SPRING

REAR DRIVELINE MODULE

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BI-DIRECTIONAL OVERRUNNING CLUTCH (Continued)

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