1990 fuel gauge not working 15 gallon tank with 4.2 motor

I don't think real power goes to the sending unit. The fuel gauge is really just an ohmmeter. It is powered by the ignition, but it just senses the amount of resistance in the sending unit. Don't put power to it, it can make the resistor coils glow red hot in the tank.

if you replaced the sending unit and the gauge and it still doesn't work, I would first look at the sending unit connector from the existing harness. That side of the connector may be corroded and need cleaning. That would add much resistance and screw with the reading. Next I would look at the harness connector that goes to the gauge, same thing, needs to be shiny metal and squeeze on tight. I clean them with a small piece of 400 grit paper rolled up and shaped to fit. It loads up quick if there's corrosion, so use a new piece if you need. Hose it with some contact cleaner and the put corrosion preventer on and snap it together.

Also check that the tank is properly grounded. An ohmmeter should read very little resistance between the tank (bare metal) and the frame. If it is high, clean the ground.

I second on the voltmeter. Check your grounds. I have had similar problems, and found it was faster just to rewire the the sender to the gauge. Are your gauges illuminated?