Here's the circuit that you need to verify proper resistance values:
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0 ohms should give you 0 psi indicated.
45 ohms should give you 40 psi.
90 ohms should give you full scale deflection.
The tan wire physically runs from oil pressure sending unit Pin A, through C200 in the firewall, to Pin 15
at the Instrument Cluster connector. In order for that electric Oil Pressure gauge to work properly,
you need solid power and low resistance ground to the instrument cluster, also the correct 0-90 ohm
resistance coming in at Pin 15. I've attached the Theory of Operation blurb for the Oil Pressure Gauge
taken from the FSM.
Since you've already replaced the sending unit, I'd try to monitor the oil pressure signal and see what
kind of resistance readings you get. And if that gives you rational readings, I'd want to see those
same readings at the instrument panel connector, pin 15.
Q: Does this wiring diagram help?