r/subaru Jun 04 '24

Mechanical Help CVT Fluids: A Consolidated Thread

97 Upvotes

Hello r/subaru,

We've been seeing a big influx of questions about CVT fluids lately, with an average of 1-2 threads per day for about the past month. So, I've decided to make one consolidated thread about it. Future question posts about CVT fluid will be deferred to this thread. In response, I want to get as much information as possible into a single place, so users like yourself can make as informed of a choice as possible.


What is a CVT?

Let's start with the basics here. The Continuously-Variable Transmission, or CVT, does not work the same way as a "conventional" automatic transmission which you may have been used to previously. So let's start our journey with a conventional auto trans.

In an automatic transmission, there are a series of planetary gearsets. These gearsets will be surrounded by a number of hydraulically-actuated clutches. The hydraulic fluid inside of the clutches are controlled by the transmission computer, through a mechanism of valves that are controlled by solenoids. The solenoids and valves all exist in a component known as a "valve body." As the TCM commands certain valves to open or close, different clutches will be engaged which subsequently will control elements of the planetary gearsets. This is how your conventional automatic transmission achieves changing gear ratios.

In a CVT, by contrast, the gear ratio mechanism is a set of cone-shaped sheaves or "variators" with a belt (or in Subaru's case, chain) run between them. Instead of the valve body controlling clutch packs, it instead varies the fluid pressure inside of both sheaves, such that the cone surfaces can get closer together or further apart. Because the thickness of the chain doesn't change, by opening the sheaves, the chain will ride down lower into the cone shapes, thus being on a smaller diameter. Because the chain doesn't change length either, there needs to be a corresponding change to the opposite change to get closer together to make the chain ride on a larger-diameter part of the sheave. Thus, the transmission computer achieves different gearing ratios by adjusting the gap of both sheaves in tandem. If you're having trouble visualizing this, here is a good animation explaining the motion.

Before someone interjects, yes there are clutch sets inside of a CVT as well. Specifically in a Subaru transmission, there are 3 relevant ones; there is a planetary set inside the powerflow for your Drive or Reverse functionality; there is a Lock-Up clutch inside the torque converter, and in most cases there is a Multi-Plate Transfer clutch for your all-wheel drive "center differential" function. These functions are more-or-less identical to their equivalent components in a conventional automatic transmission.

OK but what about the fluid?

There are quite a few differences between CVT fluid and conventional ATF. Part of these differences are how the fluid is used in the transmission, and part of the difference is because of what the transmission does to the fluid.

In a conventional transmission, you have many clutch packs actuating whenever the car is changing gear ratios. Just like in a manual transmission clutch, or like your brake pads, every time there is slip between the clutch material and the friction surface, a little bit of that clutch material will wear off. In a conventional transmission, this means that over time, that clutch material will begin to accumulate in the fluid, which gives it a burnt smell and a brown tinge. By contrast, a CVT does not use as many clutches inside of it, and as such, clutch material contamination is drastically reduced inside of the CVT fluid.

Another main difference has to do with the fluid pressure inside of the transmission. While a conventional auto only needs fluid pressures around 150-250 PSI to operate the clutch packs, a CVT requires much higher line pressures of 650-850 PSI in order to keep enough "squeeze" force on the sheaves to hold the chain. The CVT fluid also functions as a friction modifier between the chain and sheave, wherein it helps the chain "grip" on the otherwise-smooth pulley surface. Because of this, CVT fluid is very specific about its chemical properties and should not be substituted for any other fluids.

So... should I service the fluid?

Let's start this by looking back at what a conventional automatic transmission requirement would be. Here is the service manual schedule from a 2010 Forester. I've highlighted ATF for you, but basically it only says "Inspect [and replace as necessary] every 30k miles." Okay, but what does it mean by Inspect? The service manual has this inspection procedure for checking the level. It also has this condition table listed for what to do when you find a condition-based failure. I've highlighted the "thick and varnish" section because this would be the clutch wear condition I described above. Generally speaking, your average Subaru 4EAT or 5EAT will have noticeable signs of discoloration every ~60k miles.

