Quote:
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.
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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.
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