Xoutpost.com - View Single Post - ATF change, ATF lab report, ESSO no longer recommended by BMW or GM

JCL · #4 02-27-2009, 11:03 AM

Good post, and thanks for the information on the lab report.

A couple of comments from my experience with oil and hydraulic fluid sampling:

The lab report, while interesting, becomes much more valuable if you can plot a trend line from multiple samples. Isolated numbers, by themselves, don't tell very much. For the total acid number, you could take a sample of the original factory oil (from the dealer) and see what the change has been. For the other items listed, they weren't zero in new fluid and it helps to see when they climb, when they stabilize, and when they spike. You want to change it before they spike, essentially. If the aluminum got to 80 during break-in, for example, being at 90 now isn't an issue. If it was 5 for the entire life of the transmission and went to 90 last month, then now is exactly the time to change the fluid.

Did you do any large particle analysis? The dissolved solids info is fine, but I would look for a particle count as a key indicator.

My other comment is about the comparison with the GM transmission. I have no doubt that the mechanicals of the transmission are the same as the GM product (or ZF, for the ZF-built transmissions). What BMW does do is add their own control package, and electronics. Those electronics and sensors are programmed to provide different shift points, and different shift characteristics than the GM electronics (which is why BMW puts them in there). The way in which a transmission shifts has a large effect on the wear and heat resulting from a shift. Essentially, a firmer shift reduces the time the clutch packs are slipping, reduces heat, and so on. All of that extends fluid life. A soft, slow, shift that slips the clutche plates more results in faster wear of the clutch plate material and disks, and much more heat (which breaks down the fluid much more quickly). While the transmission durability is likely similar between the GM and BMW versions, the above means that not everything can be extrapolated. That doesn't mean that you shouldn't change your fluid if you want to, but it means that the GM recommendation for 100,000 is simply one more data point, and not an absolute truth.

I was very glad to see that after you changed the fluid, you did a second change. If you are going to change it, it makes sense to get more of the old fluid out and if you got 50% of the old fluid out each time, after two changes you are at a 75% change, considering the amount of fluid that stays in the transmission/torque converter when you drain the pan.

Thanks again for your post.

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