Other possibilities, all relating to the new sender.
(1) Be sure you got the correct sender for your vehicle. The sender for idiot lights is a simple ON/OFF switch, while the one for gauges is a variable resistor.
(2) Be sure you got the correct sender for your year. In 1991 Chrysler reversed the polarity of the gauges. What this means is if you take the sender for a 90 and put it in a 91, it'll start off reading very hot, and then the reading will go down as the vehicle warms up.