Thoughts on Changing Transmission Fluid

[racingbmwm3]

First, I'm slightly confused why everyone thinks the transmission is a closed system. There is most definitely a vent pipe which allows for expansion/contraction of the air/fluids as the tranny heats/cools. If it didn't have this, every seal would start leaking very early in its life. The vent is higher up in the engine compartment and shaped to keep water out, but moisture and dust can get in there over time.

Yes, the filter does perform filtering function better over time becoming a super awesome filter, right before it becomes a perfect filter (100% filtering) at which point your transmission stops working because of lack of fluid flow. And yes, this will probably not happen until over 200k miles. But, lack of any maintenance before this happens, means that a fluid/filter change this late in life will result in all those varnish/deposits getting cleaned up all at once by the new fluid, and now you have clogged passages and valves, etc. So, new transmission or a complete rebuild (which is cheaper?) will soon follow the clogged filter.
Same story applies for the viscosity increasing over time. Everything has to work harder, which is accounted for by the adaptations, until the viscosity can't be accounted for anymore at which point you start having shifting issues. This should be at higher mileage again, and change the fluid, it will shift better again, until the the new fluid frees up the varnish/deposits and you need a new transmission or rebuild.

But, since there aren't many X5's out there over 300k miles that have regular 60k transmission servicing, JCL is correct that nobody knows what the actual life of a well maintained transmission will be. So maybe you spend $2000 in maintenance to get your transmission to last 300k miles. at which point it needs to be replaced or rebuilt. If you would have replaced the transmission at 200k without any servicing, you haven't really saved any money. And $1200 (3 fluid changes, 180k) over several years are lost earning potential.

Now, if it can be proven that the fluid changes double the life expectancy of the transmission, it will totally be worth it. (neglecting the lost earning potential of the maintenance money)
But, we need some guinea pigs to prove this case. And ideally, more than 100 vehicles doing regular maintenance to get some basic stats. Plus we need to find another 100 vehicles without doing any maintenance. And we need all 200+ vehicles to drive 400k miles.

But, if you drive your car hard as in lots of redline shifts, heavy towing, etc, you definitely should look at extra maintenance beyond typical mfr. recommended. Everyone else, probably doesn't matter.

[jgold47]

obviously there are too many variables to do a consistent test. In my mind you would have


GM/ZF

Miles

change interval (if any)

Fluid choice

Driving style

Filter/no filter

enviroment.



would be tough to get a one size answer for these.

[upallnight]

Originally Posted by racingbmwm3

First, I'm slightly confused why everyone thinks the transmission is a closed system. There is most definitely a vent pipe which allows for expansion/contraction of the air/fluids as the tranny heats/cools. If it didn't have this, every seal would start leaking very early in its life. The vent is higher up in the engine compartment and shaped to keep water out, but moisture and dust can get in there over time.

Yes, the filter does perform filtering function better over time becoming a super awesome filter, right before it becomes a perfect filter (100% filtering) at which point your transmission stops working because of lack of fluid flow. And yes, this will probably not happen until over 200k miles. But, lack of any maintenance before this happens, means that a fluid/filter change this late in life will result in all those varnish/deposits getting cleaned up all at once by the new fluid, and now you have clogged passages and valves, etc. So, new transmission or a complete rebuild (which is cheaper?) will soon follow the clogged filter.
Same story applies for the viscosity increasing over time. Everything has to work harder, which is accounted for by the adaptations, until the viscosity can't be accounted for anymore at which point you start having shifting issues. This should be at higher mileage again, and change the fluid, it will shift better again, until the the new fluid frees up the varnish/deposits and you need a new transmission or rebuild.

But, since there aren't many X5's out there over 300k miles that have regular 60k transmission servicing, JCL is correct that nobody knows what the actual life of a well maintained transmission will be. So maybe you spend $2000 in maintenance to get your transmission to last 300k miles. at which point it needs to be replaced or rebuilt. If you would have replaced the transmission at 200k without any servicing, you haven't really saved any money. And $1200 (3 fluid changes, 180k) over several years are lost earning potential.

Now, if it can be proven that the fluid changes double the life expectancy of the transmission, it will totally be worth it. (neglecting the lost earning potential of the maintenance money)
But, we need some guinea pigs to prove this case. And ideally, more than 100 vehicles doing regular maintenance to get some basic stats. Plus we need to find another 100 vehicles without doing any maintenance. And we need all 200+ vehicles to drive 400k miles.

But, if you drive your car hard as in lots of redline shifts, heavy towing, etc, you definitely should look at extra maintenance beyond typical mfr. recommended. Everyone else, probably doesn't matter.

Sign me up for the Group not performing the maintenance. Currently 148,000 miles or 238,182.912 Kilometers.

[JCL]

Originally Posted by racingbmwm3

First, I'm slightly confused why everyone thinks the transmission is a closed system. There is most definitely a vent pipe which allows for expansion/contraction of the air/fluids as the tranny heats/cools. If it didn't have this, every seal would start leaking very early in its life. The vent is higher up in the engine compartment and shaped to keep water out, but moisture and dust can get in there over time.

Fair comments. To my thinking, it can be considered a closed system and still have a vent for thermal expansion. I think of transmissions, differentials, and transfer cases as closed systems, but that is really by way of comparison to engines, which are not closed systems. Regular engine oil changes are commonly used as a parallel example supporting regular transmission fluid changes. I don't think that comparison is valid though. Engines have contaminants by way of combustion by-products and daily condensation in the intake tract, so they are very open. The vents in the closed systems are open, agreed. I would leave differentials for 100,000 km or miles of highway use without a worry , but have changed differential fluid when the vehicle in question has been used to ford creeks and so on, and has a risk of water ingress.

[racingbmwm3]

Reminder that the transmission vent is at the same height and almost same location as the differential vents...

[JCL]

As I never partially submerged my X5, I didn't worry about that, whether for differentials or the transmission. If one is doing so, checking for water ingress would be a good idea, in all the compartments.

[RRPhil]

Just for info., on the 5HP24 the air breather is mounted on top of the oil pump housing









Phil