[Randejo]

Hi All, I'm a new member who recently bought our first Subaru for my son. It's a 97 Legacy Outback automatic trans.

Yesterday the car suddenly developed a loud banging and vibration. The banging happened whether we pushed the car or started the engine, and I put it on my hoist (single post) and had him start the car and put it in drive while I watched underneath. Loud banging and shaking of engine/driveline during banging noise, and the front tires also shook while the noise was happening. It seemed to bind up after a minute or so, as he pressed that gas harder and the front wheels didn't turn. I thought it might be the front differential, but doing more research here and elsewhere, it may be the center diff (since the tires aren't all the same size... guess I need to fix that!).

Do center differentials fail so suddenly? Apparently there have been no noises or strange behaviors until it suddenly failed.

Thanks,
Randy

 

[plain OM]

Your Subaru with 4-speed automatic transmission does not have a center differential. That's found only in the manual transmission, and in a planetary form in the later 5-speed automatic. Instead, the 4EAT has a multi-plate clutch that connects the output of the transmission to the rear propeller shaft. (The output of the transmission is also connected through a pair of gears to the front differential.)

What happens if you manually turn one or more wheels with the car up on the hoist and the engine off? If the banging etc is apparent when the car is pushed I would think it would show up on the hoist as well. By manually turning the wheels it should be possible to better identify where the noise/resistance is coming from. In this regard, because the front wheels stopped turning when the engine was running in gear, and the engine was shaking, it could well point to a broken front differential.

If the noise seems to be coming from the rear end (extension case) of the transmission, the multi-plate clutch hub might have separated from the reduction gear inside. It would lead to noise, although I'm less certain it would cause the drive train to seize up.

Also check the propeller shaft U-joints, the engine and transmission mounts, and the front wheel drive shafts.

 

[canubaru]

you need a trans.

this sounds like a front diff issue. and these are rare. if you had started with '' i have a bad front diff'' i would have gone on and on about how hardy they are and how it is rare for them to fail. the subaru auto trans is basically a FWD trans with a transfer clutch to send power to the rear wheels.

but if the front wheels are not turning then either the front diff is bad or the trans is bad. in either case you need a used trans. this is the most economical, fastest, surest, solution.

try Car-Part.com--Used Auto Parts Market put in your zip code and sort by distance to see what is close. then sort by price to see if there is a bargain that can be shipped for less than a local trans.

TIP: regardless of what the software at car-part.com says, you can use any auto trans from a 2.5L car 96 - 98, (outback, GT, or LSi). the only difference in these is the speedo gears in the trans. outback 96 - 98 is a perfect match. GT and LSi may throw off your speedo.

TIP: you will have to search each year separately.

good luck

 

[Randejo]

Your Subaru with 4-speed automatic transmission does not have a center differential. That's found only in the manual transmission, and in a planetary form in the later 5-speed automatic. Instead, the 4EAT has a multi-plate clutch that connects the output of the transmission to the rear propeller shaft. (The output of the transmission is also connected through a pair of gears to the front differential.)

What happens if you manually turn one or more wheels with the car up on the hoist and the engine off? If the banging etc is apparent when the car is pushed I would think it would show up on the hoist as well. By manually turning the wheels it should be possible to better identify where the noise/resistance is coming from. In this regard, because the front wheels stopped turning when the engine was running in gear, and the engine was shaking, it could well point to a broken front differential.

If the noise seems to be coming from the rear end (extension case) of the transmission, the multi-plate clutch hub might have separated from the reduction gear inside. It would lead to noise, although I'm less certain it would cause the drive train to seize up.

Also check the propeller shaft U-joints, the engine and transmission mounts, and the front wheel drive shafts.

Oddly enough, when I turn the wheels with the engine off it doesn't make the loud banging sound. There might be a slight noise coming from the front diff area. Also, when I turn either front wheel, the other doesn't turn. The rears will turn in opposite directions when I turn one or the other, like a regular diff. I drained the front diff fluid and didn't see any chunks of metal. I have the rear driveshaft out, and I was planning to pull the rear cover off of the transfer case to check the clutch packs. Would this help?
Thanks,
Randy

 

[plain OM]

when I turn either front wheel, the other doesn't turn.

