B5 rear diff tail piece seal change

[Dank713]

My '98 A4-B5 has a leak at the real differential input driveshaft seal. I'm in the middle of replacing it. eBahn talks about changing the seal in the tranny, but seems to miss the rear diff. Best I can tell, the process is similar. Found a few hits on here (forums), but no one seems to have run into my problem or I missed it. The diff in my car has the long "neck" extension piece forward of the diff to the driveshaft. I pulled the plastic round plug out on the bottom and unclipped the "C" clamp around the shaft on the inside of the neck peice on the bottom. eBahn says "just pull the shaft out" and common sense says that it should. However, it doesn't pull. I concluded the front bearing is probably a little snug around the shaft, so I put two of the driveshaft CV screws back in the flang and worked them in until they pressed on the front lip of the diff housing. The shaft started to pull out (not a lot of force, but no walk in the park either). Once the shaft pulled passed the snug of the neck peice bearing, it does slide easy, for about 2inches. There it stops. I can slide the shaft back in (until it starts to snug the bearing) or pull it out about 2" until it stops again. Whatever the problem is, it is far back inside the other inner end of the shaft. I can hear the sounds back by the differential its self. There are four bolts hold that "tail" extension onto the diff. Anybody every break a diff at that position? Can I without dorking up some critical adjustments? Anybody ever have a similar problem? So close... yet so far away.

[Dank713]

I went for it. Came right off at the four bolts with no problems.

[mtroxel]

Dank, I've been needing to do this job too. So if you unbolt that front extension piece, then the shaft comes right out so you can change the seal?

[Dank713]

Yes. I pulled the four bolts out that hold the extension piece onto the diff. The whole thing dropped down no problem. Maybe 15 lbs.? Not bad. There is a big fat oring (not gasket) between the two pieces, so it works well for when you reinstall the piece back onto the diff. You will need a puller once you get the extension piece out where you can work on it. There is a coupler on the end of the shaft (end where you split it apart) that will prevent you from pulling the drive shaft out of the seal / bearing. The coupler is just pressed on and pulled reasonably easy. I happened to have a claw style puller so I was able to get it off. A three leg puller would work best as the two leg one tends to tilt off angle as you crank up the pulling pressure. I clamped some small hardwood blocks along the clamp sides to prevent it from tipping as I pulled the coupler off the end of the shaft. As for the seal, with the extension piece in my vice on the bench, I just used a small 14" crowbar under the seal. There was plenty of room to get in there. It popped out pretty easy. Just some things to consider: Pull that round plastic plug out that is on the bottom of the longest part of the tail piece before you split the diff parts. You will see a ring clip holding it in. When you do, all the 75w90 oil is going to pour out from that point. Because you will have the bracket removed, the end of the diff will point downward some letting the oil flow. I had a "can style" jack supporting the angle. I had only changed the oil 10k ago and didn't want to waste it so I put a new "paint roller" pan under it first so that I could return the oil when done. I had the luxury of time, so I let it drain over night. It runs very slow being so thick. My oil still looked brand new. I was able to drive the coupler back on with a block of hardwood and sludge hammer on my concrete floor. I'm doing the tranny seal today.

[mtroxel]

Dank,

Thanks a lot for all the info. I gotta do this soon. That seal was leaking about two drops a month....but now it's starting to worrry me. I gotta get to this.

[Dank713]

It's a hard call. Mine was leaking pretty bad but I don't think I would have gone after it had that been the only reason. I was having a problem where when I let off the gas, I would hear this subtle "grinding" sound coming from under the car while decelerating. Long story short, it turned out to be the drive shaft rubbing on the aluminum foil heat shield. The sound never quiet sounded like that, so I never suspected that. Turns out, unknown to me, I have a carbon fiber drive shaft in my car. Carbon fiber sounds much quieter than metal when rubbing. I also had an exhaust leak in those flex couplers in front of the cats. I had the exhaust out for welding and had pulled the drive shaft out for inspection. Had I not already been that far, I don't think I would have gone for the seal. The exhaust and the drive shaft are the real efforts there. The seal replacement was actually rather easy once past the other stuff. Good luck to ya. Oh, if you don't already have one, you are gonna need one of those "oil hand pumps" to refill that rear diff with oil when done.

[mtroxel]

Got the oil hand pump. I have almost 160K on mine, so it wouldn't hurt to put some fresh Redline in it anyway...and with even a small leak I'm wondeirng how much gear oil I have in there.

FYI, I think someone's changed your driveshaft at some point. I believe they only did carbon fiber on the 96, with maybe a few bleeding into 97.

[Dank713]

Was there something wrong with the carbon fiber shaft? I was a bit surprized to see it in there because every article I read or pretend order I placed on-line showed a steel one. Weird thing is, I know of both the two owners before I got the car and neither of them are the type to stray from the path of original. They were both women / family people with no auto experience. All work was done by their mechanics. Possibly their mechanic found a cheap used one or something. Anyway, that aside, I got it all back together and all is good. No more leak at the pinion input. But... I still have that freak'n quiet grinding sound that kicks in about 1 second after dropping the throttle. At this point, it has to be in the diff. The 75w90 was clean as honey and the bearings were smooth as could be, but I'm missing something. So at this point, I don't know what it is. Been doing it for the past 25k miles (and not sure before that as I didn't own the car.) Just gonna turn the radio up louder and pretend it doesn't do it. "I'll just drive her till she don't drive no more." If anyone on here knows of or has seen this... I'm listening.

[Dank713]

Followup: Not confirmed, but identified; The grinding during deceleration is pinion and ring gear adjustment problems. The backlash is out of adjustment. Given the complexity of setting it correctly and tools required to repair, and given the miles on the vehicle, I'm just going to replace the entire diff with a used one and hope the replacement is good.

[mtroxel]

Did mine today Dank. Not an awful job, just a lot of wrenching. I took off the front piece and changed the seal on the bench. I used your trick of putting a couple of bolts back in the flange to pull that shaft out, but I had to wedge a washer between the bolts and the housing to get it to work right. Then I just took the diff out of the car to do the half shaft seals. That was a good decision. At that point, it's only 3 more bolts, and it sure is easy to do those seals with the diff out of the car.