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. In some cases, fluid replacement intervals are provided. In other cases, it’s claimed that the transmission fluid is good for the lifetime of the vehicle. It may also be recommended to inspect the fluid periodically.
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
(Please note that this entry deals with the practicalities of deciding when or when not to replace transmission fluid. It is not mainly focused on the damage that failed or failing transmission fluid can cause or allow.)
Recommendations 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.
“Filled for life” claims:
What is the lifespan of a vehicle? Is it the duration of the powertrain warranty? How about 100,000 miles? Or 150,000. How about 300,000? How do manufacturers define a vehicle’s “life,” and how might that correlate to the timeframe in which they would gladly sell you another vehicle?
Reading between the lines:
Sometimes the recommendation will be to leave the fluid alone under “normal” usage but to change it periodically if the vehicle is used for towing. Let’s say the recommended interval for towing is 60k miles. It would make sense then that fluid will last longer if it’s not being used for towing. However, the recommendation to replace under towing conditions does indicate that the fluid is not “perfect” or “indestructible.” So what if we assume that towing is “twice as hard” on the fluid and we double that 60k interval for lighter usage?This would yield a 120k interval.
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 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.