The location for Tans. Sender on a 7.3L 4R100 Transmission can be found just above the 2nd trans. pan bolt on the driver's side.  There is a test port plug here (arrow).  Remove it and insert the trans. sender.  No need to worry about excessive fluid loss.  Only a few drops will come out if the truck is level.  Image shown with an 11mm wrench in the plug. READINGS: ATF goes from the pan, thru the oil pump, and then past this port.  In essence, this location for the sender will give you pan oil temperature readings (or very close to it). Normal SAFE temps are anything below 200°F.  200°F to 225°F is the WARNING temperature.  Over 225°F is TOO HOT.