Does anyone know if there is a diode or something in the gauge cluster that controls the needles? I used to have a ranchero which had fuel gauge problems and the tech told me about a diode that goes bad. Well on my 94 the water temp quit and I replaced the sender, connector looked good as far as I could tell. Ran the truck and all the gauge indicated is to the top of the 'C' hose was not screaming hot but was warm where I think it should have been at the normal line.

Any help appreciated.

 

There is a voltage regulator mounted to the backside of the cluster. It is a sort of 'chopper' or 'buzzer' that cycles on and off to deliver a chopped, reduced voltage to the gauges. The 12v system is actually a lot more variable, and without the regulator, the gauges would wander based on the then-current voltage, headlights, wipers, blower, etc all on at idle, or nothing going down the highway. The gauges would better be called guesses in that case, so they limit it to ~7v by chopping. If it goes, the gauge will wander based on rpm, in general.
Your sending unit may have more resistance than expected. If you ground the one-terminal sending unit connector on the block, the gauge should swing quickly (QUICKLY!) to full Hot reading. Remove the ground, and it should swing back to Cold. That tests the gauge, wiring, and regulator for function. Repeatable accuracy depends on the gauge and sending unit, as modulated by the regulator.
tom