Brian;
There os a VERY GOOD reason for bypassing the in-radiator cooler
Debris left over from the previous disaster
It is sometimes inpossible to remove, even with a 'Hot-Flush' machine like a DCI
So bypassing it makes a lot of sense................and as far as running the transmission 'too cool', it's never been a problem here (western IL), we do it on EVERY 4x4 and every other truck that does anything more than haul a driver back and forth
Really HD units like the 4L80E, Ford E4OD, Mopar A518/618 get a pair of big coolers, plumbed in parallel, and for GM and Fords, a 50-50 mix of ATF and MobilOne 5-20 Synthetic Engine Oil (have to use Mopar ATF +3 or ATF +4 in Mopars due to converter clutch chatter)
*********************************************** Hi Warren. What Doc says above will keep you good for many years. One thing to add ,though it's been decades since I managed a few AAMCO shops,is that AAMCO used to install the aux.cooler in series with the original cooler and before it.I guess that was because ATF is designed to operate at certain temps and that the viscosity is set for this minimun.I personally tend to think that if you use just a stand alone aux. cooler that your tranny would be operating at too low a temp.much of the time and that this MIGHT affect shifts,lock-ups,torgue-converter operation etc. I don't think it makes much difference one way or the other and like I said it's been a long time so things may have changed.I don't know if current manufacturers ,when adding a tranny cooler option, leave the radiator section in play or not.If they do then I would too,assuming the factory engineers must know what they are doing. Since heat IS the tranny enemy #1 ,I'm sure either way will help. Best.......Brian p.s. For this advice all I require is a medium pepperoni ,easy cheese.
******************** There are three kinds of lies; lies,damned lies,and statistics.---Disraeli