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Old 04-16-2012, 02:46 AM   #1
BoMbS
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Default Rear <-NO! - Center diff is cooked.

I drive a 1993 WRX with 150kms, and there is an awful knocking noise coming from the rear of the car while I turn at slow speeds, but only after a long high speed drive.

Took some research to find out that the reason that it only happens after a high speed drive only, but I think it's the viscous coupling in the rear diff that's shot. I read somewhere that the symptoms of a failing diff are "obvious binding at low (parking lot) speeds, easily induced binding (i.e., upon less turn of the steering wheel) and heat related binding (after long high speed summer trips)."

I know that my car has a 4.111 final drive, what other makes and models have the same gearing and would fit with my axles?

*edit: Turns out to be the center diff, not the rear diff.
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Last edited by BoMbS; 05-19-2012 at 02:56 AM. Reason: Title was misleading, found actual problem.
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Old 04-16-2012, 07:23 PM   #2
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Look in the Transmission forum. There's many cars that have the 4.11 FD such as the 2nd gen Legacy GT and the 2.5RS.
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Old 04-16-2012, 09:49 PM   #3
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98-01 USDM Impreza 2.5RS with manual transmission, but only the 00-01 have an LSD.
97-99 Legacy 2.5GT or Outback (w/2.5). not sure which have LSD
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Old 04-16-2012, 10:30 PM   #4
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pull the diff, and see if it's just r&P or if it's the diff itself.

if just R&P then life will be much easier.
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Old 04-17-2012, 07:45 PM   #5
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I'm pretty sure it's not the ring and pinion, since the symptoms only occur when the diff is hot. From everything I've read across all corners of the web, I'm sure it's the VC that's failed/failing. Since tearing into my differential and trying to replace parts is beyond my DIY skills, I think I'm better off finding a used diff.

Thanks for the help finding compatible diff's, I think I have a couple options that aren't far from where I live. I will post updates as my adventure continues...

I just hope I'm not mistaking the location of the noise and that it is indeed my rear diff and not my center diff...

Any extra advice is appreciated, whether it's diagnostic or otherwise!
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Old 04-17-2012, 07:45 PM   #6
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I have the rear diff from a 2004 wrx. Axles, mounting hardware, drive shaft, etc... If this will work for you LMK

-Dave
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Old 04-18-2012, 01:05 AM   #7
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Quote:
Originally Posted by lavid2002 View Post
I have the rear diff from a 2004 wrx. Axles, mounting hardware, drive shaft, etc... If this will work for you LMK

-Dave
this will definitely NOT work
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Old 04-18-2012, 01:10 AM   #8
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Quote:
Originally Posted by elislider View Post
this will definitely NOT work
Thanks
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Old 04-18-2012, 11:08 AM   #9
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Is it possibly broken down gear oil? That would show problems after being heated up.
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Old 04-18-2012, 09:48 PM   #10
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I should have mentioned, but the gear oil is fairly new, probably only 15000 km's or so. I'm using Red Line synthetic lightweight shockproof.

Everything I know about differentials and especially viscous couplings I've learned in the past two weeks since these symptoms started, so someone correct me if I'm wrong, but the VC is a sealed unit that has it's own special oil (usually a silicone blend) inside. The gear oil in the diff is there to lubricate the ring and pinion and the bearings.
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Old 04-18-2012, 10:07 PM   #11
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Well lets start with the simple stuff,
1.Do all 4 tires match size and brand?
2. Is there less than a 1/4 inch difference around the tread circumference between all 4?
If no get matching tires.
3. If yes to above (assuming manual trans) drain the trans gear oil, Note the color, is there is a color shift from light to dark, or dark to light, and in the pan there is pooling of different liquids like oil vs water, if yes the Center transfer (viscous coupling) seals have come apart in the trans.


Its rare that a Subaru rear diff fails for any reason other than lack of lubrication or contamination (usually water). Unless it was swapped and has the incorrect gearing (would be obvious) or the tires don't match and causing the limited slip to heat up or has damaged it.

Afterthought, have you replaced any of the cv axles, aftermarket cv axles are known to clunk and pop, or if a cv boot has ripped and slung some or all of the grease out can make horrible noises as well.

Last edited by Blktrax; 04-18-2012 at 10:35 PM.
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Old 04-19-2012, 02:27 AM   #12
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This is a helpful document: http://www.northursalia.com/techdocs...rannychart.pdf

Since I've known the car (before I owned it) until now, it's always had matching tires.

I changed the transmission gear oil at the same time I did the diff fluid, and I've been checking ithe fluid regularly.

All my CV axles look and feel good and are OEM as far as I can tell.

But, now that you mention it, the car came with a spare in the trunk that was clearly very worn out.

I bet the PO drove it on for too long at high speeds and did ruin the viscous coupling...
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Old 04-19-2012, 06:58 AM   #13
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The Center Transfer is not that hard to replace and you can do it with the trans in the car. You only have to remove the back cover section, after removing the driveshaft and exhaust.

Its a matter of getting the correct measurements and shims to set the pre-load on the gears. Other than that its a pretty straight forward out and in situation.

However it may be worth it to spend a few bucks and drive it get it hot, prearrange to take it to someone with a lift, and run it in the air to verify where the noise is. If the vehicle is in the air and the parking brake set, 1st gear slowly let out on the clutch it should stall or nearly stall the car (no throttle), if it doesn't (front wheels spin) or loud clunking happens the center is done.

