Jeep XJ. Manual — part 17
STANDARD DIFFERENTIAL
DESCRIPTION
The differential gear system divides the torque
between the axle shafts. It allows the axle shafts to
rotate at different speeds when turning corners.
Each differential side gear is splined to an axle
shaft. The pinion gears are mounted on a pinion
mate shaft and are free to rotate on the shaft. The
pinion gear is fitted in a bore in the differential case
and is positioned at a right angle to the axle shafts.
OPERATION
In operation, power flow occurs as follows:
• The pinion gear rotates the ring gear
• The ring gear (bolted to the differential case)
rotates the case
• The differential pinion gears (mounted on the
pinion mate shaft in the case) rotate the side gears
• The side gears (splined to the axle shafts) rotate
the shafts
During straight-ahead driving, the differential pin-
ion gears do not rotate on the pinion mate shaft. This
occurs because input torque applied to the gears is
divided and distributed equally between the two side
gears. As a result, the pinion gears revolve with the
pinion mate shaft but do not rotate around it (Fig. 1).
When turning corners, the outside wheel must
travel a greater distance than the inside wheel to
complete a turn. The difference must be compensated
for to prevent the tires from scuffing and skidding
through turns. To accomplish this, the differential
allows the axle shafts to turn at unequal speeds (Fig.
2). In this instance, the input torque applied to the
pinion gears is not divided equally. The pinion gears
now rotate around the pinion mate shaft in opposite
directions. This allows the side gear and axle shaft
attached to the outside wheel to rotate at a faster
speed.
Fig. 1 Differential Operation—Straight Ahead Driving
1 – IN STRAIGHT AHEAD DRIVING EACH WHEEL ROTATES AT
100% OF CASE SPEED
2 – PINION GEAR
3 – SIDE GEAR
4 – PINION GEARS ROTATE WITH CASE
Fig. 2 Differential Operation—On Turns
1 – PINION GEARS ROTATE ON PINION SHAFT
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TUBE, 181, AND 186 FBI AXLE
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DESCRIPTION AND OPERATION (Continued)
DIAGNOSIS AND TESTING
GENERAL INFORMATION
Axle bearing problem conditions are usually caused
by:
• Insufficient or incorrect lubricant.
• Foreign matter/water contamination.
• Incorrect bearing preload torque adjustment.
• Incorrect backlash.
Axle gear problem conditions are usually the result
of:
• Insufficient lubrication.
• Incorrect or contaminated lubricant.
• Overloading (excessive engine torque) or exceed-
ing vehicle weight capacity.
• Incorrect clearance or backlash adjustment.
Axle component breakage is most often the result
of:
• Severe overloading.
• Insufficient lubricant.
• Incorrect lubricant.
• Improperly tightened components.
• Differential housing bores not square to each
other.
DIAGNOSTIC CHART
Condition
Possible Causes
Correction
Wheel Noise
1. Wheel loose.
1. Tighten loose nuts.
2. Faulty, brinelled wheel bearing.
2. Replace bearing.
Axle Shaft
Noise
1. Misaligned axle tube.
1. Inspect axle tube alignment. Correct as
necessary.
2. Bent or sprung axle shaft.
2. Inspect and correct as necessary.
3. End-play in pinion bearings.
3. Refer to pinion pre-load information and correct
as necessary.
4. Excessive gear backlash between
the ring gear and pinion.
4. Check adjustment of the ring gear and pinion
backlash. Correct as necessary.
5. Improper adjustment of pinion gear
bearings.
5. Adjust the pinion bearings pre-load.
6. Loose pinion yoke nut.
6. Tighten the pinion yoke nut.
7. Scuffed gear tooth contact surfaces.
7. Inspect and replace as necessary.
Axle Shaft
Broke
1. Misaligned axle tube.
1. Replace the broken shaft after correcting tube
mis-alignment.
2 Vehicle overloaded.
2. Replace broken shaft and avoid excessive weight
on vehicle.
3. Erratic clutch operation.
3. Replace broken shaft and avoid or correct erratic
clutch operation.
4. Grabbing clutch.
4. Replace broken shaft and inspect and repair
clutch as necessary.
Differential
Cracked
1. Improper adjustment of the
differential bearings.
1. Replace case and inspect gears and bearings for
further damage. Set differential bearing pre-load
properly.
2. Excessive ring gear backlash.
2. Replace case and inspect gears and bearings for
further damage. Set ring gear backlash properly.
3. Vehicle overloaded.
3. Replace case and inspect gears and bearings for
further damage. Avoid excessive vehicle weight.
4. Erratic clutch operation.
4. Replace case and inspect gears and bearings for
further damage. Avoid erratic use of clutch.
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TUBE, 181, AND 186 FBI AXLE
3 - 19
Condition
Possible Causes
Correction
Differential
Gears Scored
1. Insufficient lubrication.
1. Replace scored gears. Fill differential with the
correct fluid type and quantity.
2. Improper grade of lubricant.
2. Replace scored gears. Fill differential with the
correct fluid type and quantity.
3. Excessive spinning of one wheel/tire.
3. Replace scored gears. Inspect all gears, pinion
bores, and shaft for damage. Service as necessary.
Loss Of
Lubricant
1. Lubricant level too high.
1. Drain lubricant to the correct level.
2. Worn axle shaft seals.
2. Replace seals.
3. Cracked differential housing.
3. Repair as necessary.
4. Worn pinion seal.
4. Replace seal.
5. Worn/scored yoke.
5. Replace yoke and seal.
6. Axle cover not properly sealed.
6. Remove, clean, and re-seal cover.
Axle
Overheating
1. Lubricant level low.
1. Fill differential to correct level.
2. Improper grade of lubricant.
2. Fill differential with the correct fluid type and
quantity.
