Help me troubleshoot my oil pressure guage

[72farmer]

I have a 87 Silverado with a LS swap engine and trans.My electric oil pressure guage has never worked.I tore in to it today and I need some help.
The guage needle does not go back to zero when you turn it a little and let it go.It will go all the way around without stopping anywhere.There is resistance when turning the needle.How does it work on voltage or resistance?Do you think I need a guage or a sending unit on back of motor?I read 12VDC on one of the three terminals in dash.