Drive Axles, Bearings and Joints: Testing and Inspection

ROAD NOISE
Some road surfaces cause noise that may be mistaken for tire or axle noise. Road noise is usually the same while driving or coasting. Driving on a smooth surface helps determine the cause of the noise.
Perform the following maintenance/diagnostic checks:
- Check the tires for irregular wear.
- Check the tire pressure.
- Check the front axle lubricant level.
- Drive the vehicle to warm up the front axle.
- Drive the vehicle at various speeds in drive float, coast, and while cornering.

TIRE NOISE
Tire noise continues with lower tone as the vehicle slows down. Tire noise may easily be mistaken for axle noise. Tire noise changes with varying road surfaces. Axle noise does not change, although, it usually stops when coasting at speeds under 48 km/h (30 mph). Temporarily inflating all the tires to 345 kPa (50 psi) pressure for test purposes only will materially alter noise caused by tires, but will not affect noise caused by the axle.
Perform the following maintenance/diagnostic checks:
- Change the tire pressure to minimize noises.
- Drive over different road surfaces.
- Drive on smooth blacktop to minimize tire noise.
- Cross switch the tires, if necessary.
- Remember that snow tire treads and tire studs cause added noise.

ENGINE AND TRANSMISSION NOISES
In order to determine which unit is at fault, take note of approximate vehicle speeds and conditions under which the noise is most pronounced. Stop the vehicle in a quiet place to stop interfering noises. With the transmission in neutral, run the engine slowly throughout the range corresponding to the vehicle speed at which the noise was most pronounced. If a similar noise is produced, the noise is caused by the engine or transmission and not the axle.
Perform the following diagnostic checks:
1. Drive slightly above the speed where the noise occurs. Place the transmission in neutral.
2. Let the engine speed drop to idle.
3. Stop the vehicle.
4. Run the engine at various speeds.

WHEEL BEARING NOISE
Raise the vehicle on a twin post hoist. Spin the wheels by hand. Listen for a knock or a click about every two revolutions of the wheel. The bearing rollers do not travel at the same speed as the axle and the wheel.
Perform the following diagnostic checks:
1. Drive the vehicle at low speed on a smooth road.
2. Turn to the left and turn to the right. The noise should change with the cornering loads.
3. Jack up the wheels in order to verify roughness at the wheels.

GEAR NOISE
There are two basic types of gear noise. The first type is produced by broken, bent, or forcibly damaged gear teeth which is usually quite audible over the entire speed range, and presents no difficulty in diagnosis. Hypoid gear tooth scoring is one example that generally results from the following conditions:
- Insufficient lubricant
- Improper lubricant
- Improper break-in
- Insufficient gear backlash
- Improper ring and pinion alignment
- Loss of drive pinion nut torque

This scoring will progressively lead to complete erosion of the gear teeth or gear teeth pitting and then eventual fracture if the initial scoring is not corrected. Another cause of hypoid gear fracture is extended overloading of the gear set. This will produce fatigue fracture, or shock loading, which will result in sudden failure.
The second type of gear noise pertains to the mesh pattern of the gear teeth. This form of abnormal gear noise can be recognized because it produces a cycling pitch (whine) and will be very pronounced. It tends to peak in a narrow speed range, appearing under drive, float, or coast conditions and will remain constant in pitch. Bearing noise will vary in pitch with vehicle speeds.
- Rough running or a whine noise should increase with speed.
- Noise pitch should be higher than differential noise.
- Front pinion bearing noise may be louder on deceleration.
- Rear pinion bearing noise may be louder on acceleration.
Perform the following diagnostic check:
- Road test the vehicle on a smooth road in order to minimize tire noise.
- Road test the vehicle at various speeds in drive, float, and coast.

NOISE DIAGNOSIS

Notice: The front suspension SHOULD NOT be allowed to hang free. When the CV joint is run at a very high angle, extra vibrations can occur, resulting in damage to seals and joints. Always follow equipment manufacturer's instructions.

Any gear-driven unit produces a certain amount of noise that is normal and cannot be eliminated by conventional repairs or adjustments. Slight noise that is heard only at a certain speed or under unusual or remote conditions is acceptable. For example, a noise that tends to reach a peak at speeds from 60 to 100 km/h (40 to 60 mph) depending on road conditions, load conditions, gear ratio, or tire size does not indicate trouble in the axle assembly.
When an axle is suspected of being noisy, check to determine whether the noise originates in the tires, road surface, wheel bearings, engine, transmission, propeller shaft, or axle assembly. Raise the tire pressure to eliminate tire noise (although this will not silence the tread noise of mud and snow tires). Listening for the noise at varying speeds and road surfaces, on drive, float, and coast conditions will help to locate the source of the noise.