Quote:
Originally Posted by e30cabrio
I am interested in the argument against replacing old fluids that no longer provide protection.
Can someone please explain how it is better to leave it in than possibly dislodge unknown contaminants that may or may not be in the system.
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Sure. Transmission fluid is designed to last a long time. It is in a sealed system, so it doesn't get contaminated by things like engine oil does. It has additives that fight oxidation, and foaming. There are also friction modifiers. Over time those additives become depleted. The fluid does wear out over a long enough period of time. It doesn't stop lubricating, but it does lose some specific properties. Recall that lubrication is the lowest requirement of a transmission fluid. It is a very undemanding lubrication requirement. That is why it is a straight 10w oil. It is primarily a hydraulic fluid. The question is, does it wear out before the transmission expires of other, unrelated causes? Most failures we are seeing are related to things not related to lubrication (wiring harnesses, sensors, etc). Those failures won't be impacted by a fluid change. So, there is a big question as to whether changing the fluid will have any effect on average transmission life.
Now, what happens when a transmission wears (naturally) over time? There are sediments created by clutch plate material, generally non-metallic, and they sit in the pan on the dirty side of the filter. Not the clean side. They look bad, but they don't hurt anything. They are also distributed throughout the transmission. There, again, they usually don't hurt anything. Now put in new fluid. Very high detergent levels, by design, that is what makes it last so long. Those detergents clean out the transmission. Same reason we use ATF in an engine to clean out combustion deposits. What happens when those deposits are loosened? They go through the transmission. They end up in the valve body. If they make it to the pan, no problem. If they get stuck in the valve body, in a sensor, in an actuator, etc, they can cause an early transmission failure. It happens. Not always, but often enough to show up in the statistics.
So the debate is, is the risk of damage (lets call it x) greater than y (the benefit of clean fluid, which is generally a good thing). If you believe that clean fluid is such a benefit that it overshadows the risk of failure through introducing new fluid, then change it. If you acknowledge the risk (small on a total population basis, but real) and don't see a history of failures due to aged fluid then you would not change it, save the money, and put it aside for an eventual transmission rebuilt not caused by a fluid issue.
There isn't a right answer. The only error, IMO, is not acknowledging that there is some risk, and that there is no demonstrated benefit. This isn't to say that all preventative maintenance is bad. It is usually good. But where there isn't a correlation between PM activities and extended component life, then it is better and cheaper to run it to failure. That is counterintuitive to some.
Hope that helps.