[77newell]

There are four things required for proper operation: 1) 12vdc supply, 2) ground connection, 3) signal from alternator (probable source), 4) the gauges internal circuits are working.

I would check the connections on the back of the instrument to make sure they are all connected and then verify voltage and grounds. If those are OK then check for voltage on the signal wire with engine running. If this checks OK, replace the gauge.