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Are you positive (I am not)?

My understanding is that the dark color of transmission fluid was due to heat-caused oxidation, not just murkiness. Oxidation definitely affects performance. The heat is not from direct combustion, but it still can be quite high.

And 150-200k miles is definitely outside the operating limits of transmission fluid.
I cannot find a document of ZF recommendations for oil service, but I remember it says fluid change at 100k.
My wifes car gearbox wasn't touched till we sold car on 230 K - or 260, can't remember. my convertible transmission wasn't touched till 470K, when I needed to fix torque converter, it is where fluid was changed - and I am pretty sure now workshop didn't fill it properly and I was driving with underfilled transmission for another 150k , till it just went to safe mode and died - and I was driving my car in very same pattern - so no symptoms, no fails. Absolute most of all time I drive it with manual mode, using engine to break - not revving into red zone, but def using car the way BMW meant to be. checking gearbox just shown worn disks, that is it, that was a reason, nothing wrong with solenoids, etc. I
have to mention, I live in Dubai, so half of the year temp is above 40 C, and summer time it is around 50C. I am not chemist so I cannot say whether oxidation by temp affects hydraulic characteristics of fluid, but I seriously doubt it has obvious effect. changing fluid suppose to wash passes and solenoids, true, although there is not much of dirt in there - not like in engine, where washing features of oil are vital.
My point is - if you want to change fluid, just do it, but don't expect any major improvements, and of course follow exact procedure to change it. If you have some troubles with gearbox already, it is very unlikely that new fluid would fix it.
 

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I cannot find a document of ZF recommendations for oil service, but I remember it says fluid change at 100k.
My wifes car gearbox wasn't touched till we sold car on 230 K - or 260, can't remember. my convertible transmission wasn't touched till 470K, when I needed to fix torque converter, it is where fluid was changed - and I am pretty sure now workshop didn't fill it properly and I was driving with underfilled transmission for another 150k , till it just went to safe mode and died - and I was driving my car in very same pattern - so no symptoms, no fails. Absolute most of all time I drive it with manual mode, using engine to break - not revving into red zone, but def using car the way BMW meant to be. checking gearbox just shown worn disks, that is it, that was a reason, nothing wrong with solenoids, etc. I
have to mention, I live in Dubai, so half of the year temp is above 40 C, and summer time it is around 50C. I am not chemist so I cannot say whether oxidation by temp affects hydraulic characteristics of fluid, but I seriously doubt it has obvious effect. changing fluid suppose to wash passes and solenoids, true, although there is not much of dirt in there - not like in engine, where washing features of oil are vital.
My point is - if you want to change fluid, just do it, but don't expect any major improvements, and of course follow exact procedure to change it. If you have some troubles with gearbox already, it is very unlikely that new fluid would fix it.

470k. Wow, must have been a transmission from Krypton.

Levity aside, by your own account I'm not trusting a non-chemist on their opinion on heat-related fluid oxidation characteristics. There are however actual chemists that have opinions opposite to yours and I think I'm going that route.
 

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I cannot find a document of ZF recommendations for oil service, but I remember it says fluid change at 100k.
ZF recommends 50K mile service intervals as optimal and even puts a time criteria on it.



As for the fill procedure, cycling through the gears is still SOP per the manufacturer:



 

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470k. Wow, must have been a transmission from Krypton.

Levity aside, by your own account I'm not trusting a non-chemist on their opinion on heat-related fluid oxidation characteristics. There are however actual chemists that have opinions opposite to yours and I think I'm going that route.
If you read my post you might find that I do not insist on not changing fluid. I said and I could repeat it again- changing fluid will not improve performance of gearbox significantly and I doubt it will greatly prolong transmission life. Unlike engine, where oil‘s main work is to eliminate friction, transmission fluid prime task is to provide pressure required.
I would be grateful if you could post an opinion of chemist on how working temp and mileage and oxidation caused by those criteria affect condition of fluid. It would really help to determine how critical it is actually to change fluid and how often we have to do it.
 