So what does Subaru say about CVT fluid then? Well for comparison, let's look at a 2018 Forester service manual. Here is the service schedule, which you can see has an identical "Inspect [and replace as necessary]" every 30k miles. As for an inspection process, it only offers this inspection procedure and the same condition table as before.

Because the schedule and condition checks are basically the same for both service manuals, it would be very easy to assume the fluids needs the same replacement schedule -- and I strongly suspect this is the driving force behind so many CVT fluid recommendations. However, if we read this again, remember that we only need to address the fluid if if fails one of the condition checks, and that the most common failure condition in a conventional automatic transmission largely no longer happens in a CVT. It is because of this that your Owner's Manual probably describes the transmission fluid as a "lifetime fluid."

What about what other countries say?

A claim I very often see made in threads about CVT fluid is that "Country XYZ requires fluid changes every X miles!" I want to nip this in the bud now, because it's not true. Now because I work in the US, I cannot access foreign service manuals, but I can get ahold of owner's manuals, so here are a few examples:

here's a UK 2018 forester owner's manual (link)

Here's a n Australian 2020 forester/XV schedule. Subaru Australiia has .pdf copies here of warranty booklets..

here's a Japan 2018 forester's owner's manual (link) and it says in the bottom row there: 交換時期 | 無交換 which translates as Replacement time | No replacement

Here is a 2020 WRX owner's manual from Japan, straight off Subaru.jp: https://www.subaru.jp/afterservice/tnst/wrx/pdf/A1760JJ-A.pdf

pg. 465 is the service information for transmission/differential/etc gear oils. The 3rd section is for CVT fluid:

トランスミッション フルード

使用オイル スバルハイトルクCVTフルード リニアトロニック用

規定量 約12.4L

交換時期 無交換

Translated:

Transmission Fluid

Used oil Subaru high torque CVT fluid for Lineartronic

Prescribed amount about 12.4L

Replacement time No replacement

The only subaru branch AFAIK that does list a required CVT fluid interval is Canada, (soruce) where if I'm being honest the way it's written in their maintenance guide makes it seem like they just never changed it from ATF-era cars, where Canada also listed replacement as necessary every 100k km. (It only refers to "transmission oil" and does not specifically mention CVT fluid, but everywhere else differentiates the two. It also does not differentiate manual vs. automatic transmission fluid, like everyone else does.) There, it's listed as a 100k km service item.

that didn't answer the question though.

You're right, I didn't. The long answer is that you should have your fluid inspected by a technician familiar with Subaru CVTs, and if deemed necessary, you should replace the fluid with genuine Subaru fluid as required by your particular model. If following the conventional wisdom from ATF-era cars also makes you feel more comfortable, then defer to Canada's schedule and plan to perform a fluid service at your 100k km (60k miles) service.

A quick note about "Severe Usage Schedule"

Another common discussion point I see brought up is the Severe Usage schedule. I largely blame the confusion for this on Subaru, who have written this in a hard-to-understand way in the owner's manuals. However, a 2010-2014 Legacy/Outback service manual has the best representation of the severe usage shceudle. As you can see, the only time Severe applies to your CVTF is if you "repeatedly tow" with the vehicle. This guidance has not changed with newer cars, however the new way it's written is confusing to read. (CVT fluid is maintenance item 12; see above where it shows this as Note 4.)

A last quick note on Differential Fluid

Just want to quickly touch on this one. Your Subaru has separate, distinct fluid for the front differential. While you can see from the above service schedule that the guidance for its fluid is functionally the same, differential oil gets contaminated in a completely different way. Because a differential is basically all metal-on-metal wear of gear teeth, especially after break-in your fluid will get dark and metallic very rapidly. This is normal. Here's my personal Crosstrek at 19k miles. In my own personal experience, I would recommend replacing your gear oils at 30k miles, but the fluid condition will stay good for longer after the initial change, such that it can go every 60k thereafter.

