So I found out that my differential needs to be replaced as well as my transmission. There's a local junkyard that has several of each, and pretty good prices.

My first question is: when I replace the tranny (3.8L, 5 speed), are there any seals that I should replace before putting the new one in? Also, on the differential, are there seals other than the axle seals and the pinion seal that need replacing? In both cases, I'd like to have all new seals so that I know I'll never need to rip this crap back out.

My second question is: if I'm just replacing the diff (not the whole rear axle), are standard and limited slip the same size and mating? I'm wondering because I have the regular open diff, and the junkyard has offered me either one for the same price.

Third: I read somewhere that you should change the tranny mount any time you take the tranny out of the car... I can get either a "solid" or "hydraulic" transmission mount, but I don't know what the difference is. For the standard M49 5-speed, which mount should I get?

Fourth: If I do upgrade my diff to limited slip, would I need a new tune on my PCM to adjust for the gear change? I'll be replacing the PCM soon anyway, and this decision might affect where I get the new one from. On the same note, I've read that a new PCM would need to relearn VATS and crankshaft position... VATS is a simple (although time-consuming) sequence of on/off with the key, but is the crankshaft position something that I'd have to take it to a dealer for?

Thanks for any help that you guys can offer! Also, if anybody's selling a tranny or diff near the north Alabama area, let me know! I don't mind driving a little ways for a good deal.

 

The first question i would ask you is have you ever install a differential before? There is a pretty exacting process that needs to be followed to the letter. A lot of mechanics screw it up unless they shown how to do it. What makes you think the rear diff is bad? are the gears shot? You may want to replace the wheel breaing while you are in there.

Are you going to address the clutch? I generaly dont worry to much about the rear seal in the transmission since its so easy to do at any time but that is up to you.

 

The tranny and diff both have 290k miles on them. I can get both used (approximately 70k miles) for way less than it would cost to rebuild the ones I have.

The differential is leaking fluid out of both axle seals and the pinion seal, so at minimum those need to be replaced. Both the tranny and the diff work, but both are making noises that indicate they need some work.

The tranny has some internal bearings that have gone bad, causing a rattling and occasional sticking when shifting. The differential roars like a lion with an amped up megaphone. If I take a really sharp turn at 10 or so mph, it makes one giant THUMP, like a gear is jumping a tooth. (Edit: This last noise could possibly be a bad ball joint on the back left suspension... it made the noise again, and this time I had both windows open, and it sounded like it came from the left side rear, not center rear.)

The clutch is relatively new (was replaced shortly before I got the car, so about 2 years ago). I'll probably take it off to inspect (and probably replace) the rear main seal on the engine. There's oil behind my engine, dripping onto the exhaust pipe, that looks like it might be coming from there, and my transmission guy said that it could be caused by the transmission's input shaft rattling around. Anyway, if there's anything on the clutch that needs to be replaced, I'll do that while I've got the transmission out. But I'm 90% sure the clutch is all in working order.

 

What's the difficult part about replacing the differential? I know it's a breeze to take the axles out to get those seals replaced... how hard would it be just to replace the seal between the driveshaft and the differential? The seals definitely need replacing, whether I replace the diff or not.