The sensor is for the temperature display. If it is unplugged or there's a cut wire, it will say "OC" for "open circuit", but it will still light up. Start by checking for 12 volts on any wires in the unit's connector with the ignition switch off. I seem to recall there should be none, but there might be one. Next, check if there's 12 volts on any other wires with the ignition switch in the "run" position. If there is, you likely have a broken ground wire. Most commonly the feed and ground wires develop corroded terminals in a connector down low. If there's rust holes near the bottom of the cab, that's a good place to look first.

Hi there thanks for the reply,but i do not think that i have a ground problem because i test it with a tester,because the roof lamp get its main power and ground when that plug is in the digital display. The display is just not coming on when truck on,it sometimes give u the direction of east or west.north etc and temp out side. is there a sender relay out in the engine compartment.

There's no relays and nothing under the hood that will cause the display to not light up. There's two different ground wires. One is for the lights and one is for the display. I drew a diagram of the connector to show the wires to test, (pins 9 and 7). The closest I have is a '98 service manual but it should be the same.

Hi again, well it nite time now so in the morning i will check both ground... and see if its grounded.