Voltage coming out of Oil Pressure Gauge

[Odoyle]

Recently installed a "used working" veglia oil pressure sender into my 77, which had an oil pressure gauge with no sender from the factory. Oil pressure is around 40 psi on start up and around 50 when at speed, but my question is what should the voltage be at the G/B (grey and black) wire that connects to the oil pressure sender? I measured mine and got 7.5V coming out of the wire which I though was low. I tested the voltage going to the oil pressure sender in my 83 and the voltage was 12V. Could the oil pressure gauge be faulty?

[18Fiatsandcounting]

The oil pressure sending unit doesn't produce any voltage of its own, but rather acts as a variable resistor to control the current that flows through the oil pressure gauge and thus moving the needle. The two ends of this "resistor" are the ground (engine block where it screws in) and the G/B wire you noted. So, if the resistance dropped all the way to zero on the sending unit, the G/B wire is fully grounded and the gauge would read maximum deflection even though the voltage at the G/B wire would be zero. In other words, the voltage at this wire doesn't really mean anything.

40 psi at startup and 50 psi while underway sounds about right, so I think you're good.

-Bryan

[spider2081]

Could the oil pressure gauge be faulty

I don't know if the gauge has an issue.
It appears the sending units for early spiders and 83-85 Pininfarina's have different part numbers so maybe the entire system is different. Maybe not even the same manufacturer.
Do both gauges indicate o psi when the engine is not running but the ignition is on?
You state you measured voltage on the gray/black wire for the sender. Was the wire connected to the sender when you measured it or was it disconnected. Also did you measure the voltage going into to both gauges? If you did was it similar? I would expect both cars would have battery voltage feeding the gauge when the engine is not running.