What it Does
Transmission fluid fulfills multiple roles for the transmission. It lubricates, carries hydraulic pressure, affects friction characteristics, and cools the transmission.
Why This Matters
Transmission fluid condition can impact a transmission’s longevity. If degraded or failed fluid causes, fails to prevent, or fails to discourage internal transmission failure, repair costs can be severe. (Even if the failed component is small and low cost, the work required to remove open, repair, and reinstall the transmission can require many hours.) Guidance for replacement intervals is often lacking, and “filled for life” claims do not address the difference between a vehicle that gets traded in at 100,000 miles versus a vehicle with a desired 200,000 (or 300,000, or even 400,000) mile service life. Outside of lab analysis, recommendations to inspect are not much help.
Possible Problems and Concerns
Loss of Lubricity:
Lubricity refers to the ability of lubricant to prevent surfaces from wearing against each other. Loss of transmission fluid lubricity can allow internal transmission bearing failure. Bearing replacement can require both opening up of the transmission and removal of metallic debris that is introduced into the transmission via the bearing failure.
(A Note on Transmission Operation:)
Since continuously variable transmissions don’t shift gears, thay have to be able to achieve gear ratio changes while still maintaining power transmission. Imagine if you will, holding a strong rubber bungee cord around an agressively tapered cone and trying to work the cord up the cone while holding it tight. You can probably imagine that this will be easier if the cord and cone are lubricated. Now imagine that you have to do this while pulling that cone against the weight of the vehicle! (Or imagine the forces involved with thousands of pounds of vehicle moving at 50mph! The transmission is what transfers engine power to make the vehicle move. it works hard! The nature of CVT transmissions makes it critical to use fluid with excellent lubricity and friction modifier characteristics.
Loss of Friction Modifier Performance:
Friction modifiers are important for automatic transmissions but probably even more so for these CVT’s. Friction modifier break-down in a CVT transmission can result in high internal temperatures, component wear resulting in debris loading, and transmission failure.
Torque Converter Shudder:
Torque converters are transmission components that include a lock-up clutch.This is a lubricated clutch assembly. Degraded transmission fluid may result in a shudder as this clutch repeatedly grabs and fails to hold. Too much of this behavior could result in a failed clutch as well as clutch material contaminant in the transmission fluid. This contaminant could have an abrasive affect on internal transmission components.
Mis-Diagnosis:
Faulty transmission operation due to degraded fluid may be mis-diagnosed as a mechanical fault in the transmission. With that said, many transmission operational faults do result from internal wear and will not be remedied by fluid replacement.
False Hope:
When transmission operation deteriorates, people sometimes seek out a fluid service in hopes that it resolves the symptom(s). There are cases in which in which it will, and torque converter shudder is a good example of this. However, in many cases, the transmission is acting up because it has an internal mechanical problem that no amount of fluid replacement will correct. Nor will torque converter shudders always be corrected by transmission fluid replacement.
Recommendation to Inspect:
A visual inspection of transmission fluid does not reveal much information. Color is often not a reliable indicator of condition. A visual check does not communicate the lubricity of the fluid nor whether the fluid’s friction modifiers have broken down. It will tell you if the fluid level is low. Other than that, if the fluid is clearly burnt, contaminated, or carrying debris the transmission may already be on a path to complete failure (inability to reliably move the vehicle) or substantial failure (it moves the vehicle, but it does not shift, engage, or hold correctly.) Substantial failure may be followed by complete failure.
What is Fluid Replacement?
Most transmissions will not give up all of their fluid when they are drained, and many won’t even give up most of it. It’s common to see 30-60% of the transmission fluid drain each time. Thus, the decisions here do not simply concern whether it’s worth it to do a low cost drain & fill. It may be necessary to go through multiple steps to achieve a “complete” (we like 90% or better) fluid replacement. The cost involved here makes an intelligent approach and good decision-making even more important.
Recommended Parts Brands
The only transmission fluid that should be used is fluid that meets the original required specifications, or has been tested and confirmed to be an acceptable (or better) replacement. GM’s transition from Dexron III to Dexron VI is an example of this. Usage of the wrong fluid can damage transmissions, accelerate transmission failure, or inhibit proper driveability and smoothness of transmission operation.
A Recommendation
In the absence of manufacturer guidance for replacement intervals, we tend to advise replacement of transmission fluid every 100k miles or so. This can vary depending on the vehicle, its history, and originally-equipped fluid type.