question about transmission fluid cooling

[smaring]

I've been having a slow coolant leak in my 2001 4.6L Eddie Bauer and decided it was probably time to replace the radiator and heater core. While disconnecting the transmission lines from the old radiator I noticed a bit of ATF around the connectors but nothing flowed out of the radiator like I was expecting. That got me wondering how the transmission fluid even gets to the radiator. Is there a pump inside the transmission? Does fluid only flow through the radiator when the vehicle is running?

 

[JamaicaJoe]

There is actually a pump inside the transmission to circulate ATF fluid. I don't think the coils for the transmission cooler are very large. Did you notice any coolant on the transmission side or vice versa?

 

[smaring]

OK. I guess I will assume it is normal for the ATF to flow back down from the radiator to the pan when the engine is off.

I haven't seen any evidence to suggest that ATF and coolant are mixing in the radiator. It has been a couple years since I rebuilt the engine. At that point I dropped the transmission pan, replaced the filter, replaced the lines going between the transmission and radiator, and put in new ATF. Every since then I've learned to expect that when I get to the stop sign at the end of the street the transmission may take a few seconds to engage when I try to take off. After that, it is usually fine for the rest of the drive. I had an old mechanic guy tell me that I should have kept the original ATF and put it right back in. I have a tough time wrapping my head around the rationale for that one.

 

[Dustin Gebhardt]

A typical drain and refill changes about 1/3 of the fluid capacity on these transmissions. Since you are having shifting issues, I'd suggest another drain and fill, or do a shade-tree flush by disconnecting one of the hoses going to the radiator/cooler and connecting a temporary reservoir to the return line and place the drain line into your catch container. Turn on the engine and keep the temp reservoir filled. When the drain line fluid goes from dark(er) to bright red, turn off the engine and button it back up.