I have a '99 Subaru Impreza Outback Sport with 158,000 miles. It runs great, looks good, and has been very reliable. I get this intermittent shaking/shuddering on acceleration and sometimes on tight turns at low speed. Any ideas?
Automatic or manual transmission?
When the AWD transfer valve in a Subaru automatic transmission starts to wear out it doesn’t allow the clutch pack to disengage during tight turns, and the vehicle will shudder as the front and rear wheels fight each other for traction.
Insert a fuse in the FWD receptacle under the hood. This should temporarily disengage the AWD. If the shuddering goes away with the fuse inserted, you’ve found the culprit.
Replacement of the AWD transfer valve and clutch pack requires removing the rear section of the automatic transmission. It’s about a $700-800 job. Ignoring the shuddering can result in internal transmission damage, which costs even more.
If your car has a manual transmission it uses a viscous coupling, rather than a transfer valve and clutch pack. Sometimes changing the transmission fluid will eliminate this shuddering with a manual transmission.
So, is your car automatic or manual?
Automatic. Is it possible to just disengage the AWD for a while? What about the torque convertor? I’ve found some posts on Google suggesting that. Also, I’ve had the trans fluid and filter changed recently. Is there any other fluid I can change like the rear, etc.?
Are all 4 tires precisely matched as to brand, model, and size?
Do all 4 tires have the same amount of tread wear (or does it vary from tire to tire)?
If the answer to either or both of these question is “no”, then it is very likely that you damaged the clutch pack for the center differential.
Thanks for the responses from both of you. Things seem to be pointing to the trans. I also had a mechanic look at the car yesterday. They found intermittent issues shifting from 1st to 2nd (automatic trans). They weren’t able to high speed test drive but that’s what they found on DC city streets. As far as the tires are concerned, I’ve also heard that mismatched tires could contribute to a trans issue. Right now, all tires are exactly the same make, model and size. The fronts are newer than the rear, though. Prior to me buying new tires in the front, there were different make tires between the front and rear. Same size but different makes. Thanks again for your responses. I got a quote yesterday for a rebuilt trans for $2200. I’m not going to be sinking that kind of money into this car right now. I’ll see if I can get another opinion/quote. My concern is if I find someone who will do the clutch work for $800 and I end up having other trans issues. It’s a shame. The car is a good city car for me right now.
My Subaru mechanic tells me there’s no reason you can’t insert a fuse in the FWD receptacle and drive the car that way. You’ll have a front wheel drive Impreza, but that’s fine.
I’d try it and see if it makes a difference. Driving it while it’s binding sure won’t help anything, and can cause damage to the inside of the transmission case.
I will definitely give that a try. I can live with just FWD if that’s the case. It sure beats $2200 bucks! 
Do you happen to know where this receptacle is? Thanks again.
I stuck a 20amp fuse in the FWD socket in the main fuse box under the hood. It appears to have fixed the problem - no more shaking on tight turns or acceleration in the test drive tonight. I’ll be driving a bit this weekend so it’ll have a good long road test. And as per the Owners Manual, I get a FWD light on the dash indicating I’m in FWD. I can definitely live with this. If I get stuck in the snow somewhere, I’ll take out the fuse to give me temporary AWD. Thanks again for sending me down this path - I didn’t even have to buy the fuse.
It sounds like my wife’s 1998 outback, auto trans, <40K miles, very conservative driven, has a similar problem.
Where do I find the FWD fuse block ??
TIA
BillB
I found the fuse block… Near the fire wall by the windshild wiper motor and behind the shock tower on the passengers side…
BillB
I would like to tap the wisdom of those on the forum. I don’t expect to try to fix this myself as I have almost no AT experience or special tools but I would like to be knowledgeable when talking to a repair shop. Sounds like I have a similar problem.
At low speed like pulling into or leaving a parking place and when turning our Subaru Outback has a pronounced drive line chatter or binding.
1998 Subaru Outback, 2.5 L, Auto, Very conservatively driven ( wife’s car ), less than 40K miles half of that on basically flat suburban roads in Arizona. There are very few times where the AWD actually served.
