Subaru All Wheel Drive System Failures & Repair

Subaru AWD systems are pretty well designed and failure is not all that common. When you have a failed or failing system there are some things to look for and this article will explain these symptoms and how to repair them without even replacing or removing the transmission. Here are some thing that we will cover.

  • Subaru AWD system common failures

  • How to Identify a Failing Subaru AWD System

  • How to Repair a Subaru AWD Transfercase

Over the 12 years that I have worked on Subaru’s and Subaru transmission repair there are two main parts that have usually fail when you have a problem with the transfer case viscous coupler. The first one is the AWD duty solenoid for applying and releasing the viscous couple when needed. The system needs to allow slip in the clutch when the front and rear tires are rotating at different speeds. This is usually only on turns. When the duty solenoid fails it usually will turn on the AT Oil Temp lite. To retrieve codes for the TCM on older models without a Subaru scanner is a tricky procedure but not that bad once you know it. If you need help in retrieving the diagnostic trouble codes for the TCM contact me for the procedure.

AWD Solenoid Top AWD Solenoid Bottom

Above are a couple pictures of the AWD duty solenoid. Notice how the solenoid comes with its own miniature valve body assembly. This is replaced with the solenoid as an assembly when purchased from a Subaru Dealer. This is the only way I’m aware of how it comes when purchasing a new one and also if there is an aftermarket solenoid available with or without the valve assembly I still wouldn’t recommend it.

The next failure is the most common and even when I have to just do a solenoid you will usually see this problem anyways. The inside viscous clutch drum gets grooves worn on it where the clutch frictions ride on the grooves from the clutches releasing and applying constantly in both directions. What happens when this occurs is as you take a sharp turn under acceleration the clutches bind in the applied position and cannot release. In return you will get a chatter or vibration while turning and acceleration at the same time. This is usually the most common customer complaint that I will hear of. Now regardless if you have solenoid failure or just a worn drum I still recommend to replace both the solenoid and drum. You will also need to replace the clutch pack. The clutches in the old set are usually not burnt up, but they also get worn in the area where the ride against the drum. In extreme failure the drum may even brake like the one I will show you below.

AWD Drum Broken

Broken Subaru AWD viscous clutch drum.

AWD Drum Notches

Worn Subaru viscous clutch drum. Take note of the worn grooves from the clutches.

Subaru All Wheel Drive Transfer Case Repair

Now as I said earlier when you tear down the transmission to repair this system these parts are better off replaced together. So lets go over the needed parts for the repair.

  • AWD duty solenoid and valve assembly
  • solenoid/valve gaskets (2)
  • AWD viscous clutch drum
  • viscous clutch pack set (frictions with steels)
  • transfer case gasket

Now first things first, gets access to the transmission transfer case. Here are the steps I do it in.

  1. remove exhaust
  2. remove heat shield then driveline
  3. remove crossmember and lower transmission down
  4. drain transmission
  5. remove back half of the transmission with transfer case parts (see gallery below)

Now that you have these parts removed it will be pretty self explanatory on the repair now that you know what to look for. Note that the viscous clutch drum has to be pressed of the shaft even though it looks as if the snap ring just holds it on the splines. look over the picture below to get a better idea of what you will encounter and also take note of the park pawl upon removal and assembly, it will only fit one way but it’s possible to get confused here.

This post was written by: Martin Hand


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Martin Hand

About Martin Hand

ASE Certified L1 Advanced Mastertech. Martin Hand has over 15 years experience in Asian and European Import Auto Repair. Specializing in electrical diagnosis, engine performance, AT/MT transmission repair/rebuild. Martin is also pursuing a degree in Computers Science & Information Systems starting at Portland Community College while he plans to transfer to OIT. Certified in Java application level programming, experienced with other languages such as PHP, Ruby, JavaScript and Swift. Martin has future plans of automotive diagnostic software development.


  • Simon Leary says:

    Hi Martin,
    What a fine website! One of very
    Is it so that most viscous couplings from Subaru are all the same, meaning that you can use one of an Impreza in a Legacy, or one from the automatic in the manual driven car? I had the feeling after numerous pictures and youtube movies that they are the same. I too have beginning problems with the viscous coupler. Alltough it does its job in transmission oil, I read that the actual plates are closed-off and rely on a particular silicon oil to have friction. Does that oil have a name? or number? I saw that you wrote here above that the retainer clip can easily pop of. I fear that this is quite lethal for the functioning of the viscous coupler or can it be saved?
    And is there a kit to renew the viscous coupler if the casing is not too damaged?

    Thanks in advance,


    • Simon Leary says:

      One of few concise that is out there . (I was about to say)

    • Martin Hand Martin Hand says:

      The viscous coupler for MT transmission is different then AT transmission. MT transmission viscous coupler has to be replaced as complete unit, individual parts for repair not available.

      • Simon Leary says:

        Thanks for the swift reply,
        Here and there pop stories up about repaired MT viscous couplers but not real succes stories. I now know for sure then that I can only buy a new one (second hand is too much of a gamble for only half the price). Have a good one!

