Mystery vibrations maybe center diff?

[super subbie]

I have a mysterious vibration problem; hoping the gurus can help.  This is a 95 outback, with a 90 EJ22 in it.  The tranny locked up in reverse, so I replaced it with a 93.  (That might be the problem right there, but this is a FrankenSubbie for the teenagers to drive to school). 

The new tranny ran fine for about 20 miles.  Suddenly the whole drive train seems to vibrate.  Driving straight on pavement.  I pull over to the shoulder, wait a while, and it's fine.  This has happened repeatedly, forcing me to drive home at 5mph.  I park the car overnight, and then it runs fine.  The vibrations are severe.  Very alarming at high speed.  It feels like the whole drive train is binding or fighting against itself.  And I noticed if I go over a speed bump, or otherwise unweight one of the wheels, I can hear it make a little screetch or skidding sound.  I assume it's a bad center diff, but this happens when driving in a straight line.  This is undoubtedly a severe mechanical interference problem, but it goes away if I park the car over night.  That's the part that perplexes me.  Any advise is appreciated. 

[heartless]

did you make sure the tranny and the rear diff matched (final drive ratio)
if not, then yeah, you have a pretty serious problem

[1 Lucky Texan]

that^^^^^

must be same final drive ratio

and, same size/model tires all 4 corners, did tires change with the work done?

 

also, 5 spds viscous ctr diffs usually fail such that, stone cold they are OK, but bind when warm.

[idosubaru]

Oh yeah - the screech noise is binding.  

You need to verify final drive ratio and install a rear diff that's the same gear ratio as the transmission you installed. 

 

The stubby trans shafts may not be fully seated.  They will drive a few miles then when a bump/turn pulls them out - WHAM WHAM WHAM car falls apart unless you're limping 5 mph. 

Check that those stubby shafts are fully seated and the circlips are intact.

If that's not it pull the front axles and check the inner joints - they can pull apart during trans swaps and pull the cage past the retaining ring and exhibit similar symptoms.

 

Edited July 9, 2022 by idosubaru

[idosubaru]

22 hours ago, super subbie said:

This is a 95 outback, with a 90 EJ22 in it.  The tranny locked up in reverse, so I replaced it with a 93. 

I think the 95 is a 3.9 final drive and the 93 is a 4.11 final drive.

So you changed your front differential final drive ratio and the rear needs to match. Of course you should double check since it's been previously swapped and still undiagnosed.

You probably need a 4.11 rear differential.  Do it now.  That's a lot of straining buzzing down the road and screeching tires on bumps. 

Edited July 9, 2022 by idosubaru

[el_freddo]

23 hours ago, idosubaru said:

I think the 95 is a 3.9 final drive and the 93 is a 4.11 final drive.

So you changed your front differential final drive ratio and the rear needs to match. Of course you should double check since it's been previously swapped and still undiagnosed.

You probably need a 4.11 rear differential.  Do it now.  That's a lot of straining buzzing down the road and screeching tires on bumps. 

Other way around Ido, the ‘93 manual would most likely be 3.7 ratio and I’ve known the outback a to be 4.111 ratio. 

The ‘93 auto would be 4.111 ratio, I agree with that. But since we’re talking about a centre diff I assume the OP is dealing with a manual gearbox. 

Match that rear diff and you shouldn’t have anymore problems. The earlier centre diffs seem to be sturdier than the later model units, you might have dodged a bullet there. 

Also the phase 1 gearboxes are diff stub arrangement - so double check that you have your roll pins in place, not that I think these are causing the issue. It has to be miss-matched diffs for sure! 

Cheers 

Bennie

[super subbie]

Thanks guys!  Both trannys are 4.11, at least according to all the charts out there.  One of the CV joints on the stubby hyper-extended itself.  I popped it back in, but it wouldn't turn after install.  Bought a new one, and that worked.  But I think I need to have a second look at that (as soon as the temp in my garage drops below 100F..).

Also, I pulled the center diff out of the old tranny.  Having a hard time seeing the viscous coupling function of this thing.  Looks purely mechanical.  I was thinking VSC might fail intermittently with temperature.  But I've been driving this thing around town for a week, trying to get it to fail again without luck. 

