1998 dodge transmission temp. light...HELP!

[drwatson]

I just put a used tranny in my 1998 4 wheel drive with 5.9.
works great, the trans. temp light and the O/D off light keep coming on.....seems to be random, I blew the cooler out , that was fine, the check valve operates fine, my dad has a computer.....said engine temp was 212, trans temp was 298....however when i touch the engine oil pan, it is hotter than the transmission pan.....
i think it is a faulty sensor......because of this that it is random.
i drive it ten miles....lights are on.....shut it off run an errand , get back in........it is off again....then my overdrive switch works ....anyone have any clues?
where is the temp sensor?