Thoughts on Changing Transmission Fluid

[Brandon002]

Im not sure, but they guaranteed they changed all of it.

[BMW_E53]

Interesting. Mine has 80k miles and when I looked at the pan it states that fluid is lifetime. Also awhile back a BMW service adviser told me that they'll refill the transmission if it was low at 100k miles.

[Brandon002]

Do you honestly believe any fluid in a car can be lifetime?

[Whitecat]

Originally Posted by Brandon002

Im not sure, but they guaranteed they changed all of it.

Ok, bcause they (transmission shop) changed 16 liters on my 4.4 with torque convertor and cost associated (similar than yours) but with Wynn's oil ..versus a lot of folks here doing DIY with 5 or 6 liters ?? ...my tranny works very good since the changed at 70k miles....here is more from that thread :
http://www.xoutpost.com/bmw-sav-foru...2-4-4i-21.html

[JCL]

Originally Posted by Brandon002

Do you honestly believe any fluid in a car can be lifetime?

Yes. Lifetime of the transmission. Which will likely experience an eventual failure not prompted or impacted by the state of the transmission fluid.

[JCL]

Originally Posted by pezho405

Lifetime .... of the gearbox..... but if you want it to last a lifetime of the car, you gotta change it

That is only true if the fluid condition precipitates a transmission failure.

[Brandon002]

So don't change the transmission fluid... because it won't cause a problem?

I'm sorry, but no fluid is going to last forever and regular changing of the fluid is going to prolong transmission life. This is a fact. It's called "preventative maintenance" for a reason.

[srmmmm]

Originally Posted by JCL

Yes. Lifetime of the transmission. Which will likely experience an eventual failure not prompted or impacted by the state of the transmission fluid.

I agree with JCL 100%. Automatic transmission failures are typically caused by:
1) Mechanical abuse;
2) Overheating;
3) Fluid contamination;
4) Electronic malfunction.

Hydraulic fluid, which is the basis of ATF, essentially does not wear out. The additives can be damaged though by excess temperatures and contamination. The BMW units are for all practical purposes, "sealed", so contamination is not an issue unless new fluid is introduced, whether by using the wrong type or performing an incomplete flush. As to overheating, the fact the vehicle is rated to tow 5000-6000 pounds without additional modification is a testament to the cooling capacity of the transmission heat exchanger. (For peace of mind though, I'd probably add an additional cooler if I were towing on the highside regularly.

2002 X5 3.0 243,000 miles (no fluid changes)
2004 325i 108,00 miles

[Brandon002]

By not servicing transmission youre rolling the dice. New fluid will always be better than old fluid.

Brake systems are hydraulic and for all intensive purposes sealed. They need service. A transmission is a mechanic component. Why would bmw put magnets in the trans pan if there was no worry of metal particals contaminating the fluid? Those metal particals need to be removed periodocally as they can clog passages or cause even more rapid wear on othe components.

Its your own choice ultimately, but it is a gamble that can more than likely have the odds swung in your favor with preventative maintenance.

[JCL]

Originally Posted by Brandon002

So don't change the transmission fluid... because it won't cause a problem?

I'm sorry, but no fluid is going to last forever and regular changing of the fluid is going to prolong transmission life. This is a fact. It's called "preventative maintenance" for a reason.

No, don't change the fluid unless there is a reasonable expectation that doing so will prolong transmission life, and proportionally more so than it costs to change it.

Preventative maintenance does not consist of changing out things just because. And transmission fluid doesn't wear out based on miles or km, it wears out based on incidences of overheating. Transmission fluid does get oxidized when it is overheated. But it never stops being slippery. And the lubrication demands on transmission fluid are very low, which is why a 10w hydraulic fluid provides sufficient lubrication. Yet most proponents of changing it talk about the mechanical wear that cleaner hydraulic fluid will somehow prevent.

If the fluid was typically wearing out, considering all of its critical characteristics, across a sample of BMW vehicles, and leading to subsequent transmission failures, you would see a normal distribution (bell curve) representing transmission failures related to fluid degradation. We don't see that. And we see transmissions running hundreds of thousands of miles on original fluid without drama. Those are real data points. Changing fluid on a transmission that is working fine, and then pointing out that nothing bad happened, doesn't support doing preventative fluid changes.

The failure modes we do see tend to involve sensors, wiring harnesses, and so on. There are lots of random nuisance issues. None of them appear to correlate very well to fluid degradation.

If the fluid goes out of spec due to abuse or whatever, it is reasonable to change it. That appears to be what happened to your transmission. But if there are no shifting issues, particularly at higher mileages when there are likely to be deposits built up throughout the transmission, then the economics of regular fluid changes are very questionable. And the added risk of introducing a new, high detergent fluid into a closed system with varnish, etc, means that there is a risk of a transmission failure precipitated by the fluid change. It won't always happen, but it happens frequently enough to be recognized as a known failure mode, even with the correct fluid correctly installed.

It all seems counterintuitive, because people want to do what is best for their vehicle. We used to change oil at 3000 miles, something not at all necessary on these vehicles. We used to do tune ups. We used to adjust valves. There are lots of maintenance activities that are no longer relevant.

The thinking on preventative maintenance, and predictive maintenance, has evolved a lot in the past few decades. It is no longer intended to prevent failures. It is intended to produce the lowest overall operating and owning cost. And that means that fluid changes that cost money but don't produce an improvement are in fact a liability, not a benefit.

And I told myself I wasn't going to wade into this particular topic again. Oh well.