texas008

I've had this question for a while. reporting what I've found.
As the last step of radiator replacement, I performed a transmission fluid top up:
drained 2 QT of old fluid, pushed in 3QT, start engine for transmission fluid to reach 113F, drained the excess (1QT) and tighten the drain plug.
So, there is basically no loss of transmission fluid during radiator replacement
​​​​​​​(transmission cooler line connector was plugged during radiator replacement to avoid transmission fluid loss).

BlackML550

I agree - You'd think there was still some trans fluid in the radiator even after plugging all the lines. Only thing I can think of is that maybe you were somehow overfull to begin with? I'd also bet the radiator doesn't hold too much trans fluid.

DennisG01

I agree, if the size of the internal tranny cooler is roughly the same size as typical external coolers, they really don't hold much. Plus, I would imagine that once the car is turned off that some (?most?) drains back to the tranny? Although before I really thought about this, I probably would have thought there would be loss, too.

BlackML550

It could drain back but youd think its pressurised to some extent anyway. I can see why this is messing with you lol. But you warmed up the fluid as part of the refill process, so it would have cycled through the radiator. I assume you went through the gears when doing that. I'm not sure if the pump works when in P or N.

eric_in_sd

When I replaced my radiator, I disassembled the old one to salvage parts. The heat exchangers are tiny, a single skinny tube, with some fins, traversing the left and right tanks. You wouldn't notice that amount of fluid missing.