With an oil change and transmission fluid swap out of the way, it's now time to exchange the oil transfer case lube for some fresh stuff. For those of you with manual transmission, you'd follow most of the same steps to service that unit, so please don't feel slighted (there will be a few in-depth posts on manual transmission service in another month or so).
(Click on through the jump for the full walk-through)!
For the most part, the lube in a part-time transfer case lives a relatively easy life. During normal usage, there is no significant gear reduction occurring in the case, so heat build-up is minimal. Keep in mind, however, that the fluid is not filtered, and as such there is really no means for trapping wear particles. Additionally, contamination of the fluid can occur in off-roading situations, or even through normal usage (condensation being the big enemy here). And if you have a full-time 4WD or AWD system that utilizes clutch packs (such as GM's Autotrac system) or a gear-type limited slip (the Torsen unit that's used in many AWD vehicles), there is an additional level of stress placed on the lubrication. Consider it a good idea to swap out the transfer case fluid every 25,000-30,000 miles or 2 years (whichever comes first). It's also wise to check the fluid level in the transfer case during every oil change, but we'll admit that this is highly inconvenient. If the unit is at least examined for leaks during each oil change (remember, the reason you're getting down-and-dirty with your own vehicle is so that you can check for such things) and if the fluid is replaced on a regular basis, you've greatly reduced the chance of incurring a problem.
Minimal supplies are required for this task; basically, we need some simple hand tools, and about three quarts of the correct lubrication. Older transfer cases that were not part of a shift-on-the-fly system often used gear lube, while modern cases typically utilized automatic transmission fluid (ATF). Since our '96 GMC K2500 has the oft-infuriating pushbutton 4WD system, our case was filled with ATF. The tag on the exterior of the case may provide a clue as to the required lubrication (although that was not the situation with our NVG 261).
To begin, we had to remove the transfer plate skid plate. These fasteners had "frozen" over time, so a substantial amount of penetrating oil (we prefer PB Blaster) was used to free them from the confines of corrosion.
Since individual nuts were used on the topside of the crossmember (instead of "weld nuts" or threaded holes), we had to position a box wrench between the skid plate and transfer case while removing the fasteners.
With the skid plate removed, we can see the drain (bottom, circled in red) and fill (top, circled in blue) plugs. While these fasteners in this case had a 1 1/8" hex head, it's not usual to see ordinary pipe plugs with either a male or female square drive head. Smaller pipe plugs may not be immediately noticeable under several layers of grime, so use some solvent and a few rags to wipe down the exterior of the case (a good idea in any situation, really, as to minimize the chance of allowing contamination to enter the case).
Removing the top and bottom plugs will quickly result in an empty transfer case. Due to the light "weight" of ATF, it's not really necessary to warm up the vehicle before performing this task, although it doesn't hurt. Those with units containing gear lube will certainly want to take a few trips around the block before attempting to drain the lube.
Once the fluid flow slows to a trickle, the drain plug is reinstalled and torqued to the correct value (admittedly, we left the torque wrench in the tool box and simply aimed for "snug"). Leave the drain pan in place for now.
Select the appropriate filling device (we used a screw-on nozzle, but your situation may vary) and begin filling the transfer case with fresh fluid.
You're done filling the case when fluid begins to overflow from the fill hole, as observed above. There's no benefit to cramming additional lubrication into the case, so don't try to give the bottle once last squeeze before re-inserting the fill plug. Once again, the plug is torqued to the factory spec or snugged-up until it feels properly tightened.
ATF and gear lube can usually be recycled with ordinary motor oil, but please ask your local recycling center for specific instructions, or else a large amount of waste material could be contaminated.