does the needle move at all? how is your heat?
there's a coolant temprature sensor on the block that sends the signal to the cluster. it could be screwed, or the wiring might be disconnected/bad.
otherwise, the thermostat might be sticking open. with the big ol' fan on the front of the rangers and some cold weather your tuck might not be heating up at all. (it sounds weird, but i had a 92 chevy 1500 in the shop the other day with this exact problem)