If it works properly then the noise is likely from the rear as you stated before. With the suspension unloaded and the wheels spinning free (parking brake off 3rd gear @ idle), you would likely be able to hear rear end or cv noise pretty clearly with an automotive stethoscope.
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Old 04-23-2012, 02:40 PM   #14
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Thanks Blktrax, that's exactly what I wanted, some good way to diagnose where the problem is coming from, as it's really hard to tell when sitting in the car.

I will try what you said, and make a post if I find anything out. Hopefully it will still make the noise without turning. If only I could lock just one rear wheel...
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Old 05-19-2012, 02:48 AM   #15
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Update time:

Blktrax, you were right, it was my center diff, not my rear diff.

I tried the test you suggested, but couldn't hear any abnormal noise from anything. I'm assuming it's because the clunking would only occur when cornering, and thereby forcing the wheels to spins at different speeds, which I couldn't duplicate in the air.

Anyway, since my brother recently exploded one of the gears in his 95 STi I had a free center diff for the taking. As you said, it's a lot easier to swap the center diff than change the entire rear diff. So I figured, why not try that first, and after 6 slow paced hours, the car was ready for a test drive.



Happy to report that after a long highway trip on a hot day, I did about 10 victory donuts without a sound from the drive train.

Afterward, I dismantled the old center diff to have a look at the viscous coupling, and it was oozing some thick yellow goo, which I assume is the special silicone based fluid that's not supposed to come out. The case of the VC was also very black compared to the shiny one I put in, which I've read is a symptom of it getting 'burnt' out.



Very happy to be rid of this problem before I did some serious damage.
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Old 05-19-2012, 10:41 AM   #16
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same thing happened on a 95 STI drivetrain i put into a USDM impreza.

i just pulled the center diff out of the stock 1.8l trans. it was the same. and it was all better...

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Old 05-19-2012, 12:29 PM   #17
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Glad you got it fixed.

I would as a preventative , drive it for a few hundred miles and change the gear oil again. That silicone fluid thins out the gear oil and usually wipes out the bearings in the front diff as thats where it tends to settle.

I would also note to others, that center diff when assembled is pressurized, then sealed, obviously this one leaked because it would have shots parts and fluid all over the place. Just becareful, Subaru Repair manual says, (not yelling just quoting) DO NOT OPEN: No servicable parts inside.

Last edited by Blktrax; 05-19-2012 at 12:42 PM.
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Old 05-22-2012, 09:15 PM   #18
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So I've kinda got the same issue. It started about 3 weeks ago, I was at a stop turning onto a street and as i was starting to go there was a brief (vibration/ thump, thump, thump) feeling. It felt like it was coming from the center of my car, kinda where the center console is and just behind my seat or next to it. Now at first this was just when i turned. It seemed to happen more often when the car had been driven for a bit. It now does this not only when i go from complete stop, to turning.... but also when I go from complete stop to driving straight. The thump has also started to increase... so instead of thump thump thump.... it's thump thump thump thump thump. There is no noise or vibration that I notice while driving. I've also tried slowing the car (almost to the point of stalling the engine, about 2-5 mph) without putting in the clutch and speeding up again and i'll get the vibration. it always seems to happen just between 1-5mph range.... so I talked to a Subaru specialist and he felt it was most likely the center diff. I put the car up off all 4's - hand brake on - 1st gear - slowly let out clutch = front wheels spin, back wheels stay put, engine didn't die. now am I correct in assuming that my center Diff is toast? If my center diff was working correctly, can i assume that my car would have stalled when I tried the above?
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Old 05-22-2012, 09:52 PM   #19
Blktrax
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Pretty much, is should idle down significantly almost stalling or actually cutting off as long as you're dealing with a 5-MT.
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Old 05-22-2012, 09:55 PM   #20
Benja
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Hmm, well it did idle down quite a bit, but the front tires were definitely moving and backs were not
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Old 08-13-2013, 05:56 AM   #21
sirrado
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Default did you try to open the viscous

Quote:
Originally Posted by BoMbS View Post
Update time:

Blktrax, you were right, it was my center diff, not my rear diff.

I tried the test you suggested, but couldn't hear any abnormal noise from anything. I'm assuming it's because the clunking would only occur when cornering, and thereby forcing the wheels to spins at different speeds, which I couldn't duplicate in the air.

Anyway, since my brother recently exploded one of the gears in his 95 STi I had a free center diff for the taking. As you said, it's a lot easier to swap the center diff than change the entire rear diff. So I figured, why not try that first, and after 6 slow paced hours, the car was ready for a test drive.



Happy to report that after a long highway trip on a hot day, I did about 10 victory donuts without a sound from the drive train.

Afterward, I dismantled the old center diff to have a look at the viscous coupling, and it was oozing some thick yellow goo, which I assume is the special silicone based fluid that's not supposed to come out. The case of the VC was also very black compared to the shiny one I put in, which I've read is a symptom of it getting 'burnt' out.



Very happy to be rid of this problem before I did some serious damage.
I found some methon to rebuild vw viscous, do you think it could be help to rebuild Subaru viscous ?
Did you try to open the viscous ? to check what could be wrong inside ?
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Old 08-14-2013, 01:06 AM   #22
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I found a place in the UK that says they can rebuild subaru viscous couplers but its nearly the price to replace with a new one. Their process supposedly involves laser welding the vc unit back together once its been filled
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Old 08-14-2013, 03:09 PM   #23
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Hi Thank for answer.
Is this shop have a web site ?

I found it video to rebuild vw viscous:

http://www.vwsyncro.co.uk/data/forum...?TOPIC_ID=3048


and the special oil there:

http://www.silikon-profis.de/index.p...e3d4a138a215c5
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