Ok, as most of us who have the NP231 transfer case have found out at some point in time, the rear seal where the slip yoke is is prone to leaking. When it gets bad enough, the ATF inside the case leaks out onto the driveshaft and is in turn flung up against underbody of the jeep creating a nice grease slick. While it’s great rust protection, this slick burns against the muffler and we all know that topping off the case once a week gets to be a hassle. The good news is that changing this seal is easily accomplished and is a cheap fix. (under 20 bucks in parts) You don’t even need to jack up the jeep. Here’s how.
As I just mentioned you don’t need to raise the jeep to do this fix, but if you want some extra room to work in, feel free. The first step to this project is to drain the t-case of fluid. Easy enough if you have a 30 mm socket. If you don’t have one, get one. Those drain plugs are a pain to get at with a wrench and there’s no sense rounding them off trying a tool that isn’t quite the right size. A piece of flashing formed into a trough, or a wide mouth funnel will help keep the fluid from dripping all across the cross member and everywhere but your drip pan. Once you’d drained the fluid, remove the rear drive shaft. Do this by unbolting the straps holding the u-joint to the rear axle pinion yoke. These bolts were 8 mm on mine. Once the straps are loose, the u-joint may just drop out (depending on how clean it is) so watch it. Those bearing caps can come off pretty easily and then you’ll be left looking for all your needle bearings. Not fun and more often then not you’ll end up buying new u-joints because you can’t find them all. Incidentally, you may just want to change out that joint while you’ve got it out. If the drive shaft doesn’t want to drop out, a few taps with a rubber mallet will knock it loose. Put your pan under the slip yoke (where that leaky seal is) and gently pull the rear drive shaft out of the t-case. You may or may not get fluid dripping out, depending on how well it drained.
Once the shaft is removed, locate the 3 bolts holding the rear extension housing to the main part of the case. That’s it, only 3 bolts. If I remember right, they were 15 mm. Crack them loose and remove them. Once the bolts are out you can remove the extension housing. It’s siliconed on there so the rubber mallet may be needed here too. (If you’re using a regular hammer, remember that the case is just aluminum. Don’t go hitting it too hard). Once the housing is off, you’re done under the jeep for now. Take your parts to your work bench.
Here’s a shot of the entire case on the bench with the housing removed.
There’s two parts that you are interested in for this fix. The first is obviously the real seal. The less obvious one is the bushing just behind the seal. This bushing has a tendency to wear out, allowing your drive shaft to move laterally while it’s spinning. If it wears too much, then the seal won’t work properly and you could replace it 7 times and still have leaks. Since it’s a $9 part, it pays just to replace it along with the seal as long as you’ve got it torn apart. Leave the bushing alone for the moment and focus on the seal.
As you can see from the pic here, mine was pretty far gone.
Getting the seal out can be a challenge for someone without a seal puller (me), but it can be done. What I found worked best was to take a pocket knife and work the edge into the lip of the seal where it meets the housing on the side. I’m afraid I didn’t take any pictures showing where I mean, but if you have the new seal then you can see what I’m talking about. Once you have a bit of a gap there, take a screw driver and work it around the entire edge of the seal, trying to push it out of the housing. Most likely though the lip of the seal will bend before it comes out. Don’t worry about that too much. Once you’ve got the lip lifted, Slip one of the ears of the housing (the flare around the bolt holes) into a vise, seal end down. Tilt the housing so the end with the seal is touching the vise to it doesn’t spin. Take a long, heavy screw driver and slip it done the inside of the housing and put it against the inside of the seal. Try and tap the seal out from this direction. Unfortunately, this didn’t work for me either. In the end, I ended up totally mangling the seal, bending the lip almost double before the tension gave enough that the seal could be pulled out. It took a good half hour of fighting with it before I got it loose. Just keep at it and don’t be afraid of bending the seal. As you can see, mine put up quite the fight. The top right part of the seal shows where I bent it over to free it up.
See part 2