I have a 1994 ford ranger and my tempature gauge is constintly going up and down?

When I drive the temperature gauge will rise up above the half way point and then it will go down Almost all the way and will just keep doing that until I turn off the truck. Anyone know what could be the cause of this?

Two things most probably are a bad thermostat or simply low on coolant. If coolant is low, find the leak and fix it. If its full replace the thermostat then add coolant to the radiator until full, wait a few minutes and add more as it will keep going down for a few times as air is displaced. When full, add more coolant to the recovery tank, drive and check it for a few days to see that it stays to normal level in the recovery tank as there will be more air displaced as the engine heats and cools. This same refill procedure applies whether you replace the thermostat or just adding to top off a low cooling system.

Faulty thermostat?

The thermostat should open and close to regulate the engine temperature. Part open once the vehicle is warmed up. Fully open under a heavy load, or when stopped in traffic. Part closed when under a light load at highway speed.

If the thermostat is stuck 1/2 open then you have some cooling, but the engine may run too hot or too cold depending on how you are driving.

It's also possible it's a head gasket problem, but that's serious and will usually have other symptoms.