Front differential Issues, grabbing/skipping after transmission is heated up

  • V8
  • AWD
  • 95,000 MILES

I have differential lock-up problems when the transmission gets heated up (after a thirty to forty five minutes of normal driving or after anytime towing). The front wheels, when turned all the way to the left or right start 'skipping' on the pavement as if the limited slip differential is locking the wheels. The dealer stated that despite encountering this problem with other cars, they cannot accurately pinpoint the problem without going through a number of 'fixes' as a process of elimination, at my expense. Any suggestions?

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have the same problem?
Thursday, July 28th, 2011 AT 4:43 PM

1 Reply


Since the vehicle is all wheel drive, you cannot remove the front prop-shaft to isolate the front differential. There are problems with front differentials in those, eating up bearings, but I have not seen one have the symptom you are describing. The transfer case on the other hand, has a fluid coupling inside of it, that if in a locked condition, provides no slipping to allow for speed changes (i.E. When turning). I cannot see a number of attempts, but my first attempt would be the fluid coupling in the transfer case. It is all mechanical/hydraulic, and not controlled by any outside source. The way it is designed to work, is the slippage from front to rear, in theory heats up the viscous fluid, causing more transfer of power to front/rear. Normally, the transfer is 68% rear, 32% front. Here is the description and operation of that transfer case.

Transfer case description and operation :

The NVG 149 RPO NP3 is a single speed, single mode transfer case. The mode is full-time all wheel drive. It has a planetary differential gear set that splits the torque, normally 38 percent to the front wheels and 62 percent to the rear wheels.

The NVG 149 utilizes magnesium housings. Proper fasteners, brackets, and fill/drain plugs must be used to prevent galvanic corrosion. The planetary differential uses the carrier (6) as the input. The annulus gear (4) connects to the rear output shaft (5) and rear wheels. The sun gear (3) connects to the front output shaft (7) and front wheels through the chain (8) and sprockets. The viscous coupling (2) consists of a sealed housing filled with a high viscosity silicone fluid and thin steel plates alternately splined to the inner and outer drum. The inner drum is connected to the input shaft (1), and the outer drum to the sun gear (3). Whenever there is a speed difference between the front and rear wheels, the inner and outer plates of the viscous coupling spin relative to each other and the silicone fluid provides resistance. The resistance was tuned to be high enough to bias power quickly to the wheels with traction, and low enough to prevent binding in a tight turn on dry surfaces. This is the most common way the viscous coupling is activated, the shear mode. If the speed difference is high, the coupling can lock or hump. This "hump" occurs when the heat generated, expands the fluid inside the housing, changing the fluid dynamics between the plates. This results in pressure between the plates, forcing them into contact with each other, similar to a clutch pack. In the hump mode, the coupling can bias torque 100 percent to one axle, if required. Situations requiring this are extreme such as backing up a steep gravel grade or climbing over off-road obstacles. The viscous coupling is not serviceable; it must be replaced if defective. This is because each viscous coupling is calibrated for optimum vehicle performance for both the shear and hump modes. If the viscous coupling is in the "hump" mode too long, severe damage will occur. To prevent damage to the viscous coupling, do not:

Tow with only two wheels down.

Drive without one prop-shaft.

Drive with a "donut" spare tire for an extended period of time.

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Thursday, July 28th, 2011 AT 10:05 PM

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