How to select fuel sending unit?

[Viper]

Sounds like a poor connection somewhere. Make sure all your connections are tight and clean. Some are daisy chained from one gauge to the other so a loose connection could affect a few gauges. Check to see if there are main harness plugs, if there are, split the plugs and make sure the pins aren't corroded or partially pulled out.

[ColRon]

I agree with Viper. Start tracing and checking your wiring, be patient and take your time. In the end you may not only fix your fuel and tach problem but others that haven’t shown themselves along the way.

[jrafaello]

I checked everything and it all seems to be working. Shorting it sends the needle past full, as does removing the ground wire. So I guess I just need to cruise for a while and see if it moves.