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Granted they are selling a used oil/at analysis service, but ...

 

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I'm no chemist or engineer but have been at this trade professionally since the early 1980's. I also used to rebuild automatic transmissions (far too many) back in the day of the dreaded 4HP22-24-EH family of automatic transmissions from ZF at the time.

Beyond any doubt, the hardest working fluid on any car is the automatic transmission fluid. (I know such and have been taught such a couple of times.) You're correct that it's primary function is to be a hydraulic fluid, but it also has to do so much more. It must provide the above in a very broad set of temperature conditions, and must function as such for a egregiously long period of time. Take a look at the size of the filter and compare it to your engine oil filter. Would you run your engine oil that long with such a filter?

Likely not, so a large part of modern transmission fluid is heavily dependent upon the additive package. The base oil is still there, but the many additives can/do break down, oxidize and face the contaminants of the wear of the friction surfaces. Much of the additive package is a stabilizer to provide anti-wear and anti-heat stabilizers for that brief period when one clutch set becomes uncoupled and another one (sometimes 4 at a time) become active. Much like a manual transmission there is a brief period when the clutches are coming together and there's some slippage involved until full pressure is attained within those clutch packs. This is the time when maximum wear/heat happens. The fluid must have additives to deal with such as they happen constantly over and over again as the transmission continually shifts (changing multiple clutch packs) all the while during your drive.

With most (not all) automatic transmissions the above will see the filter become less effective and also as the additive package ages out, wear will accelerate. At an incremental pace but it does happen.

Have I seen automatic transmissions go a very long time w/o service, sure. Have I seen engines go a long period of time with egregiously extended service intervals, well yes to that also.
But why would one? If ZF issues a bulletin countermanding the directive of BMW's edict of "Lifetime" (That bulletin BTW changed BMW's attitude when it was released, and let me tell you they weren't happy with ZF at the time.) it gives one insight as to why it "should" be done.

Why would ZF care? Lets be pragmatic, they're in the business of selling transmissions and parts. Servicing would only see such bringing extended life to any transmission.
For the reasons I've cited above and the many classes I've attended taught by ZF that strongly advocate fluid/filter changes, I for one feel it's necessary.
 

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Not to mention, changing tranny fluid is way cheaper than oil changes when you take distance into consideration. You need a tranny to move the car as much as you need the engine. If you change the engine oil every 25,000kms as BMW recommends, at 100k you will have changed the oil 4x costing about $400 total. If you change the tranny fluid once in that period it only costs about $250. Anyone that doesn't change it is an idiot.
 

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Not to mention, changing tranny fluid is way cheaper than oil changes when you take distance into consideration. You need a tranny to move the car as much as you need the engine. If you change the engine oil every 25,000kms as BMW recommends, at 100k you will have changed the oil 4x costing about $400 total. If you change the tranny fluid once in that period it only costs about $250. Anyone that doesn't change it is an idiot.
Well, pretty serious statement. Considering 25K oil change by BMW recommendation ou just mentioned, wouldn't be logical to follow BMW recommendations regarding lifetime oil in transmission?
May I know if you buy a car brand new and if yes, did you change trans oil after 100K?
 

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I'm no chemist or engineer but have been at this trade professionally since the early 1980's. I also used to rebuild automatic transmissions (far too many) back in the day of the dreaded 4HP22-24-EH family of automatic transmissions from ZF at the time.

Beyond any doubt, the hardest working fluid on any car is the automatic transmission fluid. (I know such and have been taught such a couple of times.) You're correct that it's primary function is to be a hydraulic fluid, but it also has to do so much more. It must provide the above in a very broad set of temperature conditions, and must function as such for a egregiously long period of time. Take a look at the size of the filter and compare it to your engine oil filter. Would you run your engine oil that long with such a filter?

