What it Does
Transmission fluid fulfills multiple roles for the transmission. It lubricates, carries hydraulic pressure, affects friction characteristics as components grab and let go of each other, 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 or 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.
Loss of Friction Modifier Performance:
Friction modifiers in transmission fluid affect the interaction of grab and release components inside the transmission. This includes, for example, the clutch packs that are applied and released as a transmission shifts gears. This also includes the torque converter lock-up clutch. Friction modifier break-down may lead to reduction in shift quality, wear of transmission friction materials, and torque converter clutch shudder.
Failed Clutch Packs:
Clutch packs are used to shift automatic transmission gears. They are designed to both apply and release and contain friction elements accordingly. Degraded transmission fluid risks premature wear of these clutch packs. If a clutch pack wears out, the transmission will lose its ability to shift properly into one or more gears.
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 a 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 and 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.