Test the gauge by grounding the sender wire and see if the gauge pegs high with the ignition on. DON'T leave this ground connected long, just long enough to see the gauge rushing to the peg. Control current with the ignition switch. A mechanical test gauge with a BTP thread into the sender hole and cranking the motor with a starter button hooked to battery cable at starter and starter solenoid wire spade connector will let you lie under the truck and see the gauge as you crank with the ignition and truck electrical system off. Did you drain and change the oil?