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I previously installed an aftermarket transmission cooler in a 2006 GMC Envoy (4.2, 4L60E). I installed it "after" the factory oil-water heat exchanger built into the radiator (that is, the hot transmission fluid would first pass through the radiator, then through the aftermarket cooler, then return to the transmission). At the time I installed it, I remember that "everyone" said this was the right way to do it because it provided more cooling capacity.

Thinking about it more I am not so sure. The 4L60E does not have an internal thermostat, therefore it would rely on the radiator to regulate the fluid temperature. Therefore it would rely on the radiator to cool AND heat the transmission fluid, since the radiator water temperature should reliably be around 195 F during normal operation.

By placing the transmission cooler after the radiator, it seems like the transmission fluid might get too cold. At the very least, this seems like it would cause excess fuel consumption, but a cold transmission is also bad for longevity.

Wouldn't it make more sense to place the aftermarket cooler BEFORE the factory radiator, therefore ensuring that the transmission fluid temperature matches the radiator water temperature?

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Transmission fluid cannot be "too cold". There is one enemy of transmission fluid and that is heat. Having cooler temps in tranny fluid is a good thing. Your transmission isn't like your engine. The reason your engine needs to be "up to temp" and "temperature controlled" is to ensure proper combustion is occuring. The reason your transmission is not is because it needs to be as cool as it can be kept, which will increase its longevity.

The way you are suggesting to do it, with the aftermarket cooler after the installed one is the general way to do it. This is to ensure the coolest fluid is going back into your transmission. I could see it where when the radiator is running fully at temp, if you ran the tranny fluid back into the radiator cooler, you'd actually be heating it up. The only other way I've seen an aftermarket cooler done is by bypassing the regular one altogether.

EDIT: I realize some of my "logic" is circular, so I thought I'd add someone else's wisdom in here. Here is an article which pretty much says the same thing I'm saying ... well ... only better :o)

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  • If trans fluid cannot be "too cold" then why do some transmissions (such as Ford's 5R55E) have an internal thermostat? – masospaghetti Nov 20 '20 at 19:29
  • @masospaghetti - The "too cold" statement should be considered "within reason" ... Frozen would not be a good thing. Still, heat is still the enemy. – Pᴀᴜʟsᴛᴇʀ2 Nov 20 '20 at 19:39

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