I'll report out when (if) I get it figured out.

[GeneralDisorder]

90,91 is 4.111 

92-94 is 3.90

Outbacks are 4.111

You need to change the rear diff to a 3.90

The VC is sealed and not serviceable on the all the 5MT center diffs. Your ability to see it is irrelevant to it's existence. 

GD

[super subbie]

Still working on this.  The tranny I removed is a TY752VAAAA.  Final drive ratio of 4.11 according to the chart that is all over the net.  The tranny I put in is a TY752VA2AA.  It's not on the chart, but numerous sources online indicate it is also a 4.11 (Google says so, so it must be true?).

By rotating front and rear wheels in various configurations, it looks like the center differential is functioning smoothly and easily.  The viscous coupling device seems to be very stiff, compared to the one I have on the bench. 

Before I tear the center diff / VCD out, I have a question maybe you gurus can help with.  Attached is a picture of my old center diff, on the bench.  The rear drive shaft is connected through the drive transfer shaft, shown at the bottom.  This shaft has two different spline sections, which fit into each of the beveled side gears inside the center diff.  Not the spider gears, but the ones connected to the front and rear drive shaft.  This locks the two gears to each other, which seems to defeat the whole function of the differential.  It's like it's got a center differential, but it's locked.  Any insights?

 

[88SubGL]

I don’t see any pictures.

[super subbie]

Me neither.  I'll try it again. 

[super subbie]

[Numbchux]

IIRC. The viscous LSD unit is separate from the spider gears. If you were to cut that shaft in half, it would be like an open diff.

[super subbie]

Yeah, I think I'm missing some something here (aka: I'm an idiot).  With the rear drive transfer shaft (bottom of pic) inserted, both the rear and front drive shafts are effectively locked to each other, meaning there is no differential action at all.  They are both locked to the center diff housing as well, negating the need for any kind of LSD.  I'm going to replace the unit with the spare from the old tranny and put it back together.  But it drives me crazy to do that not understanding how it's supposed to work.  If anybody has any insights, please let me know.

[1 Lucky Texan]

brief search found these, maybe one will help?

 

 

 

 

[super subbie]

Thanks for responding LTexan.  I had also found those vids, but watched them all again to see if I missed something.  Especially the first one.  I'm going to tear my failing unit apart and see if a learn something.  BTW, sliding the center diff and rear drive shaft into the transfer case, on the vehicle, is in no way as easy as that guy does it in the second vid.  Took me about an hour...

Just can't get around the rear drive transfer shaft locking the front and rear side gears together, inside the differential.  On the bench, when I insert this shaft, the differential is locked together as if it's one big fused shaft.  But there's no way that's how it was designed to work. 

[super subbie]

OK, problem resolved after taking the old center diff apart. The rear drive transfer shaft, shown with the two spline sections, doesn't actually engage with the front drive beveled gear in the differential.  The bevel gear, and the front drive shaft do rotate with respect to the spider gears and the rear drive beveled gear (open differential).  Just not in my specimen.  I needed to use vice grips to rotate the front / rear drive shafts because my viscous coupling device is nearly frozen.  I heated it up a little to 120F, and it locks up completely.  Put a used center diff back in the tranny, and it seems to work fine.

For prosperity, here's how to check if your center diff / LSD is failing.  Jack up the vehicle from the side, so that the front and rear wheels on that side are off the ground.  Parking brake off, tranny in neutral.  Rotate the front wheel forwards, and the rear wheel should also rotate forward.  Now put it in gear and repeat.  This time the front wheel forward should result in the rear wheel rotating backwards, reverse. 

In neutral, the center diff housing rotates freely, allowing both drive shafts to move forward together.  In gear, the center diff housing is locked by the tranny and engine resistance, forcing the front and rear drive shafts to rotate in opposite directions.  When I do this, it's very stiff, but the wheels rotate in opposite directions.  I'm assuming under normal conditions the two drive shafts only need to move a little, and slowly, with respect to each other