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.

597 Comments

  • Natalia says:

    Thank you for the great site!
    I would be grateful for your advice – I have 1998 Subary Legacy 4WD auto /turbo / Japanese built (64K). In a morning when car is still cold – I shift the gear to D and car jerks/ makes a little jump. I had a couple incidents when gear was in drive, but car would not move – I shifted gear back and forward and car started to move. It didn’t repeat after I’ve changed transmission fluid.
    One shop told me that I need engine tune up, another guy told me that my transmission should be replaced. Just want to know what you think

    • Martin Hand Martin Hand says:

      That is not good advice. You need to have the problem accurately diagnosed first. Keep driving to see if problem is progressively more consistent or intermittent still. I wouldn’t worry about it for now unless it’s consistent problem.

  • philip potts says:

    Hi just bought a 2006 auto legacy from auction only 108000k’s But ,there’s always a but, it has the engine malfunction light and the blinking cruise light on.As well it has the chatting at slow speed turn not noisy but you can feel it I feel this may have been covered but could you supply any tips

  • Michael Levine says:

    Hello from California:
    2006 Outback 2.5XT Limited. 5 speed automatic trans., 135,000 miles, original owner. All service has been done per schedule since car was new. Last week started to notice something I’ve never experienced before with this car. Holding freeway speed (60 – 70 mph) in 5th gear with pedal only or with cruise control and then accelerating with moderate pressure on gas pedal I feel one “buck” or “lurch” (sounds like a “clunk) as the car downshifts. It doesn’t do it all the time. Normally the car will smoothly downshift as power increases. I’ve checked for codes with OBD II scanner …. none, and no other warning lights have come on. Should I be concerned and what should I do?

    Thanks in advance for your help/comment on this.
    Mike

    • Martin Hand Martin Hand says:

      The transmissions will do this sometimes and have nothing wrong. Probably just related to the specific driving conditions. I wouldn’t worry about it unless the problem becomes progressively worse and more consistent.

  • jaeger engberg says:

    i jut put a new transmission in my 2005 subaru sti. when i drive it it has a strong shake unless i let off the gas or push in clutch. if i take my dccd out of auto into the open position the shake is alot less significant.(basically gone) if i move the dccd into lock the shaking happens again. i have put after market cv axles new excedy stage one clutch and act lightened flywheel. any suggestions on where to start feels like diffrent gear ratios almost!
    thanks in advance

    • jaeger engberg says:

      this is the second transmission ive put in with the same problem

    • Martin Hand Martin Hand says:

      Start by checking the obvious: cv axles, driveline carrier and u-joints, rear differential, even tire/wheel size. Try running it in the air to see if there is uneven wheel speed. I don’t ever replace the transmissions and less I have to. Do you have the original transmission? I would repair that one

  • alan liu says:

    Very insightful and helpful site! I recently replaced the solenoid in my 02 forester. It used to bind to the point where the car does not move unless you give some gas. But after replacing the solenoid i can still feel a binding feel when I turn. I’m thinking its the drum and clutch pack set. Would you happen to have the part numbers for the drum and clutch pack set?

    • Martin Hand Martin Hand says:

      I don’t keep part numbers for that type of repair they change between models and years too much. Contact Subaru dealer and pick out what you need from the parts diagram.

  • Nick says:

    Hay, i am having issues with my 2005 Subaru impraza AUTO transmission. the problem starts when i am going at low speed and taking a sharp, left or a right turn. and the whole care shacks, but when i am going strate every thing is fine. i also did a test by putting my subaru in to 2WD and had no issue when i took a sharp turn, every thing went back to normal. but puting 4wd the problem comes back, pleas HELP.

    • Martin Hand Martin Hand says:

      Sounds like the same problem that’s described in this article. Are you near Portland Oregon? I would need you to bring it bye so I can check it out. Check for transmission diagnostic trouble codes.

  • Rick Gargano says:

    I have a 2006 Subaru Forester manual transmission . I was driving it on the freeway and now it will engage in all gear but it won’t go. Can you tell me what this could be?

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