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.
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.
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.
- remove exhaust
- remove heat shield then driveline
- remove crossmember and lower transmission down
- drain transmission
- 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.