Thoughts on Changing Transmission Fluid

[Whitecat]

As for the price of $400 for the oil changed at the shop....seems a bit low if oil used was OEM ZF at $25 per liter X 16 (including torque conv) ...just the oil totaled $400....my transmission shop bill showed 16 liters ... Is somebody here can confirm that a full flush need 16 liters ?

ATF/Automatic Transmission Fluid By ZF OEM 1 Liter 5HPXXX

[Brandon002]

The shop charged me cost for the fluid. Im sure they get it well below retail.

[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

[bcredliner]

Originally Posted by srmmmm

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

Nay, the horse never dies in this debate.

Friction is good for friction services. Level of friction is calculated for optimum performance and durability. Less friction is not good.

The fluid and the filter become less effective over time. The degree of degradation may not be enough to cause transmission failure but changing the fluid every 50,000 miles means the fluid will be far closer to the initial specifications for optimum performance and durability.

Even at 50,000 miles I don't think the fluid smells or feels like new.

What source says---whatever you do, DON'T change the transmission fluid?

[upallnight]

Originally Posted by bcredliner

Nay, the horse never dies in this debate.

Friction is good for friction services. Level of friction is calculated for optimum performance and durability. Less friction is not good.

The fluid and the filter become less effective over time. The degree of degradation may not be enough to cause transmission failure but changing the fluid every 50,000 miles means the fluid will be far closer to the initial specifications for optimum performance and durability.

Even at 50,000 miles I don't think the fluid smells or feels like new.

What source says---whatever you do, DON'T change the transmission fluid?

JCL

[JCL]

Originally Posted by bcredliner

The fluid and the filter become less effective over time. The degree of degradation may not be enough to cause transmission failure but changing the fluid every 50,000 miles means the fluid will be far closer to the initial specifications for optimum performance and durability.

Even at 50,000 miles I don't think the fluid smells or feels like new.

What source says---whatever you do, DON'T change the transmission fluid?

The fluid and the filter have many critical characteristics. They don't vary linearly, or necessarily in synch, so 'effective' is a nebulous term.

In terms of lubricating ability, we have seen reports (linked above) that oils actually lubricate better after thousands of miles than when they are new. That means less friction. For mechanical wear, that is a good thing. In terms of just the lubricating aspect of transmission fluid, I think the SAE papers are relevant.

In terms of heat conducting ability, I don't think there is a significant change. And in terms of compressibility (ability to serve as a hydraulic fluid), ditto. So for two of the most critical requirements of an automatic transmission fluid, the fluid doesn't degrade over time.

In terms of viscosity, I would expect the fluid to get thicker over time due to dissolved clutch material. The transmission has a built-in adaptation to that process with a feedback signal, so the fluid degradation in terms of higher viscosity doesn't matter until it goes beyond the transmission's ability to adapt.

Now to filters. A filter that is partially blocked will tend to filter better in terms of particles stopped (because the passageways are smaller). There will be a correspondingly higher pressure drop, and potentially a reduced flow. The pressure drop doesn't matter until the drop exceeds the ability of the pressure regulation circuit to compensate. But the filtering efficiency (amount filtered per unit volume) will rise. Until the filter can not flow sufficiently. Then it needs replacing.

My point is that a new fluid and filter isn't necessarily the best. They don't necessarily start out perfect and then degrade over their life. They can be designed to be optimum at mid-life, or at end of life (immediately prior to failure), depending on the characteristic involved. It is the same with many mechanical components.

Interesting debate.

And who says don't change the transmission fluid? That is a very broad question. I don't. BMW service manual reportedly does, up to 100,000 km if a technician has to drop the pan for another reason. Instructions I read were to reuse the old fluid. I say don't change the fluid if it is working fine, but it is the first thing to try (after checking the level) if the transmission is not shifting correctly. And I agree with your approach in that if you are going to change it, I think it is better to do it at shorter intervals than wait until 100,000 miles. That doesn't consider the value of changing it, just that it is a good way to manage the risk of higher mileage changes.