Thoughts and maybe Mark will chime in. 99 F250 4x4 V10, 4r100. These early year has no trans fluid thru the radiator to cool or warm up the fluid. Typically my use is 60+ miles++ each way where it will hit 150ish, but the occasional local run to the gas station is 20 miles each way. Trans temp never goes over 120 if that in winter. On the other end It never goes over 190 when used as hard as I'll ever need it to in summer. I've considered putting the radiator in the loop but if it aint broke dont fix it im thinking.

 

Installing a transmission cooler in the radiator will make the trans run cooler. That's what a transmission cooler does. It does NOT heat the trans fluid, ever, in any condition.

What works to warm the trans quicker in cold weather is to put a piece of cardboard in front of the trans cooler.

 

How bout cardboard in front of the radiator? That has to warm the trans fluid, unless what you're saying is that the trans cooler will bleed more heat than the engine coolant can add it.

 

The engine coolant can't add ANY heat to the transmission fluid. I've had thermocouples inside the radiator to measure this and prove that the transmission cooler is a cooler and never a transmission warmer.

The transmission cooler is located in the part of the radiator that has coolant that has already passed through the radiator and has been cooled. In cold temperatures the coolant will be within a few degrees of ambient temperature. So if it is 0°F outside, the coolant temperature around the transmission cooler will be around 5-10°F even when the engine is fully warmed up. Coolant at those temperatures will not warm the transmission.

I'm not just thinking this is how it works. It was my job for several years to measure this. I had cars and trucks instrumented to measure hundreds of temperatures throughout the transmission system and ran tests in ambient temperatures ranging from -40°F to +120°F.

I never found any condition where the engine coolant warmed the transmission fluid. Interestingly, the closest the coolant came to warming the trans fluid was at higher temperatures. But even when the engine is running hot, the cooler side of the radiator is still cooler than the ATF it's cooling.

 

I was asking the question assuming that you had real world testing/data to relay what you *observed* (as opposed to a theory).

That makes sense, I didn't think about how the trans cooler is at the base of the radiator where it's nice and cool. I still wonder if there are conditions where blocking the radiator would warm a trans.......like very cold ambient temp. I have blocked the radiator in many diesels i've owned, but it was to allow the engine to get to operating temp - I can't imagine the coolant in the radiator gets hot enough to become a trans warmer, like you say.

 

I now recall reading Marks posts some time ago RE: heat transfer etc. Based off that info I will call it good