Subaru CVT Fluid Case Study | 30,000 Miles

CVT Fluid Comparison

I remember back in 2010 when Subaru started using the 1st Generation CVT in the Legacy and Outback. Of course, we mourned the passing of the 4AT, but hey fuel economy is more important right? All of us Subaru technicians were in for a surprise though, somehow there was a disconnect between the sales department explaining how the CVT's operate. We had so many people coming in to service with complaints of "transmission slipping in 1st gear". We would perform tests and drive the vehicle, only to tell the customers this is normal. 

Transfer Clutch

Once we started seeing actual issues arise, they surfaced as noises, electrical faults, and stalling. A vast majority of the CVT problems are covered in either a recall or TSB, so the odds are good the repair will be covered under warranty.  There isn't much you can do to prevent the electrical issues, since they all occur inside the valve-body solenoids. However, you can do your part in fluid and software maintenance...

Secondary Pulley

Over the course of my day as a Subaru technician I would take CVT fluid samples from vehicles at different mileage intervals that had a CVT issue, and after several months I had a collection of fluid samples ranging from 11,000  to 100,000 miles. What I noticed is that the fluid stays very clean all the way up to 25,000, then you can start seeing the change. Not only does the fluid start changing to a brown color, but you can see a lot of suspended material moving around. I don't know for sure what the debris is, but we can assume is wear from the input/reverse/AWD clutches.

CVT Fluid Sample Comparison

New fluid on the left, 30k fluid on the right...

​I change my CVT fluid every 30,000 miles, because its right there where the fluid starts looking bad. Also, knowing that the valve-body can become clogged with that debris makes even more sense to just stay on top of it. The other very important element I would suggest is that you pay attention to your shift lag time. When you shift from reverse to drive, does it take a long time to actually move the vehicle? If so then you need to have the clutches re-learned, this is where the wear on the clutch packs is compensated for. If you neglect this issue it can slip the clutches every time you take off before feeling the CVT engage drive or reverse. 

Input Clutch

The final element to CVT maintenance is the Differentials, well technically only the front differential is part of the CVT but you know what I'm saying. It's super rare for a differential to fail, but you should still change the fluid every 30k as well. Gears meshing together always creates some wear metals in the fluid, so you can't just never service them. If you don't believe me take a look at the fluid in a new car, there will be "some" metal debris and fluid discoloration straight out of the factory. This is because the gears create a matching pattern as they break-in, it's normal. Just like with a new engine, new driveline components can benefit from a break-in oil change. Take a look at the fluid from my Forester  below:

Differential Fluid Comparison

That's the new gear oil at the top right, 30k front diff oil at the top left, and 30k rear diff oil at the bottom left. I hope this write-up has helped you understand how to take care of your Subaru CVT and differential better, if you have any feedback just leave a comment below. Take a look at the short time-lapse video I made on this transmission: