NP249, NV249 Viscous Coupling.
FAQ Contributed by Miklos
The main symptom of Viscous Coupling (VC) failure include chirping or spinning of the front tires during sharp slow turns on pavement where you would not expect any chirping. After I changed my VC, it will not "spin" a tire even on gravel. However, the need to use a little extra fuel during a sharp turn is still there which I believe to simply be the the resistance of the VC as it is doing its job.
It seems to be the general concensus that since the VC is a sealed unit and does NOT operate using the fluids withing the transfer case, changing the fluids within the case will NOT fix the problem. Changing the fluids will, however aid in keeping the rest of the transfer case in good shape.
Heat buildup inside the VC is both what kills it and what operates it. Whenever the rpm of the frant and rear drive shafts do not match perfectly, the plates inside the VC are slipping in the internal fluids, creating heat. Minimizing that heat is the key to long life.
Ways to keep your VC in good shape:
a) All tires must be the same diameter all the way around. To insure this, rotate tires frequently to keep the wear even. Only replace tires in sets of four or five.
b) Keep the rolling radius matched between all frour tires when determining tire inflation pressures.
c) Never ever use a spare tire that is a different size, even for a short distance.
d) Do not use chains on a single axle. That can change your effective rolling radius and have the VC constantly trying to adjust torque to between the axle with traction and the axle without. Use chains on both axles only.
Here are the VC replacement directions.
<This link appears to be dead - keeping it here in case it returns>
Order VC here.
Here is another link to replacing the VC.
Another usually more expensive alternative is to swap to a NP242 or NP231 transfercase. These do not contain a VC. For the hard core four wheelers this is a far better choice. Ski bums and bad weather drivers will usually find the full time mode of the NV249 to be the best choice.