[Aarongriffey]

Why would a six cylinder passenger car take a different sending unit than a V8?
Just because a gage or sending unit is 6 volt doesn’t mean it is comparable with every 6 volt gage or sending unit.
I can only suggest what others have already posted, especially what Kube has said.
Positive or negative ground also makes no difference.
They work off from ohms.
Keep us posted on what you find. I for one would like to know what the prblem is.