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Axle Noise Diagnosis

Part 4 - Isolate the Condition
From MasterTech October 1999
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A good way to check for exhaust sounds is to perform a static test in the service bay. First, apply the parking brake. Then start the engine and leave the transmission in Park, or in Neutral for a manual transmission. Run the throttle up to the engine speed at which you noticed the sound during your road test and check for the presence of exhaust sounds or vibration. If none is detected, move on to the next step.

To determine which axle is producing the sound, get the vehicle up on a hoist and remove the front propshaft if the vehicle is equipped with four-wheel drive. Then repeat the road test through the different drive modes and speed ranges.

If the sound decreases or changes pitch, the condition is probably in the front axle. If the sound does not change, reinstall the front propshaft, and remove the rear prop shaft.

When doing this, keep in mind that you have to plug the transfer case output with a sealing plug to prevent fluid loss. Use Snap On kits YA321 or YA2340 (Fig. 10) for the appropriate sealing plug. Also, remember that after repairs are completed, the fluid must be changed in an NV 247 transfer case.

With the rear propshaft removed, you may experience a shudder as you start out on this leg of the road test. Try to keep stop-and-go driving to a minimum.

Figure 10 Snap On kits YA321 and YA2340 contain the sealing plugs needed to plug the transfer case output when the rear propshaft is removed.

Repeat the drive mode and speed range tests once again to see if the sound decreases or changes pitch. If it does, you have isolated the problem to the rear axle. If the sound does not change in pitch or intensity, the condition is not axle-related and may be coming from other drive- line components or elsewhere on the vehicle.

If you need help, call the STAR Center for assistance, or contact your zone technical advisor. If you have the con- dition isolated to an axle and know whether the sound is a howl, growl or whine, an inspection and analysis of the axle is your next course of action.

Part 5 Axle Inspection and Analysis

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Last Update: December 20, 1999