Oil light not working

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Oil light not working

Post by thessler »

First post on this site.
I have a new to me 1956, 312. It sat for 15 years it took some work and I got it running the oil light on the dash worked fine.
Front transmission was gushing oil, so I pulled it and did the seal and bushing. Yesterday I got it running again and now the oil light doesn't work.
I do not know how to diagnose the problem, the wire is on the sending unit. I took it off and on a few times no change.
Can someone please explain how that circuit works so I can figure this out.
Thank you, Tom
T-Bird Bob
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Re: Oil light not working

Post by T-Bird Bob »

Hello Tom,
first: Are you sure it is plugged in?
Happened to me, the light bulb holder with bulb in the dash was fallen out of the hole in the dash. The oil light is not bright enough to illuminate the foot well, so you won't notice except by sticking your head under the dash - I hope you are good at yoga.

If that is not the case, the sending unit provides contact to ground for the light when the pressure is low. So you could easily test it with a stopped engine and a multi meter or test light. It should have low Ohms to ground. Or just switch on ignition and hold the wire to ground (block,heads, any bolts, etc.), then the light should come on.

If that doesn't fix it, you might just have a bad bulb - swap with the charge light if that works.

For any other test, you need a wiring diagram, a multi meter, a very flexible back, additional joints in your arms and patience. You need to follow the circuit from the ignition switch to the bulb, to the sending unit and measure voltage everywhere.
I ordered a reprint like this one:
https://www.macsautoparts.com/ford_thun ... -1956.html
It helped a lot!

Have fun!

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Re: Oil light not working

Post by CSavaglio »

The oil light is an extremely simple circuit. A black-green tracer wire provides power to the light socket in the dash, a white-red tracer wire goes from the dash to the sending unit. The sending unit is a switch that closes when there is low oil pressure, grounding the circuit to the block and making the light come on.

Not much to go wrong in the circuit....

Bad wiring/connections
Bad sending unit that's staying open all the time
Bad/corroded bulb socket
Bad bulb

That's it. You can test the circuit by disconnecting the connecting at the sender and grounding it. If it lights up, everything in the circuit works except the sender. Otherwise, get the multimeter out and check for voltage at various spots with the key on. Should have 12v everywhere with the neg lead of the multimeter grounded. If you find a point that doesn't have voltage, set it to ohms and test from one end of the wire to the other and look for open spots in the circuit.

More than likely, if nothing was messed with...like messing around behind the dash, it's likely the bulb is just bad although old wiring is always suspect...just driving around can create faulty connections if the wiring is no good. If the socket came loose, it will still light up since it grounds thru the sender and not the cluster, so you can turn the key on in a darkened garage and look under the dash to see if it's lighting up.
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Re: Oil light not working

Post by paul2748 »

I just changed the sending unit on my 56 (the car has been sitting a while). The light didn't do on when I turned the key. Since I didn't do anything under the dash I suspected the sender. Bought a new one from CASCO (15.00 including shipping) and installed it. Light working again.
1956 Fiesta Red 312
1954 Ford Victoria 312
1948 Ford Convertible Street Rod 302
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Re: Oil light not working

Post by thessler »

Thanks guys
I figured it out. The socket fell out of the dash, it always worked.
I did check it before I posted but it turns out I was looking at the wrong socket.
Thanks, Tom
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