charlieaf92

It seems DIY posts are much less common for the front differential specifically so I'm going to document this process. Also, from a selfish standpoint, recapping the process helps me make sure I didn't forget anything and also serves as a great resource if I can't remember how to assemble something when re-installing.

I noticed immediately that my recently purchased C4S was leaving a small oil spot on the garage floor. I was planning on doing the IMS Solution, clutch, pressure plate, etc so I used this opportunity to track down the source and found that both output shaft seals on my front differential are leaking.

My plan is/was to remove the differential completely, drain, replace the seals, and refill - then reinstall.

In this case my transmission is already out of the car. If you're just trying to remove the front diff you will need to unbolt the driveshaft. The is a rubber coupler (forget the technical name) between the transmission and the drive shaft. The easiest way to get it disconnected is by removing the 3 bolts that go in from the front, towards the rear of the car. Once those are removed you can separate the pieces.

The front differential is held in place by a horizontal support/bracket where the drive shaft enters it - and another mounting bracket at the very front. The front bracket has two bolts (I believe 15mm) going through the bracket and into the diff (from front to rear). However, the bracket is very close to a 'wall' in the front of the car and you probably cannot remove them completely without first unbolting the bracket itself and moving the entire diff and bracket toward the rear of the car.

The rear bracket has a plastic retainer bolted to it that connects to coolant lines as well as a small black tube. Be very careful with this and take your time to remove it.

Before removing the diff there is a plastic pan on the bottom of the fuel tank (the black object spilling down on either side of the diff at the front of the car). To remove the plastic pan you need to unbolt one side of each of the metal straps holding the fuel tank. I did them very slowly to ensure the tank didn't fall - but it seemed to stay in place even when both straps were pulled out of the way. Once I removed the plastic pan, I re-bolted the straps to be safe.

In addition there are two cross-members that come across the front at an angle. They both need to be removed as well. Mark the top as well as the front/rear because one side is slotted and the other is round.

Next step was to unbolt the cups that connect the CV joints and axels to the diff. These are allen head bolts and I believe smaller than the ones for the rear transaxle. I found that using a u-joint and extension I was able to break them loose fairly easily by using one hand to hold the front wheel in place and the other to turn. This was the hardest part of the process and that was just because there are so many and its an awkward angle to do it so my arms and shoulders started to ache. Take your time with these and make sure your allen head is fully seated prior to putting pressure - these are not bolts you want to strip!

Before fully loosening the bolts/nuts on the rear mount I put my floor jacket below the diff with a block of wood to help spread the weight and prevent damage and put some upward pressure on it. Then I removed the rear mount and was able to slide the entire diff backwards enough to access the 15mm bolts that go through the front support. Once those were removed I pushed the front support back to where it started and the diff was 'free' for the most part. Be VERY CAREFUL when pushing the diff towards the rear of the car not to crush or damage the silver coolant lines.

With the diff more maneuverable I was able to get the 'cups' unseated from the output shafts and push them up and above. From there, I had someone operate the jack and very slowly lower it in small increments while I moved around from side to side to ensure nothing was being pinched/pulled/crushed. It is a very tight fit between the coolant lines so take your time to make sure it doesn't get hung up. It came out really smoothly and to be honest the entire assembly is much lighter than I expected.

I read in the factory service manual that the boot/cover for where the driveshaft enters the diff should be replaced anytime the shaft is removed. Therefore, I just left the shaft in and pulled the entire thing out from under the car.

That's it for now. My parts should be arriving later this week and I'll be able to do more.

Cheers
Charlie

Petersa9

Are you planning on inspecting the diff while you're in there? I am interested in pic's and more detail if this is even a DIY item.

Subscribed!

charlieaf92

Hey Petersa - I'm planning on checking for any obvious signs of wear or play with the output shaft bearings but nothing more than that. This entire project has turned into an exercise in burning cash and I need to draw a line in the sand when it comes to replacing things or I'm going to drive myself crazy. Thus far, the only 3 parts on the entire car that I've come across that aren't in good working order have been the IMS bearing flange seal, two front diff seals and one on transmission. The DM flywheel, clutch, pressure plate, t/o bearing, etc, etc, etc, were all in good shape but I replaced because of how difficult they were to access. The front differential was actually fairly easy to remove so if it begins to show any other signs of failure down the road I'll address it then.

That being said, in my research I came across this article that has some great pictures and information about disassembly and repair. It is for a 997, but from what I can see they are very similar. It should at least work as a starting point: http://porsche.gen.in/info/porsche-9...g-replacement/

Cheers
Charlie

charlieaf92

I got the call today around 4pm from the Porsche dealer that my parts arrived so, I only spent about an hour actually working on the car before dinner. Here is a quick day 2 progress update.

