I spent a couple hours doing some work on my 91 yj this weekend. I replaced the gear oil and gaskets on both my front (D30) and rear (D35) axles. While I had the covers off, I noticed a couple of things that were worth remembering to ask on here.

First, on the D30, there are two threaded holes, one on each side, that aren't used and the cover doesn't have the holes. Was this a previous design that they just got cheap and stopped using the bolt holes?

next... The front has never had it's cover off, but the oil seemed to be in really good shape! Good thing! I did notice that there is a little slack on the short side's shaft. Not enough to "see", but I could feel a little play, more like hear... It's around the bearing right there in the diff. Is this normal or something to be worried about?

next... The front axle had no metalic residue in it whatsoever. Another good thing, and no chips or anything either. The outer edges of the spider gears seemed a little rough, but seemed to have been like that for a really long time - perhaps from the factory? They don't bind or hang or anything, just don't look right. Am I over-worried about them?

next... how much fluid hides up in the tubes? Specs indicate that I should have put 2.5 quarts in each dif., but I barely put in 2 before it started flowing out. I drove around the block a couple of times and let it set and checked, full, up to the bottom of the fill hole, so, still wondering what the capacity should be.

Next... "if it ain't broke, don't fix it" aside... how often should one replace the gear oil in the diffs? same note, how often should one change the fluid in the x-fer case?

lastly. the rear D35 oil was grayish and there was a little sludge in the bottom, no chips and nothing severly magnetic. How many miles of moderate to hard driving should the oil go through before it starts looking like what? This is a Jasper rebuild from 1998 and has over 100,000 miles on it....

Overall, this was a messy, but fun weekend. Just left me with a couple of questions....

how long have you had your jeep?

at places like jiffy lube and some others im sure they have pumps that suck out the old gear oil. what doesnt make sense to me is that the front would have been changed and not the back, as you said the rear diff oil was much grayer.

the only thing i can think of here is that you have a little more wearing in the rear end for some reason, it is used much more than the front end but unless you have hubs or something like command track they should be fairly close to the same color.

as for the fluid levels.

is it lifted?

my brother had a 76 GMC Jimmy with 12 inches of lift and because of the pinion angles we had to overfill the diffs (according to the checkplug, not the specs) to get the proper fluid levels in them.

the unused bolt holes may be due to a manufacturing process at the factory or some sort of jig they use to drill other holes or something like that. i dont believe that they have changed the diff covers at all..


i personally dont know how often to change the fluids in the front and rear diffs and the transfer case but as much as i like to go offroad and use 4wd. i change them more often than is called for by far because i know im using old equipment and i want to keep it in tip top shape. and i always change the transfer case at the same time i do the front and rear diffs. and if the tranny is manual i would do that too.


Wing Nut : 2 cents

Okay, I have owned my YJ since it was new, two miles on it actually. It now has 206,000+ miles on her....

I had to replace the rear axle in 1998, thought that I made that clear. At that time, it got new fluid, where-as the front didn't because it was fine. So, the reason why one got new fluid and the other didn't.

The grayish coloring the rear has looks almost like the sludge that you would find in an automatic transmission pan after 150,000 miles. Metalic, but extremely fine, and thinner, not thick like you would see in a transmission.... The overall consistancy of the oil was about the same as new gear oil, so I don't want to give the impression that it was thin.

Yes, it is lifted, only 2.5 inches, but when I had each end jacked up at a time, the pinion angle was parallel to the ground when I did the service. So, that shouldn't have made a difference for the fill plug vs. the fill level compared to the lift and it's angle on the axle. Clear enough?

The unused bolts are a strange thing. Really cureous about them....

I've changed the fluid in the transfer twice and once when it was overhauled. I guess I should do that more frequently?
I don't recall ever doing the transmission. It is an AX15 and it has been overhauled once and I'd bet that is the only time it has ever seen new fluid.

So, waiting on some interesting comments back on all this. Thanks WingNut76 for your $0.02.

Everything sounds perfectly normal. Since the rear axle does 95% of the driving, it will have a lot more wear on it then the 30 will. Also, because the 30 is freewheeling on one side nearly its whole life, the spider gears will have some amount of wear at 204K, but unless they are chipped or have a lot of play, they are more than likely fine. Dirty diff fluid always has the looks like almost burnt ATF, or metallic motor oil. This is normal; there is lots and lots of iron to wear and not a problem. As long as the fluid is to the check hole, then the gears are getting lubed and you have nothing to worry about.

I change my diff fluid whenever I go through water that goes 3/4 as high as the tires, and the t-case and tranny fluid whenever I go in water as high as the doors. Diff/tranny/atf fluid is so cheap that even if I don't go mudding or through creek crossings, I do it every year at the end of the rainy season.

Thanks for all the input so far. I have several cases of ATF fluid around here some place, but I don't think it would be of any use on my Jeep, it has a manual shift.... Gear oil, on the other hand, is taxed to death here and runs about $3/qt. Still not that bad in the long run....

You should buy the gear oil by the 5 gal bucket. <img src="/forums/images/graemlins/cheers.gif" alt="" /> It'll keep and you'll save some bux!

ATF is used in the t-case, and can be used as power steering fluid. All ATF is is 10 weight oil with firction modifiers for the clutches in an automatic. ATF is great stuff. I use it for a pre-lube on everyhting but engines when assembeling stuff.

Even though jeep calls for 75/90 I always use 80/90. It is a lot cheaper, and on my older tranny, it helps it shift a bit smoother.

I had no idea that my 231 took the ATF. Just looked it up and sure as crap, wow.... Imagine that. Since that stuff does go bad, guess I will be changing that out soon. Does it have any sort of servicable filter or what?

As for the diff fluid. I had originally asked about the level based on a flat and level axle because it didn't seem to have gotten enough. After a couple hundred miles, I guess some of the fluid made it in it's holes and would up showing low, none leaked out, so am putting about 1/2 quart in each. Strange. my original thought that there would be some up in the sides around the shaft worked out....

There is a filter in the T-case, but it requires the T-case to be removed and taken apart. It is one of those life of the unit filters. I've seen people with 120+K on a t-case atf and not have it go bad. I change mine becuase it is a water vacume. and ATF is so cheap that it just makes you feel good to change it.

Some fluid is trapped in the pumpkins passage ways, and some fluid does go to the axle tubes. I don't recommend it, but you can run an axle like ours on 2 pints.

danke, will do some more measuring,, now, should I use a gasket or just goo when "changing" the pumpkin fluid?