AudiForums.com - A4 B5 DIY: Replace the Rear Differential Input Flange Seal

Is your front differential mount covered in fluid? Are you leaving a trail of gear oil behind your car (I hope not!) If so, your rear differential input flange seal may be leaking.

This is part two of my differential seal replacement DIYs that will be covering replacing the rear differential input flange seal. If you want to replace the axle seals CLICK HERE.

The input flange seal replacement is definitely a more involved job than replacing the axle seals alone. If you need to replace all the seals on the diff, you might as well pull the differential out. If you just need to do the axle seals they can be done with the diff in the car, and if you need to only replace the input flange seal then you can get by with just pulling the differential snout off the housing. It's a little bit more work, but it's much easier to work on the bench than under the car. The choice is yours.

--------

Part numbers

Rear differential input flange seal: 857525275
Large o-ring to seal differential halves: 010325381A

--------

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.

Differential on its way out.

Attachment 23829

Freed from its home and ready for surgery.

Attachment 23830

The first thing you need to do (after draining the fluid) is to separate the two halves of the differential. There are four bolts (8mm) that hold the snout to the rear section. With the four bolts removed you'll have to slide the snout off the pinion (I tapped it off with a soft hammer.)

Attachment 23831

The next step to removing the shaft is to pull the spline coupler off the back end of the shaft. A two or three jaw gear puller is your friend here. It will take quite a bit of force to get the coupler off.

Attachment 23832

Two jaw gear puller in action.

Attachment 23833

Coupler sliding off the splines.

Attachment 23834

Removed! (Note that the chamfered side faces the rear section of the differential, flat side faces towards the front of the car.)

Attachment 23835

Bare shaft of the snout.

Attachment 23836

The next step is to remove two c-clips that are holding the shaft to the bearing at the front. The first c-clip secures an access hole that is somewhat hidden under the front diff mount.

Attachment 23837

Attachment 23838

With the plug removed you can peek inside the housing to locate the c-clip that holds the shaft in place against the bearing. You'll need a slim set of snap ring pliers with 90 degree tips to get the ring off.

Attachment 23839

Snap ring removed.

Attachment 23840

With that clip removed you can pull/press the shaft out. I chose to start it by inserting two driveshaft bolts and driving the flange out against the housing. I finished popping it out by giving the back end a couple taps with a soft hammer and out it came.

Attachment 23841

With the shaft finally out you can attack the large (and expensive) input flange seal. I used the same method I did with the axle seals to remove the input flange seal. By locking a pair of vice grips on the seal and bending the seal back and forth eventually you'll release the pressure holding it in and it will pop out. Easy deal.

Attachment 23842

The seal ends up looking a little worse for wear once it finally pops out, good thing we're replacing it!

Attachment 23843

With the seal removed thoroughly clean the sealing surface of the housing, coat the new seal in gear oil, line it up flat and true and tap it into place. I used a large pipe coated in tape to tap the seal into place.

With the new seal installed you can start reassembly. Slide the shaft back into the housing, reinstall the c-clip, and pop the plug and second c-clip back into place.

Attachment 23844

Use a soft hammer to pound the spline coupler back onto the end of the shaft with the chamfered edge facing towards the rear of the car.

Attachment 23845

Coupler back in place.

Attachment 23846

There is a large o-ring that seals the two halves of the housing together (part number 010325381A) that should be replaced before bolting the differential back together again. After replacing that seal you're ready to bolt the two halves back together. Line the spline coupler up with the pinion shaft and make sure the splines are engaged properly before smacking it into place. Once it's in far enough insert the four bolts and pull them together.

Attachment 23847

Attachment 23848