I started with draining the fluid from both the transmission and front diff. I've read online that it is wise to break the fill plug loose prior to draining - definitely makes a lot of sense if the components are still in the car - when they're out of the car not too big of a deal either way but I did it to be safe. And, on that subject - it is EASIER to break both the fill and drain plugs on the diff and transmission loose while they're still bolted in the car. It is also easier to drain the fluid while they're in the car. So, I'd recommend doing it in that sequence.

I, however, did not do it that way. So, I had to lift the transmission around a bit and stack up wooden blocks + my transmission jack in weird positions because the drain plug is on the bottom - in the center. The differential is much easier to work with because its light - but holding it in place while trying to break the drain plugs loose is a bit of a trick. The diff has two 17mm hex head plugs (I believe they're both magnetic) and the transmission's are allen head style plugs.

After draining both, I reinstalled the drain plugs (hand tight - just to keep it from continuing to leak and make a mess) and got started on removing the output shafts. For the differential I threaded in the CV boot/cup bolts enough to have something to pry against. It ended up being a quick jerk/hit that pops them out rather than consistent pressure. Overall they really weren't hard to remove. I made a point to mark the left and right shafts - I am not sure if its necessary but it made sense to me as there is a chance they have worn in unison with their bearings or internal gears.

I started trying to remove one of the seals on the front diff and quickly decided to invest in a seal puller rather than scratch something up. As I was trying, a small circular spring came out of the seal (it was on the back side) and I removed it. So, keep an eye out for these and make sure you remove them with the seal and don't leave them in your diff.

I'm only removing one output shaft on the transmission since just one side is leaking. These shafts are held differently. There is a bolt in the center that you must remove - then the shaft slides out - there are not retaining rings on this one like the fronts. I attached a picture showing the configuration I used with a breaker bar, two of the bolts offset from the center and the transmission jack itself to keep things from turning. Again, really not too hard - just need to get things setup correctly.

dporto

Nice thread - good pics! Thx

charlieaf92

Quick update. I received the new seals from Porsche and packed the inner channel with grease (per factory manual instructions). I rented a seal driving kit from AutoZone (I'd highly recommend doing this for this job). Installing the two seals on the front diff was pretty straight forward and uneventful. These go in until they are flush with the outer housing.

However the seal for the transmission output was tricky. I felt like I had it lined up pretty square but one edge 'rolled' up and I saw some of the seal material that had sheered off being squeezed out. I removed the seal to inspect and confirmed it had rolled and was damaged. That meant waiting another few days for a replacement.

The replacements (I ordered two so I'd have a spare in case I messed up again) came today and I got it installed without much trouble. I did notice a very, very small amount of the seal got shaved off as it was going in. The seals for the rear have little ribs on the outside and I believe it was a bit of those. These things fit so tightly I think that is unavoidable - but time will tell. The rear seal on the opposite side looks like it goes in a few MM deeper than the other housing so I matched this ones depth as best as I could visually.

Next up was to re-install the output shafts. The rear was easy - just slid it in and torque the bolt using the same holder method I pictured above.

The front shafts have C clips that are supposed to be replaced. In the manual it shows using the new one and a pair of channel locks to push the old one off as the new one goes on. This did not work for me, didn't even come close. I spent quite bit of time and eventually was able to twist the old one off and then put the new one on - however at this point the new one was bent slightly out of its original shape so I question whether it was even worth the effort. For the second output shaft I left the original clip in place. Reinstalling these shafts was pretty easy after that - you just slide it in, ensure the gear mates up properly and then smack it firmly with your hand. Both popped back in place without issue.

With the new seals installed I replaced the drain plug on the front diff, then filled. It seems the front diff doesn't have any washers on the drain or fill plugs. The transmission fill/drain plugs have aluminum washers that should be replaced.

Both are filled with Porsche transmission fluid. Roughly 2.8l for the rear and 1.4l for the front. All together I used 4.4l but a decent amount is on my garage floor.

charlieaf92

Alright, yesterday I finished re-installing the front differential. It went in just like it came out. A couple tips: when installing raise the front half up into its spot first - its necessary to angle it down and back a bit so that it will clear the support bracket. Then, get the front support bolts started before hooking up the rear mount. When actually fitting it up into its space, I got it most of the way there with the floor jack, but then I essentially bench pressed it up by hand so that I could have a feel for resistance and make sure it wasn't crushing any of the coolant lines. Once I'd pushed it up into place by hand, I had someone raise the jack up to support it. I had to work it back and forth a bit to get the CV hubs seated properly but it was pretty easy. Overall it seemed like re-installing was easier than removing, but I credit that to having already gone through the process to remove it and knowing more or less what to do and in what sequence.

prok1

reviving an old thread but how do you know if the front differential is out?

Is it when not getting enough power to the front or the way it shifts?

charlieaf92

There is a test that can be done, but I believe it requires a 4 wheel dyno and special tools. When I had my car up on all 4 jack stands I put it in gear and the front wheel turned, and I've setup a camera when there is snow on the ground to verify that the front wheels do in fact spin, but that doesn't necessarily mean its fully functional. Other than that I am not aware of a way to verify. I suppose you could raise the rear of the car and put it in gear and see if the front wheels pull you forward, but unless you come up with a way to raise the rear so that it won't crash down when that happens I can't recommend doing that.