I'd focus on this, because, as you recognize, it's not what is expected. With the transmission in Park (which should lock the drive between the transmission and the front differential) turning one wheel should cause the other to turn the same amount in the opposite direction.

If you have the front of the car raised, and someone to help, turn both front wheels together (same speed, forward direction). With the transmission in Park neither should turn; with the transmission in Neutral, both should turn without any appreciable noise/clunking.

 

[1 Lucky Texan]

might be worth triple checking the axles and their stubs in the diff.

front diff /axles are definitely where this problem seems to be.

 

[Randejo]

Thanks for the replies. I have a leaky axle boot so I might replace both axles anyway. I'm also going to borrow a bore scope to see whether I can inspect the front diff. I got a glimpse of the ring gear through the drain hole but I didn't see any problems.

I probably need to replace the multi-plate clutch since the car has mis-matched tires, correct? Or is this only an issue for the viscous coupling (manual trans) cars?

 

[1 Lucky Texan]

no, you defintely need to get tires with the exact same rolling diamter on the car. If the car is jerking, particularly when turning tight as in parking, you have torque bind. Hopefully, it hasn't been driven long with an odd sized tires. They need to be the same brand/model and be within 2 or 3/32 tread wear of each other .

 

[Randejo]

The car has been driven several thousand miles with the mismatched tires, so I'm assuming the multi-plate clutch is shot. Can this be accessed by removing the rear cover on the transmission? Are these units repairable? Thanks again for all the replies.

 

[reddrexx]

Try this link http://www.subaruoutback.org/forums/99-do-yourself-illustrated-guides/27878-4eat-tail-section-rebuild-without-removing-transmission.html it is a great write up and you will get a good understanding of whats in the tail section. :17:

 

[Randejo]

Try this link http://www.subaruoutback.org/forums/99-do-yourself-illustrated-guides/27878-4eat-tail-section-rebuild-without-removing-transmission.html it is a great write up and you will get a good understanding of whats in the tail section. :17:

Thanks for the link, I may pull this apart to check things out. Would putting a fuse in the FWD fuse holder on the firewall let me isolate this to test whether the clutches are shot?

 

[plain OM]

There's two possibilities regarding the condition of the multi-plate clutch plates due to the different tires -- either the clutch plates are worn out and don't grip, or they have somehow become mechanically held together (possibly due to excessive grooving of the clutch drums).

If the plates are worn out, there won't be any significant power transfer to the rear -- you would know this is the problem when the car is stuck on a slippery surface with the front wheels spinning and the rears not turning.

If the plates are mechanically held together, you would have noticeable torque bind symptoms when making tight turns with little or no throttle.

The FWD fuse normally only disengages the clutch -- it won't help in finding out if the clutch is worn out.

In any event, a set of worn, or binding, clutch plates would not lead to

. . . a loud banging and vibration. The banging happened whether we pushed the car or started the engine, and I put it on my hoist (single post) and had him start the car and put it in drive while I watched underneath. Loud banging and shaking of engine/driveline during banging noise, and the front tires also shook while the noise was happening. It seemed to bind up after a minute or so, as he pressed that gas harder and the front wheels didn't turn.

 

[1 Lucky Texan]

^^^ good post

it would be a shame to rebuild the tailshaft and THEN discover the front diff is bad.

 

[plain OM]

Yes, it would. But then again, the linked thread in post #10 shows what might happen if the car is driven when there's torque build-up in the drive train that the clutch plate drum-to-reduction-gear welds can't sustain. Those welds might be the weakest part of the drive train. They'd be fine in normal use, but would be subjected to the twisting force if the front and rear drives aren't turning at the same speed (or trying to turn at different speeds). I'm still leaning toward the differential end, but wouldn't discount a problem at the rear, but it wouldn't be a case of a worn out clutch. Some further testing at the front end would be helpful. (E.g. why the opposite front wheel doesn't seem to turn . . .)