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 exceeding 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

Gear noise

Axle gear noise can be caused by insufficient lubricant, 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 acceleration, deceleration, coast, or constant load.

When road testing, first warm-up the axle fluid by driving the vehicle at least 5 miles and then accelerate 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 vehicle 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 differential.

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 produce a low pitch noise. Differential bearing noise is similar to pinion bearing noise. The pitch of differential 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 nut.
• Excessive companion flange 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, brackets 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 nut and companion flange.
• Excessive ring gear backlash.
• Excessive side gear to case clearance.

The source of a snap or a clunk noise can be determined with the assistance of a helper. Raise the vehicle 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.

Trac-loky differential noise

The most common problem is a chatter noise when turning corners. Before removing a Trac-loky unit for repair, drain, flush and refill the axle with the specified lubricant. Refer to Lubricant change in this Group.

A container of Mopart Trac-loky Lubricant (friction modifier) should be added after repair service or during a lubricant change.

After changing the lubricant, drive the vehicle and make 10 to 12 slow, figure-eight turns. This maneuver will pump lubricant through the clutches. This will correct the condition in most instances. If the chatter persists, clutch damage could have occurred.

Trac-loky test

WARNING: WHEN SERVICING VEHICLES WITH A TRAC-LOKY DIFFERENTIAL DO NOT USE THE ENGINE TO TURN THE AXLE AND WHEELS. BOTH REAR WHEELS MUST BE RAISED AND THE VEHICLE SUPPORTED. A TRAC-LOKY AXLE CAN EXERT ENOUGH FORCE IF ONE WHEEL IS IN CONTACT WITH A SURFACE TO CAUSE THE VEHICLE TO MOVE.

The differential can be tested without removing the differential case by measuring rotating torque. Make sure brakes are not dragging during this measurement.

(1) Place blocks in front and rear of both front wheels.

(2) Raise one rear wheel until it is completely off the ground.

(3) Engine off, transmission in neutral, and parking brake off.

(4) Remove wheel and bolt Special Tool 6790 to studs.

(5) Use torque wrench on special tool to rotate wheel and read rotating torque (Fig. 8).

Fig. 8 Trac-lokY Test -Typical

1 - SPECIAL TOOL 6790 WITH BOLT IN CENTER HOLE

2 - TORQUE WRENCH

(6) If rotating torque is less than 22 N*m (30 ft.

lbs.) or more than 271 N*m (200 ft. lbs.) on either wheel the unit should be serviced.