Cheers
Charlie

911Syncro

About a test of the VC. The 996 Viscous Coupler is the same technology that was used on the VW Vanagon Syncro in the late '80s, and the architecture of the drive train is the same as well. The long running test for the Syncro VC is just about like Charlie wrote here, except it uses a 2x4 placed in front of the front wheels, and a floor jack at the rear lifting up the engine (and therefore the entire rear of the Van). With the rear wheel in the air, and spinning freely, the power will be transmitted to the front wheels and drive over the 2x4. See here for an example of this test being performed.

Syncro Sport VC 2x4 Test:
Now the disclaimers: The Syncro will transmit 100% of the power to the front, whereas apparently the 996 VC will transmit up to 40% of the power. But that 40% assumes that power is still transmitted to the rear. Given the VC technology, I guess I would claim that the 996 will also transmit 100% of the power to the front if both rear wheels are spinning freely. Also, I have never tried this test since it requires some faith that the car will not slip off of the floor jack.

Here is a VC test that seems a little safer, just jack up one side of the Syncro (or 996), and like this video it should do that same thing for the 996. But I believe it gets very tough to detect a partially failed VC on the 996, since there isn't sufficient resistance in this test for that determination.

Syncro VC (STATIC) Test:
By the way, the Syncro VC can fail in 2 different ways: 1. it becomes open (no power transmitted), and 2. overly aggressive or semi locked. Overly agressive is bad because under the right circumstances like a tight turn, there is insufficient play in the VC, and first the tires will scrub on the pavement, and then an axle or CV will break. I haven't yet seen anything about VC failure modes on the 996.

charlieaf92

Thanks for posting - that is very interesting! My own test (in the snow) yielded different results. My front wheels had a brief lag before they started getting drive power. It also may have been that the front wheel on the opposite side got the power for a second since my video only shows one side of the car. I believe the VC system works with a special fluid that increases friction between the clutch plates as it heats up - so once they start to spin at different speeds, that generates heat and friction increases with that, causing them to start locking together more and sending drive power to the front.

I'll try to post a video of what mine did soon.

911Syncro

I haven't studied this in some years, but it really isn't the friction or heat, but it is the sheer forces within the fluid that causes the fluid to radically increase in viscosity, and therefore the plates lock together. There is very little that I've found about the 996 VC, but plenty on the Syncro VC. The fluid is some silicone based fluid that has certain properties. If the fluid gets stretched / agitated, then the viscosity increases. It takes a split second, and that is why you saw the lag on the 996 before the front wheels get fully "locked" power delivery. It is kinda like the good old silly putty; remember when you pull the putty real fast it gets hard and breaks, if you pull it slowly it remains flexible. The Syncro community has researched all of this to extraordinary lengths and even some learned how to refill a damaged VC --- if you can obtain the specific fluid. Here are a few links to learn more, but the details are long and gory. They even found the VW engineers who first designed the system, and found them in retirement! Here are some good links for more info about how the Syncro VC works, which I still think it very much applicable to the 996 VC.

A very good overview description of the VC: http://syncro.org/the-viscous-coupler/

Perhaps more detail that anyone might desire: http://users.rcn.com/derekdrew/vanag...gon_syncro.htm

The most prominent Vanagon forum where all of this has been hashed out for years (search on "VC" or "viscous coupling"): https://www.thesamba.com/vw/forum/viewforum.php?f=20

The long standing email group (from the early 90s, before online forums!) where many of the experts still participate (look through the archives for VC stuff): http://gerry.vanagon.com/

prok1

what if i am taking to the dealer- what test should they run?

911Syncro

Well, I suppose they can use the 2x4 test (as above) if they are willing to jack up the rear (or find some way to get the rear tires off the ground). This test alone is good enough.

Jacking up one side of the car, and spinning one of the wheels will reveal some information. However it will probably only tell if the VC has completely failed in the open condition, it can't detect a weak VC since our arms cannot provide as much torque as the engine.

Both of those will reveal a VC failed in the open condition. To see if the VC has failed in a closed condition, go a parking lot and start making slow but very tight turns. If the VC is permanently locked, the tires will scrub and chirp because the front wheels cover a larger distance (greater turn radius) than the rear wheels. Again I've never seen a description of the 996 VC failure modes, so can't say much more about that. And I've never heard of some 996 VC test that dealers might use - would love to hear / learn about it if anyone knows.

Or another thing, on my C4S I "imagine" I can feel the front wheels pulling when I floor it while coming out of a curve. It just has this pulling feeling is which noticeably different than a 2wd 911.

pdxmotorhead

On a wet skid pad the car will understeer if the VC is bad, it will slightly oversteer if its working.