Quote:
Originally Posted by HPIA4v2
To me, looking at all fluids colors from the dirtiest/nastiest compare to virgin fluids (I did all these fluids at least twice so this is not based on lab tests but just looking at color)
1) trans fluid
2) PS
3) xfer case
4) brake
5) diff
6) coolant
I am not arguing you should change the trans but if you do the last 5 and think the first one is a waste, I need to hear some good reasoning.
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Colour doesn't mean anything, unless it is an oil and is milky white (indicating water contamination). Lubricating fluids are designed to hold contaminants in suspension, so they get dirty looking by design. That doesn't mean they are worn out.
Your #4 should be done every two years, because it is a hygroscopic fluid. It gets contaminated with water, which can boil (soft pedal) and cause corrosion in the brake system ($$).
Your #6 can be checked for the level of conditioners it still has in it, but it is just as easy to replace it every four years or so.
Your #s 2, 3, 5 can be changed as you like, as there is no downside (other than cost). Clean fluid is generally a better thing. The fluids are unlikely to be worn out at 100,000 or whenever you do them, but since we all here tend to like cars and like working on them, we feel good when we change them. And since there is no downside, even a slight benefit is a good thing.
#1 is the only one on the list that has a possible downside to changing it. It isn't designed to be changed, so you can't get more than 40% - 50% of it out. If you change it several times in quick succession, you can get most of it changed, but that drives up the cost. It is a sealed system, so it doesn't get contaminated. The fluid gets thicker over time, and the transmission adapts to that (by monitoring clutch engagement). When you change it you introduce a new fluid that has a lot of detergent in it (by design). That new fluid is thinner, and the detergents tend to flush out the transmission. If there is any crud in there (and there will be) it can create a blockage in one of the small passageways or check valves in the valve body. It won't necessarily happen, but it is a risk. If there was a clear or measurable benefit to changing the fluid, then it would be worth evaluating the benefit over the risk. But since there is no measurable benefit other than the feel good factor, and there is a small but real risk of doing damage by changing it, many of us leave it alone. The way to avoid the risk of disturbing sediment is to decide that if you are going to change it at all, to change it early and often. If you went every 30,000 miles, for example, there would be very little risk of disturbing sediment. But it would cost a lot. And when you got to 150,000 miles and had done 5 changes ($$) 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.
For those that view this as a cost-saving issue, I see their point. But I would also comment that BMW and ZF service instructions say that if as part of a repair in a dealer shop you have to drop the transmission pan and thus drain the fluid, to reuse it. That actually costs them quite a bit, given the cost of labour. Why do they do it? Because the fluid that is worn in is actually better for the transmission than new fluid (as long as it is not burnt). They avoid the risk of fluid changes caused by adding high detergent fluids.
I listed all six points because I don't see them as comparable. Your transmission fluid has nothing in common with your brake fluid. And the failure mode of brake fluid is well understood. That is why it is a 2 year change cycle. It is the most cost-effective way to maintain your brake system. Just ask all those who had to rebuild calipers early.
That is the short version of why many of us who believe strongly in preventative maintenance, and who have worked with vehicles for years, won't change the transmission fluid in our BMW automatics. And there are others who believe the contrary position. That's fine, but since you asked, there are my own reasons.