Changing a centre diff...

For the past 6 months, my Legacy (GT-B, E-Tune, Twin-Turbo, 4WD, manual) has been suffering from a faulty centre diff.

When the car was cold, it was fine, but as things got warmer it started to “clunk” when you turned the steering wheel due to the coupling slipping. Here’s the Wikipedia description of a centre diff:

“In a four-wheel drive vehicle, a viscous coupling unit can replace a centre differential entirely, or be used to limit slip in a conventional ‘open’ differential. It works on the principle of allowing the two output shafts to counter-rotate relative to each other, by way of a system of slotted plates that operate within a viscous fluid, often silicone. The fluid allows slow relative movements of the shafts, such as those caused by cornering, but will strongly resist high-speed movements, such as those caused by a single wheel spinning. This system is similar to a limited slip differential.”

Obviously this wasn’t ideal, as it meant I couldn’t drive the car far.

After a bit of research I found that this was a known fault in the 1999-2002 centre differentials in Subaru’s, so I took it to Subaru who quoted me $2000 to replace it with an exact model, which may break down again. I also discovered that an older Subaru WRX STI centre diff would slot straight in, and that they don’t suffer from the same issue. Bingo! The hunt for a STI diff began.

It took a few months and lots of hunting around by my cousin Alex, but in early February, for the small sum of $300, one arrived at my door.

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Then last weekend we got around to installing it (Huge thanks to my cousin and mate from Street Customs). We planned on dropping the transfer case off, changing the diff, and chucking it back together. Sadly it were not to be, as the transfer case was under the gearbox cross member, and the driveshaft was covered by the exhaust. So off came the exhaust and driveshaft…

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  1. Gearbox
  2. Cross member. The transfer case is under this.
  3. Rear diff

Once the above was removed and the gearbox linkages were removed, the centre diff was surprisingly easy to access. We simply removed the back of the transfer case (the internet says you need to remove the whole transfer case, it lies. Simply use a small screwdriver to slowly and smoothly pull the diff towards you. Do not force it).

{<3>} #####Inside the gearbox

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Slot the new centre diff on to the splines, again without force, and the job’s done.

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  1. New centre diff in transfer case
  2. Gear linkage

Now the fun started of putting it all back together. It took us a good 4-5 hours to get access to the centre diff, but we knew how it went back together. Ish.

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  1. Drive shaft
  2. Primary turbo downpipe
  3. Secondary turbo downpipe
  4. Exhaust from ‘Y’ back

Around 12am, everything under the car was back to its original place, and looking good. The heat shields around the turbos are tricky to bolt back on, but we got there eventually. The last bit to go back on was the intercooler… We were expecting this to take a few minutes. Turns out getting a twin turbo intercooler on is slightly harder than the single turbo equivalent… All three pipes need to go on at the same time, whilst being on all weird angles. It took well over an hour, lots of scraped hands, and lots of swearing to get it back in.

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  1. Secondary turbo intercooler pipe
  2. Inlet manifold intercooler pipe
  3. Primary turbo intercooler pipe

Done!

We went out for a blast down the motorway to warm things up and check everything was still working correctly.

And, apart from a small exhaust leak on the one joint we didn’t touch, everything is running smoothly. No shudders. No clicking. No weird smells. Awesome.

Total time taken: 12 hours.

Would I do it again? Not really, but I would to save $1700.

Should you do it yourself? If you know your way around a spanner, tricky bolts, sharp edges and tight spaces, then sure. A pit/hoist is also invaluable – not sure we’d be done yet without one.