Thoughts on Changing Transmission Fluid

[JCL]

Originally Posted by Brandon002

I thought I read somewhere that ZF recommends fluid maintenance on the X5 trans.

Let me see if I can find the article again.

It's been posted on this site.

ZF said they support the BMW recommendations as to service interval.

They also said that for severe service, the fluid should be changed. I think the interval was 10 years, and some specific mileage, from memory.

BMW is ZF's customer. They aren't going to contradict them. They are also going to take money from those who want to buy fluid change kits. They are not an independent bystander in this debate.

We also need to keep in mind that ZF built the basic transmission, but that BMW did the transmission cooling system, and the integrated controls that back off engine power during shifts to reduce clutch wear. Also the torque converter lock up strategy. All of those impact transmission life compared to applications of the same transmission without those features.

[Brandon002]

So which has the higher probability of failure?

Transmission A: 140K miles, original fluid, original filter, original gasket

Transmission B: 140K miles, new fluid, new filter, new gasket

[JCL]

Originally Posted by Brandon002

So which has the higher probability of failure?

Transmission A: 140K miles, original fluid, original filter, original gasket

Transmission B: 140K miles, new fluid, new filter, new gasket

Assume both are operating fine, and had the same use up to this point.

If the fluid has never been changed on either one, and it is then changed on B, then B has the higher probability of failure.

If B doesn't fail in the next few thousand miles after that change, then it is back to equal.

If we change the rules of the example, and change the fluid on B every 35,000 miles from new, then it doesn't have any significant increase in risk by changing it at 140,000. I'd give the benefit to transmission B in that scenario.

Now we come to the payback question: If you change the fluid 4 times on B, will it last sufficiently longer than A to make the investment a smart one?

[Brandon002]

So you're one of those people that thinks changing the fluid on a high mileage trans will have negative effects?

I'd love to hear the reasoning behind this one.

[JCL]

Originally Posted by Brandon002

So you're one of those people that thinks changing the fluid on a high mileage trans will have negative effects?

I'd love to hear the reasoning behind this one.

It is a failure mode that is fairly well understood. But is it not that there will be negative effects. There may be negative effects, in a limited time window. There is a difference. We are speaking probabilities here, not absolutes.

Transmission fluid, by design, has a high level of detergents. That is so that it lasts a long time. Over an extended time, those detergents become less effective. Varnish and deposits can start to build up in the transmission, in places where they are not doing any particular harm. Then, the fluid is changed, and those strong new detergents scour the internals of the transmission. The crud that is released and moved can make it to the filter. If so, fine, no problem. But if it first gets lodged in the valve body, in one of the orfices or check valves or actuators, it can precipitate a failure. There is a theory that this can also happen when the transmission is first filled, and started, and the fluid courses through, but I think the detergents matter more, as failures when they happen usually aren't on start up but in the weeks that follow.

There is no such thing as shocking the fluid. There is simply cleaning out a system that isn't completely clean, and not having any place for the residue to go. Except through a valve body with a lot of very small passageways.

If there is no failure within a few thousand miles (purely an estimate, no science here) I would say that a subsequent failure, if it occurs, isn't related to the fluid change. But if a transmission is working perfectly, and the fluid is changed at a higher mileage, and the transmission fails soon after, it is a recognized issue. We are not speaking here about the wrong fluid, or doing it wrong. And not about transmissions that already had a problem. In those cases, changing the fluid is a valid attempt to address the problem, and subsequent failures can not be blamed on the fluid change.

The above failure mode is well enough known and understood that many shops will decline to change the fluid on a high mileage transmission that hasn't had a previous fluid change. They don't want to take on the risk. Because if the transmission fails, the owner will blame the shop and demand compensation. It is often better for their bottom line to just turn down those jobs and let someone else take the bullet. This has nothing to do with believing what the manufacturer says or not about changing the fluid. It is simply a risk/reward calculation.

The way to reduce the above risk is to change the fluid frequently enough that crud doesn't build up at all. Some use the interval of 30,000 or 50,000 miles. I think that is a valid approach. It is just that if you are going to do four or five or more fluid changes before the transmission fails, you should be pretty sure that you will get a return on your investment. One change is pretty cheap. Four or five, not so much. And recall that if the transmission fails due to an electronic component like a sensor or wiring harness, as many do, or a random break not impacted by lubrication, then those multiple fluid changes will have been for naught in the economic analysis of return on investment.

[srmmmm]

Originally Posted by Brandon002

So you're one of those people that thinks changing the fluid on a high mileage trans will have negative effects?

I'd love to hear the reasoning behind this one.

New fluid changes the operating environment of all the friction surfaces of the transmission. This can cause accelerated wear of those surfaces resulting in clogged filters or fluid passageways. The greatest danger is the possibility that a non-compatible fluid was used accidentally. With today's synthetic based fluids and electronic throttle/shift controls, mechanical wear has been reduced significantly. As long as the transmission has not been overheated, it is not necessary to change the fluid.

My background includes a degree in automotive engineering from General Motors Institute in 1978, with lab experience with the original lock-up convertor turbo-hydramatic transmissions, and engines families including the Cosworth Vega, Buick 3.8L turbo, Pontiac Super Duty 455 and turbo 301. Along with ten years at GM assembly plants, I spent another eleven with Japanese and German HVAC Tier One automotive and truck suppliers.

Count me as one who doesn't change fluid anymore.

2002 X5 3.0 243,000 miles
2004 325i 108,000 miles

[J.Belknap]

Transmission fluid thread. Must be Monday again.

Servicing the valve body (where the ball bearings wear) is beneficial. If one is beating the ever loving heck out of the transmission (Non-OEM software, more torque, etc) minding the fluid is beneficial.

.02

Edit:

Originally Posted by srmmmm

Buick 3.8L turbo

I would like to have a Buick 3.8L turbo.

[BMW_E53]

I actually agree with JCL's point about transmission a and b. A transmission running on original oil doesn't necessarily mean it's bad and has a high risk of failure. BMW has done extensive testing with hours and hours of abuse to back up the claim that the oil is sealed. Transmission B can have a cheaper type of fluid or a cheap filter, which down the line can also cause a failure.

[jgold47]

One of yen things that has not always been clear is if the transmissions that failed also has their filters changed at the same high mileage. It's much easier to change the fluid only without the hassle of dropping the pan. Also a lot of the chatter is related to ZF trans. I can't imagine the gm trans being equally robust. This tranny is used in lots of gm vehicles including many Cadillacs.

I just dropped the pan and did fluid/filter t 98k. I guess we will see.

[srmmmm]

I've got 243,000 miles on the GM transmission with no fluid changes and I average 3000 miles of personal watercraft towing each summer here in Texas. The only problem I ever had was dirt and moisture getting into the harness connector causing the transmission to go into failsafe mode.

2002 X5 3.0