Oil light coming on but pressure is good and can't find a short (solved)

[oldcarnut]

My oil light came on while driving down my street.  I checked the autometer gauge and pressure was good and where it always is.  Turned around to go back home and checked for a loose wire or a short in the wire to ground and changed the sending unit but no fix.  Odd thing is that the light is off when the ignition key is on but comes on as soon as it starts.  I ran a bypass wire from the sending unit to the bulk head connection to verify that the firewall wire harness was ok and it was not the fault.  Checked all under the dash including behind the bulkhead and all the wiring is looking good in there too.  I pulled the bulb to check it and didn't see anything wrong with it.  I'm running out of things to check so anyone got any ideas to follow up on? It seems to be working the opposite of how its supposed to be. Otherwise everything else if good.

[Cooter]

Try a sending unit for a Light instead of one for a factory guage...

[resq302]

Thats weird.  All cars I know of with the engine off and the key in the on position, the oil light should be on.  Almost sounds like something is backwards.

[oldcarnut]

I'll try another sending unit. Thinking about it I swapped in the the new one I had put on the RR motor I pulled but it did have the gauge instead of a light. Didn't think about it at the time as I was wanting to take t trip and running a little late.  Cooter might be on to something. Prior to the oil light coming on nothing had been done/changed.to the car.

[Nacho-RT74]

Try a sending unit for a Light instead of one for a factory guage...

if you have both conected at same unit that could be the problem. ( of course if the autometer gauge is electric )

IF you ground the oil light wire, light must coming on with key in RUN and engine off

[gtx6970]

couple quick tests are
Key on / engine off - sender unpluged- light should be off / or if it's a gauge should read zero - if it's on you have a grounded circuit
Key on / engine off - sender plugged in - light should be on  / or if it's a gauge should read zero - if it's off you have an open circuit

Key on / engine off - sender unplugged - probe the sender wire connector with a test light it should pulse . if no pulsing light  you have either an open circuit or voltage limiter issues.

Key one / engine off - sender unplugged - ground the sender wire to a good body ground. the light should be on or the gauge should max out

and as mentioned oil pressure senders are different for gauge versus light

[oldcarnut]

Cooter made the call    Put in the correct unit and it functions as normal.  Must have had a brain freeze wanting and trying to make the 2hr trip to a car show on time and forgot about the other being for a gauge .  Thanks to all for the help   and now I can feel comfortable about putting the car on Ebay this weekend without that issue.