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.

638 Comments

  • Jake Ure says:

    I am looking at a 2002 Subaru outback sport. its in really nice shape and runs and shifts its a manual. But it doesnt move, all four wheels wont spin. I want to buy it but am curious on what the problem may be. Please email me if you can help me at all. Thankyou!

    • Martin Hand Martin Hand says:

      It could be a broken cv axle but most likely it’s a broken or burned up clutch. You will have to remove the transmission to find out for sure, just check the obvious first before you remove it.

  • Khan Powell says:

    Hello. My name is Khan Powell and I am an owner of a 2002 Forester with an automatic transmission. It is working great. I am looking at buying a second Forester w/AT (1998) but the rear housing for the transfer case section was broken from the inside out, I have been told. The current owner discarded the broken housing and I am wondering what I would be getting into if I buy the car. Are the housings available and affordable? I would probably want to go ahead and replace the drum and solenoid. My buddy has a parts car which is a 1999 Legacy. VIN: 4S3BD6851X7253703. Numbers on trans are as follows: TZ1A2ZJEBA-WZ. Please advise how to proceed.
    Thanks!

  • Joshua S Klingbeil says:

    Hi, I recently replaced the transfer duty solenoid assembly in my 2001 Forester. I now have no binding in turns, but also no rwd whatsoever. The car spins the front tires on the pavement under heavy throttle. The tcu code light is not blinking and it is not storing any trouble codes. The clutch pack looked fine during disassembly and the rwd was solidly stuck on when the old solenoid was in the car, indicating that the clutch is holding fine. Any ideas?

    • Martin Hand Martin Hand says:

      I have seen the drum crack or damaged sealing rings cause no rear wheel engagement, loss of oil pressure. Also make sure the drum is not completely broken and spinning on the output shaft.

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