Likely not, so a large part of modern transmission fluid is heavily dependent upon the additive package. The base oil is still there, but the many additives can/do break down, oxidize and face the contaminants of the wear of the friction surfaces. Much of the additive package is a stabilizer to provide anti-wear and anti-heat stabilizers for that brief period when one clutch set becomes uncoupled and another one (sometimes 4 at a time) become active. Much like a manual transmission there is a brief period when the clutches are coming together and there's some slippage involved until full pressure is attained within those clutch packs. This is the time when maximum wear/heat happens. The fluid must have additives to deal with such as they happen constantly over and over again as the transmission continually shifts (changing multiple clutch packs) all the while during your drive.

With most (not all) automatic transmissions the above will see the filter become less effective and also as the additive package ages out, wear will accelerate. At an incremental pace but it does happen.

Have I seen automatic transmissions go a very long time w/o service, sure. Have I seen engines go a long period of time with egregiously extended service intervals, well yes to that also.
But why would one? If ZF issues a bulletin countermanding the directive of BMW's edict of "Lifetime" (That bulletin BTW changed BMW's attitude when it was released, and let me tell you they weren't happy with ZF at the time.) it gives one insight as to why it "should" be done.

Why would ZF care? Lets be pragmatic, they're in the business of selling transmissions and parts. Servicing would only see such bringing extended life to any transmission.
For the reasons I've cited above and the many classes I've attended taught by ZF that strongly advocate fluid/filter changes, I for one feel it's necessary.
I would assume that BMW infamous step on lifetime oils, as well as long intervals of engine oil change was great marketing solution - giving you pretty much service-free car, when Jap manufacturers still servicing their cars every 5K km. (Now we can see that long life oil for engine is serious mistake and long service intervals imminently lead to engine permanent damage, but it is theme for separate discussion).
So it would be, no doubt, right to change atf fluid regularly every 50K, lets say. But as most of e46 now have mileage close to 200k, and more, and it could be first oil change from a day of manufacturing, effect of oil change, both performance and improving gearbox health, will be minimal, not mentioning risk of damage by not following exact procedure.
 

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I'm not sure I'd call the effects minimal. I definitely notice a difference after changing the atf, even if it's only a drain and fill that replaces only 4.5 quarts. It eliminates a certain 'sluggishness' that develops with enough mileage. With fresh atf when I hit the throttle there is less hesitation and a more direct feel between engine input and transmission output.

I am picky though and borderline hate automatics to begin with, but this one I dont mind so much and its the regular atf changes that have gone a long way to keeping me actually somewhat happy with it.

As for risk of damage from incorrect procedure, that definitely exists and can cause big problems for sure. Learning to DIY the atf changes mitigates that, but yes a shop that doesn't know what they are doing could cause more problems than not doing a change at all.
 

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I would assume that BMW infamous step on lifetime oils, as well as long intervals of engine oil change was great marketing solution - giving you pretty much service-free car, when Jap manufacturers still servicing their cars every 5K km. (Now we can see that long life oil for engine is serious mistake and long service intervals imminently lead to engine permanent damage, but it is theme for separate discussion).
So it would be, no doubt, right to change atf fluid regularly every 50K, lets say. But as most of e46 now have mileage close to 200k, and more, and it could be first oil change from a day of manufacturing, effect of oil change, both performance and improving gearbox health, will be minimal, not mentioning risk of damage by not following exact procedure.
That's a multifaceted question and answer.

Yes you're right, marketing was the reason. As I pointed out in the past, I was working at the dealers as a technician when BMW forayed into "free maintenance" two E34 540's in the shop for a 60,000 mile inspection #2. One covered under free service the other NOT.
Both had parts waiting on the counter for the servicing.
Car #1 has; air filter, oil filter, spark plugs, fuel filter, cabin filter, wiper blades, 2L of brake fluid and seals for changing the coolant and differential oil.
Car #2 has; air filter, wiper blades and an oil filter.
How does an inspection sheet (required service) have two different directives on the same car produced only months apart? Marketing.....