  • 09 Forester,75K miles,All well until stopped moving.cooled down,would work,then…Replaced w/used unit and AWD lite on after a mile.Done by good,pro mechanic.Feels like l wheel drive.Before I replace this trans,can I diagnose problem?Can you help me?Have appt w?Transmission shop for diag.later this week.What gives?Thank you.

  • Jesse says:

    I just got my clutch replaced, and I was driving about 40mph (kindof on a turn) and I heard a loud POP or BANG and my wheels locked up. I could not get the car to move, even by pushing it in neutral with the e-brake off. I had it towed, and a day later it just started driving again. Do you have any idea what the heck happened?

  • Clay Weiler says:

    Hey I got a 2002 rs. So I was driving and a sound like a bad wheel bearing whining. I was on the freeway going 75 and the noise got louder and louder. Then a pop. After that I started to feel slipping. I have gotten a new tranny a new clutch master cylinder. Brand new oem clutch. Resurfaced flywheel and still finest want to catch 100%. I decided I’m gonna laugh the car while having this problem. Felt like my front wheels spun. And still didn’t want to catch 100%. Front wheels only spinning in a awd car???? Do rear diffs cause it to feel like it’s not engaging 100%? I can’t figure this out. About to throw in the towel. If this new rear diff I’m buying doesn’t work.

    • Martin Hand Martin Hand says:

      The center differential are known to blow the retainer clip for the drum causing it to fail. This could be the pop you heard but check the obvious first, make sure driveline turning and the differential turning the axles

  • Michael K. says:

    Hi, I have a 2003 Outback VDC with 149,000.00 miles.

    I have this grinding sound when I back out of a space and start to move forward, only for a second. Sounds like it coming from the center of the car, hard to tell. Sometimes I get the same noise when moving forward after getting gas. I suspect that its an issue with the viscous coupler.

    Question: Can this be fixed with the transmission in the car?

    Another issue that might be related is a vibration when the car is in park especially when parked on a hill, preload on the transmission park lock. Ill REV the engine and Ill get this vibration between 1000-2000rpm as if the torque converter is out of balance. This does not happen when parked on a flat surface.

    Thanks for any suggestions that you might have,

    • Martin Hand Martin Hand says:

      Yes it can be fixed in the car. This is how the repair is performed in the article. Make sure the transmission is working properly before attempting an in car transfer case repair.

  • mike says:

    hello all was hoping somebody would help me try see what went wrong in my auto trans i have bigger mags on the back for one the the first sign of stress so anyway i was turning a corner and next thing i know is a here a cluck the hole front end of my car squated down asif i had slammed the brakes on and then from there out it would go less then 10 khz while banging and jumping around trying to test drive it to get home wondering if its the center diff rear diff or the front and if i can use 5 speed manual parts to fix it want to keep it how it was befor i make it manual btu have a spare box that has warn sycros so wondering if i can use bits , im about to jack it up off the grount sping the wheels maybe start it and run it to see if the front wheels are locking like it feels then remove the driveshaft from the trans and diff so i can test the front without the rear and also spin the rear by hand not sure where to turn do i pull the center diff out or with more time drop the hole trans as i want to get back driving asap

  • Mike Russell says:

    Very interesting reading. I have an on going problem with my 2005 Subaru Legacy. I purchased the car at 51km and have now done 95km. I took the car into a Automatic specialist as my car was clunking down in gear when I slowed to around 15 – 25km this was happening quite frequently mainly slowing down and then accelerating turning into a gradual rise, it gradually got worse. They said it was the transfer clutch and went a head and replaced pretty much everything without telling me and charged over double what they had quoted. reading your diagnostics they didn’t replace the drum as it was not charged for on the invoice. After taking it home it started to do the same thing and the garage have had it back twice and still the same problem. I am taking it back again and going to demand they fix it this time. Whats your thoughts !! kind regards Mike

    • Martin Hand Martin Hand says:

      It’s really hard to tell without seeing the vehicle for myself but I have never seen transfer clutch cause a downshift clunk. The transfer clutch only engages dis-engages when there is a difference between front to rear wheel speed. This only occurs in tight turns which would have to be slow speed so no downshifting would occur at that time. Sounds like some shift control problem or maybe high line pressure.

  • Paul Richardson says:

    Hi, I have a 93 legacy 4wd. I’m having what seems to be false neutral before or after 3rd gear, im not sure. Somimes it goes back in gear or try to. I’ve had days it doesn’t do it (rare days) and days it will everytime.
    If I put it in 3rd and manual button it helps alot, rarely will “slip”. Putting the fwd fuse in and problems go away. It also shifts little hard in fwd and 4wd. Any ideas where to start? Thanks.

    • Martin Hand Martin Hand says:

      The transmission is probably starting to fail. The upshift problems are most likely in the valve body but depending on mileage a rebuild or replacement would be your best option.

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