87 chevy truck oil pressure gage wiring

I have a 87 chevy truck and the oil pressure gage pegs to the right when I turn on the key.Number 6 wire on the gage harness is hot when the switch is on and the other two post move the gage in either
direction when grounded,this truck is somewhat cobbled since the cab was replaced with a 83 cab and the engine which runs fine is a 87 fuel injected 350.I would like to know the technology behind how this gage works,Thank you for any support you can give.
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The OP sending unit is mounted on the engine block and monitors OP. The piece is an electrical device which uses engine ground through resistance for gauge function. This factory system is not as good/accurate as a mechanical oil-pressure gauge. You would be spending smart to install a real OP gauge.
Dave S(Texas)
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