Any oil change be it late or proactively will be of benefit to the parts that get lubricated owing to the new oil and additive package.
The trick is how long did the transmission run with oil that offered little to no protection (additive package completely depleted) and the oil has suspended bits of wear material suspended/circulating around the trans?
Could a very late oil change bring about undesirable consequences? Yes sir.

That's why when confronted with such in my shop we do two things:
Have an intelligent conversation with the customer about the possibility of an undesirable outcome.
Open the fill plug only and drain off 2L of fluid. Top off with new and send the customer to return again on ~2K miles.
Performed a few times. If all goes well, we service it properly. It usually does but NOT always.

If the fluid is very dark/smells burnt/and has speckles in it. We return the fluid back to the trans and no further intervention is done by us.
 

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That's a multifaceted question and answer.

Yes you're right, marketing was the reason. As I pointed out in the past, I was working at the dealers as a technician when BMW forayed into "free maintenance" two E34 540's in the shop for a 60,000 mile inspection #2. One covered under free service the other NOT.
Both had parts waiting on the counter for the servicing.
Car #1 has; air filter, oil filter, spark plugs, fuel filter, cabin filter, wiper blades, 2L of brake fluid and seals for changing the coolant and differential oil.
Car #2 has; air filter, wiper blades and an oil filter.
How does an inspection sheet (required service) have two different directives on the same car produced only months apart? Marketing.....

Any oil change be it late or proactively will be of benefit to the parts that get lubricated owing to the new oil and additive package.
The trick is how long did the transmission run with oil that offered little to no protection (additive package completely depleted) and the oil has suspended bits of wear material suspended/circulating around the trans?
Could a very late oil change bring about undesirable consequences? Yes sir.

That's why when confronted with such in my shop we do two things:
Have an intelligent conversation with the customer about the possibility of an undesirable outcome.
Open the fill plug only and drain off 2L of fluid. Top off with new and send the customer to return again on ~2K miles.
Performed a few times. If all goes well, we service it properly. It usually does but NOT always.

If the fluid is very dark/smells burnt/and has speckles in it. We return the fluid back to the trans and no further intervention is done by us.
I failed to mention a salient point:
ATF by in large besides being a hydraulic fluid is usually very high in detergent additives. Take an abused and long overdue transmission that's is occasionally throwing fault codes and NOW the owner starts to think about performing a service.
If the fault is rooted in a sticking valve or solenoid; servicing the unit is a good idea.
If however the cause for such is a drum with the metal sealing ring on the back end of a drum eating away at the softer aluminium, changing out the old oil, and replacing with new oil high in detergent will likely bring about the demise (was going to happen anyway) of the unit sooner.

Hence the bad reputation of changing the oil and the transmission died. The real issue was an uninformed shop, doing routine maintenance and having the transmission fail. Then said shop had responsibility brought unto then by threat of a lawsuit and had to pay for the repair out of pocket.
That shop tells 20, which shares the story with 50, then 200. BTW the ZF/BMW 4HP series of transmission was the most notorious for such.

Being with BIMRS we have training that includes factory and beyond. We have classes every year taught by ZF, Getrag (now Magna) and many other O.E.M. suppliers. Armed with handbooks that the dealer techs don't get, we can proceed intelligently and plan the best course of action.
With the above example (worn drums) we would drain the fluid into a clean container. If very bad, stop. Sometimes it's marginal and when we drop the pan, the magnets and the bottom of the pan have copious amounts of material on it. Back on, and reinstall the fluid.

Not only am I covering my butt, also trying to help the owner of the car.
 

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Not sure if my previous post went but I was saying the GM 8L90 8-speed slushbox (for performance cars 600hp+) has a known issue of a shudder (excessive vibration) caused by seal material coming off in the factory fluid... the recommended fix is immediate triple flush of the ATF. GM issues a bulletin for it... This can happen in the first 10,000miles.

I don’t need scientific evidence (this is not Bobistheoilguy.com) to tell me you need to flush the ATF. Question is the ideal interval.
 
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