All of the rear differential seals were leaking on my brother's B5 when he got it a number of months ago. Here's my little write up on the repair. This DIY will just be about the axle flange seals, if you want to replace the input flange seal CLICK HERE.
Part numbers
Rear differential axle seals (left and right side): 016409399B
Axle CV grease seals (optional): 8D0407309
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The first step is to get the car up in the air. I chose to just back the car up onto some ramps. The ramps work fine and give plenty of clearance when working at the back of the car, however the body of the car wasn't more than an inch from my face when trying to unbolt the driveshaft bolts. You'll have a better time if you raise the front as well or throw the car up on jackstands.
It is definitely possible to replace the axle flange seals without removing the differential, however since I had to replace the driveshaft seal as well I chose to remove the whole diff which made life much much easier in the long run. I'm going to gloss over the process of removing the differential. It's pretty straightforward, and if removing the differential poses to be too much of a challenge you probably don't want to venture into further into removing the seals (especially the pinion seal.)
1. Unbolt the 6 bolts (M8 triple square) that hold each axle to the axle flanges.
2. Remove the driveshaft heat shield (4 10mm nuts) and remove the 6 bolts (6mm allen) that hold the driveshaft to the differential. Zip tie the driveshaft to the exhaust so it doesn't fall down when you pull the diff out.
3. Place a jack under the rear end of the differential (I used a transmission jack) and remove the 3 bolts (8mm allen) that hold the rear of the differential to the cross brace. You may need to unbolt the exhaust hanger from the cross member to get to the lower bolt.
4. The last thing to do is to remove the support for the front diff mount. There are four bolts that hold the brace in place. Once the brace is removed the differential is ready to come out. Make sure you have it safely supported and drop it out.
The one crappy picture I have of the differential coming out.
Huzzah!
Here's a close up of the nasty leaking axle flange seals. Nice and dirty.
Thankfully, removing the rear diff axle flanges is much easier than removing the transmission axle flanges. The rear flanges are held in with a bolt (6mm allen), after removing the bolt they slide right out. No pressing required, which also means no need to beat them back into place. To remove the bolt holding the flange in, thread two axle bolts into the flange and counter hold it with a large screwdriver, metal rod, pipe, whatever floats your boat.
After pulling the flange out this is what you'll be looking at, the target seal in question.
There are many ways to get the seals out. Audi makes a special tool that threads into the seal and allows you to pull it out. If you don't have that fancy tool you can try to pry it out with a screwdriver. The risk with that is scratching up the sealing surface. You could also try and get two screws into the metal part of the seal and use the screws to pull it out. Again, the danger here is putting the screw too close to the wall of the diff and marring it up. Cutting it out is another option, but again, you risk damaging the housing.
My solution was to use a set of vice grips to grab onto the seal and bend it up and down and side to side all the way around the seal. Eventually this breaks the seal loose from the housing, by pulling the seal inwards you'll relieve the pressure holding it in and it will pop right out. The beauty of this method is that you stay well and clear of the diff housing, no risk of scratching it up as all of your contact point is on the seal itself (just don't roll the vice grips too far forward.)
Seal #1 removed.
Seal #2 removed.
Now it's time to press in the new seal. Audi makes a special tool to press the seal in, but I just used a piece of 2" pipe coupler I picked up at the hardware store. I made sure to grind the end flat and true and then wrapped it in tape so as to not damage the seal when tapping it into place.
Clean the housing of the old fluid and debris, generously coat the outside of the seal in gear oil, line it up centered and flat over the housing and tap it into place. The new seal should be pressed in slightly further than the old seal so that it rides on a new area of the flange (you'll see a line on the flange where the old seal was riding.)
That's all there is to it! You don't need to drain and replace the fluid to do the axle seals, but it's a great time to do that if it hasn't been done recently. It's also a good time to clean out the old CV grease from the rear axles and pack them with new grease. There's a gasket that holds the CV grease in that can be replaced as well, I posted the part number above. (I've gotten away with not replacing the CV gasket before though with no problem.)
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Originally Posted by MetalMan
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