Transmission fluid is full and clean ( Original fluid ). Tires are original and have equal tread ( ~ 2/3 of original ).
Gas mileage has been approximately constant over the last several years. A test in a sloped parking lot I could move froward at 700 RPM ( AC fast idle ) in drive but would only hold position at 600 RPM ( normal idle ). In both cases there was no evidence of engine missing.
Coasting in the parking lot, engine off, in neutral, and turning hard back and forth did not indicate any sort of CV joint binding. Likewise there was no CV joint noise.
Putting a fuse in the FWD fuse socket seems to eliminate the low speed chatter.
When I start the car the “AT oil temp” light comes on and soon after the engine is running goes out. ( It does not flash afterward. )
Questions:
What is the best guess at the problem ??
What might I realistically expect for a repair bill ??
Will leaving the FWD fuse in temporarily cause any harm ??
Regards
BillB
“Transmission fluid is full and clean ( Original fluid )”
Well, since that fluid should have been changed at least 3 times already (preferably 4 times), on the basis of elapsed time, I would suggest that you start by having an independent transmission shop drain it, drop the trans pan, clean out 11 years of accumulated sludge, and refill it with the correct fluid. That may not save the transmission, but it is certainly a good place to start since this vital maintenance has been skipped.
That being said, it does sound like the clutch pack may be going, but why not start with something that is just simply part of required maintenance? If servicing the transmission does not help, then you will likely need to have the clutch pack replaced at a cost of…maybe $400.
The warranty and maintenance book that came with the car only calls for inspection every 30K unless severe use is involved - not the case here. The fluid level has been inspected much more often than 30K.
Where did you get the approximately 10K transmission fluid replacement schedule ?
Regards
BillB
It’s not a “10k transmission fluid replacement schedule”, but rather a 3 year or 30k (whichever comes first) trans fluid replacment schedule.
As most of the members of this board are aware, automakers try to make their cars seem as maintenance-free as possible nowadays, even if this hands-off approach to maintenance does not bode well for the longevity of the vehicle. One of the best examples of this is the recommendation for trans fluid changes.
Subaru, like most other manufacturers lists an “inspection” at 3 years/30k rather than a fluid change. That is fine for the person who gets rid of a vehicle every few years. For people like me, who keep a vehicle for many years, changing the trans fluid every 3 years/30k is probably the best way to reduce massive repair costs later on in the car’s lifespan. Generally speaking, automatic transmissions that have been ignored can be expected to fail anytime after ~90k miles.
First the transmission oil change schedule you used has nothing to do with the problem at hand.
The clutch pack in the automatic Subaru AWD’s is a wear item that usually lasts somewhere between 150k-250k miles but there may be an age factor here. Note mismatched tires (not stating you ever did) will accelerate the wear of this part.
To address the problem is usually $500-$1000 to replace this part.
You decide what is best for your needs. There is no clear cut answer of the FWD mode hurts the car. It is much better than running around with wheel hop as that will stress out the rest of the driveline.
I would say 150k is a better number than 90k for automatics following manufacturer recommended specs. Modern vehicles have a design lifespan of that. Many far exceed that but that is what automotive related engineers I know US supplied design components for Honda, Subaru, Nissan and GM use.
I think that at 80% of original vehicle owners are covered with that #.
Hi VCDriver
Ok, I hear you… The schedule is conventional wisdom…
BillB
Hi Andrew,
Thanks for the reply.
Original tires are on the car and have been kept properly inflated and rotated. So I don’t think that is a problem.
Does your last reply suggest that you doubt the transmission fluid replacement will have much effect on the problem ?
I am considering doing a three cycle drain the pan and refill with Mobil 1 ATF as discussed elsewhere. The AT oil on the dipstick looks new - not at all dirty or off color.
Regards
The chatter is not severe like a f150 4x4 pickup where there is actually wheel scratching on gravel or chirping on pavement during a turn in 4-wheel drive.
Transmission fluid replacement will have little effect on the problem. However if the car is really 10 years old it should be performed. Also if those tires are factory original and 10 years old they should be changed simply due to age factor.