Before I tore into my truck one thing that did work was the oil pressure gauge. Now that I have my rebuilt engine in and rewired I cannot get the gauge to work. I'm looking for possible issues. Here is a little more information:
55' 239 Y that is 6 volt negative ground. Truck was negative ground when it worked before and I left it negative ground. I followed the wiring diagram to a "T". Do you think I have to flip flop the wires on the gauge? I'm confused though because the temperature gauge is working and I wired it according to the diagram and they share a common wire. Any advise from you professionals would be awesome.
Also, why does the sending unit have two prongs on it? I only remember there ever being one wire attached to it.

 

Did the ammeter work correctly before? If that schematic is for a positive ground system you need to reverse the direction of "6Y" where it passes through the ammeter loop.
The two spades on the sending unit are common to each other, take your pick. Are you using Teflon tape on the threads, I think I see a hint of it. Don't use it, it's an insulator.

 

The two prongs makes no sense, makes me think it is not OEM. But I can't think why ANY OP sender would have two prongs.

Pos / neg ground doesn't matter. Check for power at the gauge, may just be a bad connection.

 

You know what Bob.....I did use Teflon tape......4 or 5 wraps! LOL That's the plumber in me. That will be an easy fix. I have the oil drained now so will be a day or two before I test this theory. It does make sense Thank you. Let's hope that works!

 

Ok. To all you Rookies out there. Do NOT use Teflon tape on your oil sending switch threads Bob (Mixer man) bailed me out on this and it is greatly appreciated. Also, thanks Ross for your words of wisdom. I have an oil pressure reading

Edit- I tried to add your reputations but it would not allow me too. It said I need to spread it around. That's BS

 

1948/55: 6V positive ground, 1956 thru today: 12V negative ground. 6V use much thicker battery cables than 12V

 

Ross, the sending units on both my 54 and 55 have 2 prongs.