Oil light always ON in Ford 2000 diesel

Howdy,
I am trying to catch up with a few issues that I have let go in this tractor. My oil light has been ON for a couple of years. When I disconnect the wire from the sending unit (oil pressure switch) the light remains on. Should it go off? When I test connectivity between the wire that connects to the sending unit (after disconnecting from the sending unit) and the tractor's frame, I do have connectivity. Should I have connectivity?

light should go out when wire is disconnected. the sending unit grounds to complete circuit. sounds like wire is grounding before it gets to bulb

12v is applied to the instrument panel module and the sensors (light bulb in this case) is/are looking for a ground. The switch (oil pressure sending unit)
should be short circuited, illuminating the oil press light until oil pressure is up to around 8# or so where the switch opens and shuts off the light.

If you pulled the wire off the sending unit and the light is still on, you have a short in the wiring harness or on the rear of the instrument module.

Pop the module and remove the oil pressure input wire. If the light goes out with the key on, the short is in the wire between the sending unit and the
inst panel.....rats ate the insulation off the wire, or age did it's thing and the insulation cracked and fell off.

Otherwise the problem is in the instrument panel...circuit card may have gotten wet, or contaminated with dirt and all and shorted out.

I bought 2 recently off ebay, brand new for $80 ea., free shipping, one gen. driven and the other alt. driven, tach zero on opposite sides. Solves a lot of
problems and looks great too. I don't know what they sell for at YT. Maybe I should have looked there first. I do a lot of ebay shopping. Just went
there out of habit.

Thank you both for the good suggestions. I pulled the instrument panel and could not tell for sure which one was the wire coming from the oil sender. Nevertheless, I pulled one wire out at a time to see if the oil light bulb would go out and it never did. Thus, it sounds like I have a short? If I do have a short is there an easy way to find it. It looks like a big mess of wires. I am 72 and may not have enough time left to find it. Should I invest in a new harness rather than try to find the short?

Do you have multimeter? If there is continuity to ground on the unplugged wire to the sender,which if I understand correctly,you checked,that wire has shorted to ground.Texasmark1 has it correct.What I would do is identify that wire thru color or number back at the instrument panel,and unplug it there.Recheck for continuity to ground(short to GRD),if it still is shorted,run a new wire from there to the sender.Easiest fix. If it is in the dash,on a printed circuit,pull it apart and inspect that also.There should be a ground to the panel,and you could check the circuit against it.Mark.

Excellent idea. Makes perfect sense. Thank you Mark.
I am not going to ask "why did I not think about that" because I am afraid of the response.

Well in a couple of months I'm hitting 77. I do feel the drag of old age knocking at my door. Seems "seeing the forest for the trees" is getting to be
more and more of a problem. Sometimes I put up with things for a long time and one day get tired of it and in looking at it decided to fix it and the
solution was easy and usually inexpensive......I sit back and look at it and ask myself just why did I put up with that for so long. Answer is usually
because it wasn't priority one. Okaye.

Other thing is that my mind gets to running several yards ahead of the body. Mundane tasks are forgotten before they are accomplished and that is
getting to be a problem........let's see, where did I put the keys to the truck.....where did I set my coffee cup.....

Seems you are doing quite well mentally. I guess helping people in this forum troubleshoot their problems keeps you sharp. Good strategy. I might try something like that (won't be tractors).

I read that Alzheimers progress "may" be retarded by mental exercises. Won't hurt. I don't have it nor a history in the genes. Cross word puzzles,
trying to remember things that I knew years ago , helping folks solve problems within my abilities. Surely won't hurt anything. What I find interesting is
that the more I work it the farther back it can pull up things I haven't touched for 60-70 years, in a flash. Kind of fun....crosswords real good at recovery
work.