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Originally Posted by JCL
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#1 is the only one on the list that has a possible downside to changing it. It isn't designed to be changed, so you can't get more than 40% - 50% of it out. If you change it several times in quick succession, you can get most of it changed, but that drives up the cost. It is a sealed system, so it doesn't get contaminated. The fluid gets thicker over time, and the transmission adapts to that (by monitoring clutch engagement). When you change it you introduce a new fluid that has a lot of detergent in it (by design). That new fluid is thinner, and the detergents tend to flush out the transmission. If there is any crud in there (and there will be) it can create a blockage in one of the small passageways or check valves in the valve body. It won't necessarily happen, but it is a risk. If there was a clear or measurable benefit to changing the fluid, then it would be worth evaluating the benefit over the risk. But since there is no measurable benefit other than the feel good factor, and there is a small but real risk of doing damage by changing it, many of us leave it alone. The way to avoid the risk of disturbing sediment is to decide that if you are going to change it at all, to change it early and often. If you went every 30,000 miles, for example, there would be very little risk of disturbing sediment. But it would cost a lot. And when you got to 150,000 miles and had done 5 changes ($$) you wouldn't necessarily expect to get any more life than if you had never touched it. Look at all the stories of failed sensors, wiring, solenoids, and mechanical parts that are not impacted by lubrication quality (like snap rings, or the common torque converter failures). Changing the fluid isn't going to buy you a pass on those issues. You may just get a failed transmission that you have spent $$ changing the fluid on. There are documented cases of transmissions that shifted fine, no troubles at all, then got a fluid change with the correct fluid, and failed soon after. I have experienced it myself. It isn't a myth, it can happen, including with the BMW transmissions. Some cases are documented on this board. Transmission shops clearly understand this failure mode.
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I never heard of that highlighted in red above. Hmmm, sounds like the original fluid gets better with age for that trans... hmmm. I'd like to see that rational in a BMW letterheaded service memo or BMW shop manual or the transmission builder offers this rational somewhere at BMW University and can be verified. Until I see that Then what you say is an opinion
My transmission is not a ZF, it is an XE GM 5L40-E. They only say,as far as I have seen, that you shouldn't and you should rescue it and replace the old fluid during a repair... They don't say why and they never will.
I don't know what GM is telling their new customers who buy chevys etc. with these transmissions but, I know Ford is still suggesting 30,000 mile changes at least in my '05 explorer.
I clearly don't understand, If particulates are held in suspension how can sludge build up? If particulates are held in suspension then why am I not flushing them out when I drain and fill? If particulates are held in suspension then abrasives are constantly wearing on your mating surfaces with never a break, how can that be better than new fluid?
100,000 miles is the life of the transmission after it has been prematurely chewed up by dirty, micro gritty atf,that is left in by the Motor Werks. They like... no love... to take your 6,000 clams when
THEIR dirty atf has ruiend the transmission before its time. when atf changes could have prolonged trans failure and the extraction of said $6000.
If the above old fluid theory was true then ... I don't know do I?
PS I have the next 33 bottles of atf ageing on a shelf in my garage... I rotate it and turn it every month... Orson Wells checks in occasionly to see what's up.
