Installed a trans temp guage in the Satellite today. At the advice of my trans guy, I plumbed the sending unit into the out line, instead of the pan. Went for a quick drive and about freaked out! Temp guage went to 150 as soon as I started the car (cold), and went to over 250 on a 5 minute drive! . Back in the garage, I checked the fluid at the dipstick; it was barely even warm. felt the cooler lines, they were nice and cool. The guage can't be right (??).What gives?

It's an auto-meter electric guage. wired from sender to (S) side of guage: (I) side of guage to 12v source at fuse-box. pretty straight forward. What could be wrong here?