New gas gauge troubleshooting

I'm wiring my 51 GMC from scratch... I bought a new gas tank, sending unit and gauge from classic parts. I have it wired according to the wiring diagram out of the original shop manual. I have power from the ignition switch to one post...a wire from the sending unit to the other post and a ground wire from a mount screw on the sending unit to the frame. The needle on the gauge is on 1/2 full. It was that way out of the box. The old gauge had a red tag on it which says not to connect power to that post. I took the gauge cluster apart and don't know which post it was on. The new gauge didn't have that tag. Which side should I connect the power to? I connected the wires and the needle never moved - at all. I switched the wires and the fuse blew. Any ideas? Did I blow the gauge? Maybe it was bad out of the box?

 

ground the sender side to a good ground--touch it on--should go to full--if not, gauge is NG

 

R U running 12V in the truck? The OEM gauge is set for 6V. You will need a reducer for the gauge if that is what is wrong. IIRC, I put a simple resistor in series with the power post of the gauge and the power wire itself. Accurate enough that way.

Another thing to mention is that the gauge must be physically grounded to work. This is a balancing coil gauge and has two coils, which have one side grounded to the gauge frame.

 

Didn't your GMC originally have positive ground? Is your new guage compatible with that?

 

It's a new build - negative ground and 12 volt. The gauge is a 12 volt gauge too. I tried grounding the sending unit side of the gauge like olscrounger said and got nothing. The lights worked so I know the gauge cluster was grounded. I'll play around with it again tomorrow. Maybe I'm missing something.

 

Got it figured out. I painted the inside of the gauge cluster when I rebuilt the gauges so it wasn't grounding - a little sandpaper and I'm good to go!!! Thanks for the replies.