Originally Posted by Vimes

I would suggest putting the aux cooler between the trans output and radiator cooler with the bypass only on the aux cooler. It will cool the fluid more before it hits the radiator, alleviating your concern about adding extra heat to the coolant, plus it will let the radiator stabilize the temperature of the fluid headed back to the trans. Transmissions like a steady temperature range, not varying temperatures, otherwise the bypass wouldn't be needed. I wouldn't put a bypass on the radiator cooler, nor would I bypass both coolers. If the bypass were to seize closed, the transmission would burn up in a huge hurry.