mtbtim
Elite Member
There is one, and only one scenario in which the in-radiator cooler makes a difference in the regard of heating up the ATF (in a good way). That scenario is this:
If you start you truck from cold when it's say, 15 degrees F outside, and you pull out of your driveway and get on some sort of highway within only a block or so, or only a couple minutes....(basically, the transmission only shifts up/down less than 8-10 times), then after about 10-15 minutes of highway driving, the engine has fully warmed up and the thermostat opens up. The radiator starts actually passing warm coolant through the bottom at this point, and the transmission which is probably only at about 100 degrees F at this point, will warm up from the coolant. It will take a while, but the ATF will get up to about 130-140.....where as a bypassed system, the ATF will stay down around 100-115.
Every other scenario I've witnessed (I'm talking serious miles, 20,000+ miles in both setups observing the ATF with a ScanGaugeII), what heats up the transmission is shifting. I can get the ATF up to 150 before the thermostat has even cracked open if I stop and go hard 10 times in a short period when it's cold outside. The radiator has nothing to do with it getting up to 150....just the fact that it's working and creating friction....which equals heat.
You can do what you want, I'm just trying to be helpful and say what I've observed in real life. People always want to say, "well, the engineers this and the engineer's that"....and that's fine. I'm not going to argue...I'm just stating what I see in real life.
Your last sentence, " If the trans cooler is bathed in the hot coolant, there is going to be some heat transfer via conduction. There's no denying the science behind it" is where almost everyone forgets that by the time the coolant is hot in the bottom of the radiator, the truck has already been driving for enough time to get the ATF warm all by itself. Except for the scenario I laid out above.
Well, all I know is the engine coolant reaches operating temperature and thermostat opens up wayyyy before my trans temps start to come up from driving. How much is the hot coolant helping raise the trans fluid temp, I don't know.
What I do know is people who have bypassed and didn't use an electric fan in addition to the cooler have seen some very high temps when pushing their rigs in different scenarios. Many of those people chose to change to running the external cooler in-series with the OEM radiator cooler because of it.
So, to each his own and run it like you want.