3. Bearing pre-loads too high.
3. Re-adjust bearing pre-loads.
4. Insufficient ring gear backlash.
4. Re-adjust ring gear backlash.
Gear Teeth
Broke
1. Overloading.
1. Replace gears. Examine other gears and bearings
for possible damage.
2. Erratic clutch operation.
2. Replace gears and examine the remaining parts
for damage. Avoid erratic clutch operation.
3. Ice-spotted pavement.
3. Replace gears and examine remaining parts for
damage.
4. Improper adjustments.
4. Replace gears and examine remaining parts for
damage. Ensure ring gear backlash is correct.
Axle Noise
1. Insufficient lubricant.
1. Fill differential with the correct fluid type and
quantity.
2. Improper ring gear and pinion
adjustment.
2. Check ring gear and pinion contact pattern.
3. Unmatched ring gear and pinion.
3. Replace gears with a matched ring gear and
pinion.
4. Worn teeth on ring gear and/or
pinion.
4. Replace ring gear and pinion.
5. Loose pinion bearings.
5. Adjust pinion bearing pre-load.
6. Loose differential bearings.
6. Adjust differential bearing pre-load.
7. Mis-aligned or sprung ring gear.
7. Measure ring gear run-out. Replace components
as necessary.
8. Loose differential bearing cap bolts.
8. Inspect differential components and replace as
necessary. Ensure that the bearing caps are torqued
tot he proper specification.
9. Housing not machined properly.
9. Replace housing.
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TUBE, 181, AND 186 FBI AXLE
XJ
DIAGNOSIS AND TESTING (Continued)
GEAR NOISE
Axle gear noise can be caused by insufficient lubri-
cant, incorrect backlash, tooth contact, worn/damaged
gears, or the carrier housing not having the proper
offset and squareness.
Gear noise usually happens at a specific speed
range. The noise can also occur during a specific type
of driving condition. These conditions are accelera-
tion, deceleration, coast, or constant load.
When road testing, first warm-up the axle fluid by
driving the vehicle at least 5 miles and then acceler-
ate the vehicle to the speed range where the noise is
the greatest. Shift out-of-gear and coast through the
peak–noise range. If the noise stops or changes
greatly:
• Check for insufficient lubricant.
• Incorrect ring gear backlash.
• Gear damage.
Differential side gears and pinions can be checked
by turning the vehicle. They usually do not cause
noise during straight–ahead driving when the gears
are unloaded. The side gears are loaded during vehi-
cle turns. A worn pinion mate shaft can also cause a
snapping or a knocking noise.
BEARING NOISE
The axle shaft, differential and pinion bearings can
all produce noise when worn or damaged. Bearing
noise can be either a whining, or a growling sound.
Pinion bearings have a constant–pitch noise. This
noise changes only with vehicle speed. Pinion bearing
noise will be higher pitched because it rotates at a
faster rate. Drive the vehicle and load the differen-
tial. If bearing noise occurs, the rear pinion bearing
is the source of the noise. If the bearing noise is
heard during a coast, the front pinion bearing is the
source.
Worn or damaged differential bearings usually pro-
duce a low pitch noise. Differential bearing noise is
similar to pinion bearing noise. The pitch of differen-
tial bearing noise is also constant and varies only
with vehicle speed.
Axle shaft bearings produce noise and vibration
when worn or damaged. The noise generally changes
when the bearings are loaded. Road test the vehicle.
Turn the vehicle sharply to the left and to the right.
This will load the bearings and change the noise
level. Where axle bearing damage is slight, the noise
is usually not noticeable at speeds above 30 mph.
LOW SPEED KNOCK
Low speed knock is generally caused by a worn
U–joint or by worn side–gear thrust washers. A worn
pinion shaft bore will also cause low speed knock.
VIBRATION
Vibration at the rear of the vehicle is usually
caused by a:
• Damaged drive shaft.
• Missing drive shaft balance weight(s).
• Worn or out–of–balance wheels.
• Loose wheel lug nuts.
• Worn U–joint(s).
• Loose/broken springs.
• Damaged axle shaft bearing(s).
• Loose pinion gear nut.
• Excessive pinion yoke run out.
• Bent axle shaft(s).
Check for loose or damaged front–end components
or engine/transmission mounts. These components
can contribute to what appears to be a rear–end
vibration. Do not overlook engine accessories, brack-
ets and drive belts.
All driveline components should be examined
before starting any repair.
Refer to Group 22, Wheels and Tires, for additional
vibration information.
DRIVELINE SNAP
A snap or clunk noise when the vehicle is shifted
into gear (or the clutch engaged), can be caused by:
• High engine idle speed.
• Transmission shift operation.
• Loose engine/transmission/transfer case mounts.
• Worn U–joints.
• Loose spring mounts.
• Loose pinion gear nut and yoke.
• Excessive ring gear backlash.
• Excessive side gear to case clearance.
The source of a snap or a clunk noise can be deter-
mined with the assistance of a helper. Raise the vehi-
cle on a hoist with the wheels free to rotate. Instruct
the helper to shift the transmission into gear. Listen
for the noise, a mechanics stethoscope is helpful in
isolating the source of a noise.
SERVICE PROCEDURES
LUBRICANT CHANGE
(1) Raise and support the vehicle.
(2) Remove the lubricant fill hole plug from the
differential housing cover.
(3) Remove the differential housing cover and
drain the lubricant from the housing.
(4) Clean the housing cavity with a flushing oil,
light engine oil or lint free cloth. Do not use water,
steam, kerosene or gasoline for cleaning.
(5) Remove the sealant from the housing and cover
surfaces. Use solvent to clean the mating surfaces.
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TUBE, 181, AND 186 FBI AXLE
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DIAGNOSIS AND TESTING (Continued)
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