Gauge Troubleshooting

[LILREDGMC]

Put an untested deluxe gauge set in an existing deluxe gauge truck. The untested cluster includes a tach. First the tach doesn't work or doesn't appear to, one lead to negative side of coil, second to fuse box for 12V. Do not know if the tach ever did work. Any bench tests known by anyone? ? I ran the 12v to the fuse box and piggybacked the panel lights to it. Also, the temp gauge will go full scale, with key on. Previous it read correctly. Any thoughts? Used to be ground would be ? full scale ? or zero?, and open would be the opposite. And last, the wire(black) that runs down the column with a light socket on it, goes to the upper right side of the gas gauge, correct or if not where? Majot league brain fade. Thanks for the assist. Obviously the tach dash looks great, but it would be better if it worked as well.