Quote:
Originally Posted by CHE53
I hadn't realized that the detergents were even a factor in all of this. But that definitely does make sense.
I've done some more reading on the subject, and it appears that often times transmission fluid lubricity isn't incredibly affected by time (like you said), as it is heat. If the transmission was ever run at excessive temperature the ATF will degrade at an exponential rate, because it is outside of the specific ATF design parameters.
-With degraded ATF comes slipping/more overheating (IIRC)
-Also, a possible sign of overheat would be discoloration of the ATF (though other things may cause this too? May not be a dependable method..)
I definitely agree with you though.. There isn't one correct answer, with the exception of a known-overheated transmission, it seems like a roll of the dice either way.
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By design, transmission fluids are meant to last a long time, so they have a high level of detergents. The level of those detergents drops over time, naturally. People use phrases like "shock to the system" and "particles sticking" when what is really happening is that putting in those detergents cleans it out, moves all the (normal) deposits, and that is what causes the post-fluid-change problems, as there isn't anywhere for them to go, they end up blocking tiny passages and check valves in the valve body, and then you have a shifting problem. You can do several changes, and effectively flush this all out, but you are hoping that in the meantime, nothing affects the valve body.
The additives are also there to provide thermal stability. If the fluid is overheated (abuse, slipping clutches, etc) then it will break down, and the additives won't function as designed. Overheated fluid changes colour, and smells burnt. If the fluid isn't burnt, I would say the chances are the additives are still good. There are transmissions that have run hundreds of thousands of miles on the original fluid, so it isn't like the fluid has a sell-by date. It can last fine, as long as it isn't overheated.
There are also additives in there to modify the friction characteristics of the fluid, right at the point of clutch engagement. Those are what are different from fluid spec to fluid spec (Dexron, Mercon, ZF Lifeguard 5, Lifeguard 6, etc). All the fluids are essentially a 10w hydraulic oil, with different additives. The clutches in any particular transmission design expect a certain fluid frictional characteristic and are tuned to that spec. Using a different fluid is a common way to fail a transmission, if they slip too much (now you get overheating, see above). If they don't slip enough, you get grabbing, and things break. Equally bad. There is no way to test the fluid for those additives remaining, this isn't like a viscosity test. If the transmission is shifting well, that is a good indication of the fluid being in spec in this respect. If it isn't shifting well, then you can go ahead and replace the fluid and see if that helps. It may. It may not. Depends on the actual problem. But it is a cheap first attempt before pulling a transmission. Back to the fluid change philosophy, though: if it is shifting fine, then these characteristics of the fluid are likely fine.
Then there is the viscosity of the fluid. It doesn't much matter, IMO, not like engine oil. Dexron III went to Dexron VI, with a signficant change in viscosity, and was backwards compatible. The transmissions that use that fluid spec don't care much. But, older fluid tends to be thicker (due to worn clutch material, which is non-metallic, but a thickening agent in practice). Putting in new (thinner) fluid, although returning to the right viscosity spec, means that things that didn't leak pressure (internal seals, not gaskets to the outside) may not hold. That can be another failure mode with a fluid change. No guarantee that it will happen, but it has happened before.
Finally, there is lubrication. This is the reason that many claim they want to change their fluid, to make sure everything is well lubricated. That sounds good in theory, but the first thing to remember is that oil (of which transmission fluid is a type) never loses its slipperyness. It gets contaminated, but it never stops being slippery. Even when it breaks down due to heat. The thing to remember is that transmission fluid has several functions. It is a cooling medium, it carries heat to the heat exchanger (and the other way, during warm up). It is a hydraulic fluid that can not be compressible, ie it has to transmit pressure from actuators, turbine blades in the torque converter, etc. And finally, it is a lubricant. Of all of those purposes, the lubricating is actually the least important. Transmissions fail due to heat. They function due to hydraulic actuators and pressures. But as to lubrication, that is the lowest demand put on a transmission fluid. That is why viscosity doesn't matter much. It is why a straight 10w oil works fine. It just doesn't see the same bearing loads, shear, etc, that engine bearings see. Yet lubrication properties are the thing fluid manufacturers talk about. They use the word synthetic as if it matters at all. It just doesn't. Stability over time does. Additives do. Resistance to oxidation does. But how slippery it is is way down the list. That is why a transmission can run 200,000 miles on the same fluid, and why comparisons to engine oil ("no oil is lifetime") are not as relevant when discussing transmission fluid.
Probably more than you wanted to know about transmission fluid. Again, there is no one answer as to whether to change it or not. If changed regularly, ie every 30,000 miles or so, there is low risk of a fluid change causing a problem. If changed at higher mileage, or after many years (regardless of mileage) then there is more risk of a fluid change causing a problem. And in all of those examples, at whatever mileage, you have to decide whether the benefit of new fluid is high enough to warrant the cost of a change, or if you do it right, multiple changes. Analogies to coolant, brake fluid, engine oil, etc, all of which benefit from regular changes, are not as relevant here.
If you do change it, only use an OE BMW filter (or a ZF part number), never a generic one. And only use ZF fluid in your case, or a fluid that is certified to meet the ZF standard, not one that says it is "suitable for applications that call for ..." and definitely not a univeral fluid that claims to be good for all transmissions, things like D4. They may work fine. But the odds are lower. And with transmission fluid changes, it is all about playing the odds.