I'll make a couple of comments to further the discussion along. Just remember that I'm a non tranny expert, but who has read tons of tranny posts since 2001 so take these with whatever amount of salt you want and the last time I can remember staying at a Holiday Inn Express was over 10 years ago
My understanding of this bypass based on the following information from someone I believe should know something in fact a lot about the 4R100
The bypass is a pressure relief valve. When it's very cold the fluid in the cooler gets vaery hard to move. Pressure in the cooler line goes up and nothing flows. Since the return from the cooler is used to lube the rear half of the transmission, no cooler flow causes big problems.
If the pressure gets high enough the bypass opens and sends fluid to the rear lube
The internal bypass came out with the TorqShift. The 4R100
Yes, the bypass was to fix a problem. The fluid in the coolers would get to thick to flow in extreme cold weather. This starved the rear half of the transmission for lube. The bypass allows rear lue to always have fluid.
I do agree that a bypass failure can send temps sky high and IMHO
is one reason to have a good tranny temp gauge installed, know your baseline and if things go south find out why pronto and fix the issue. But if the temps are cold enough you could starve the rear of the tranny possibly before the temps got out of control. Also if you remove it and and in warmer weather have a blocked cooler as was the situation in this thread it seems the starvation issue could add to the problem, but maybe the temps go sky high before the starvation issue becomes the major factor in a tranny failure.
I can also see if the bypass gets stuck open then you will also have a heat issue since it will be bypassing the coolers
I'm not sure what the extreme temps quoted above mean, but I seem to recall a lot of folks seeing that bypass open even in mildly cold climites when trying to "flush" their trannies when they removed the return line from their trannies to do the flush.
From my understanding during the normal "tranny" flush that we do in our driveway we want that bypass closed and during the procedure we aren't lubing the rear of the tranny so some lack of lubrication over time doesn't seem to be an issue the question is how long is too long and that I have no idea.
: My stock tranny has both the factory OTA
coolers and the bypass and personally I wouldn't just remove it.
Of course I also have a spare bypass since a new one was put on when my BTS was built and I got the old one from the original tranny that was still working.