If the 2010 Ranger is like the earlier models, it will have two coolant controls; one is a "sensor" and one is a "sender". The coolant sensor is used for the ECU so it can detect engine temps for things like when to set the idle down on a cold engine start, and switch over to other engine sensor readings for fuel/air mapping.
The coolant sender is what controls the operation of the dash coolant gauge. Either this sender is bad, the wire from it to the gauge cluster is bad/shorted, or the gauge itself is bad.
On my 2000 Ranger, I'm able to test the sender/wiring by disconnecting the single wire to this engine sender, and grounding it. With the key to on (not starting engine), the coolant gauge will peg all the way hot; with the ground removed, the gauge needle will be parked on the coldest point. This tells you the gauge and wiring are good, and the sender is bad.
Last edited by bucko; 12-19-2014 at 03:26 AM.