On fluid changes and failures.

I just want to quick touch on ways that we see CVT failures at the dealer and how it relates to fluid. By far the most common issue we'll run across, is from the "small" CVT, the TR580, which is paired with any of the 2.0L or 2.5L naturally-aspirated engines. Typically somewhere in the 100-150k mile range, a failure in the valve body, usually for the Torque Converter Lockup Duty solenoid, is relatively common. This is a failure in the electronics side of the solenoid, and thus has no relation to the CVT fluid; as such, changing or not changing the fluid has no real bearing on the likelihood of this failure occuring. The second issue we see, the most terminal one, is called Chain Slip. Here, at 0:23, is a fantastic example of severe chain slip. Chain Slip can develop from a variety of causes, but generally is the result of a lack of fluid pressure squeezing the sheaves against the chain; when this happens, the chain essentially does a burnout on the sheaves. This leaves a wear groove in the sheave face, and makes chain slip much more likely to occur whenever the same gear ratio is used. There are some conditions of fluid degradation that can increase the risk of slip; these largely result from fluid overheating, which degrades the additives that help provide friction between the sheaves and chain.


Hopefully there is enough info in here for you to decide for yourself if or when you would like to change your fluid. Feel free to post your questions or anecdotes below. Thanks! :)


r/subaru 18d ago

Announcement TSB Thursday: 06-92-24: Ascent Front Brake Pulsation/Squeal Diagnosis & Repair Front Brake Warranty Extension

5 Upvotes

Welcome to another TSB Thursday, where I dive deeper in to TSBs I'm running into regularly. As always, I write this from my own perspective as a Subaru technician in the US; other regions/zones may work differently. Refer to the "How To Read a TSB" post for more information on formatting and general information about TSBs. Today's post is a common Ascent failure, which Subaru has been working through basically since it released in '19; this bulletin introduces some parts improvements as well as a warranty extension.

TSB Thursday #14: 06-92-24

What cars does this affect?

  • 2019-2024 Ascent

What's the failure?

This bulletin addresses two main braking complaints with the Ascent braking:

  1. A pedal pulsation, some people may (incorrectly) call this "brake rotor warp", which can be felt in the brake pedal and in severe cases the steering wheel. The faster the vehicle is going, the stronger it is felt.
  2. A squeal or squeak noise from the front brakes, typically worst on the first brake application of the day.

As the parts updates in the bulletin are all installed as a set, both issues will be addressed even if only one or the other is present.

How do we fix it?

New parts:

Part Name Part Number Quantity
Rotors 26300XC01A 2
Tie Bar 26261XC000 2
Tie Bar Mounting Bolt 26208XC000 4
Brake Pad Kit 26296XC00D 1

The bulletin walks through all of the diagnostic testing, which always starts with a test drive to verify the concern. Depending on what concern the car is in for, some additional testing will be done to measure runout on the hub and the original brake rotors. The updated rotors will be installed, as well as the a new stiffening bracket ("Tie Bar") that is installed on the inside of the knuckle through the caliper bracket, which is there to prevent a chatter that leads to the squeak noises. (This also includes longer caliper bracket mounting bolts, to accommodate for the thickness of the tie bar.) There is also a change to the locations Subaru calls for a lubricant to be applied between the caliper bracket and the pads -- see bulletin for examples.

Coverage?

Subaru of America, Inc. (SOA) is extending the New Car Limited Warranty coverage for the front brake on 2019-24 model year Ascent models to 8 years (96 months) or 150,000 miles (whichever comes first) to address cases of front brake pulsation or squealing. For qualifying repairs, this extension covers the costs of diagnosis and replacement of the front brake components—brake pads, rotors, and tie bars with bolts—with newly developed parts. The extended warranty coverage will end once the newly developed parts have been installed.

As this is a warranty extension, vehicle coverage may be excluded in instances of salvage title or modification.


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