Oil light on

[1965 fastback]

The oil light on the dash of my 65 turns on upon the first turn of the key and stays on when the car starts and engine runs. I’m certain this isn’t normal. A wire is connected to the oil sending unit.

Does this mean the sending unit isn’t working? I actually don’t know how the sending unit works…or it’s there another reason why the oil light stays on. I guess the bulb behind the oil light could be in the wrong place?

[Clubman7]

Two reasons i can think of.
Sender is faulty.
You’re out of oil.
New sender is easy to swap in.

[GEOFF289]

So you've been driving it around with the oil light on? if so, stop until you've got to the bottom of it. As well as the possibilities Brett suggests you could add a dud oil pump or some sort of restriction in the oil galleries including in the sender itself.

The sender in a '65 is a simple switch. When the ignition is on but the engine isn't running it is registering zero oil pressure and energising the globe in the light. This continues briefly once the engine is started until it registers the preset pressure and turns the light off.

Certainly replacing the sender is simple and relatively cheap as Brett suggests. I have an oil pressure testing kit you can come and borrow if you like. You screw this into the little extension in place of the sender and it directly reads the actual hydraulic oil pressure. If this shows you have satisfactory oil pressure the problem is the sender or the gauge. You might find the sender is all gunked up where it screws in and cleaning it up might be all that's needed.

[1965 fastback]

Thanks Geoff and Clubman,

Pretty sure it’s the sender because I have put about 400 miles on the engine since installing. Everything sounds like it is running well with no noises.

I always thought I had the wrong light bulb in the instrument bezel which is why it stayed on… probably not that.

I watched the below YouTube just now, which provides a diagnosis process for the light being on.

https://youtu.be/85jCa0WS9z8

First test was to check power to the cable that attaches to the sender with the ignition on. This did not register on mine.

[pmb0186]

Easy to test
Turn the ignition to on
Remove the lead from the sender light should go off
Short it to the block light will go on
If as above replace sender or no oil pressure 🙁

[66FBK]

Brave bloke driving with your oil light on. If you have traveled 400 miles than it will be a faulty sender. You would not go much further than out your driveway with no oil pressure.  As explained, with the ignition on + 12v goes to your oil warning light and -ve goes to your oil pressure sender to earth of you engine. As soon as your engine has oil pressure this simple switch opens and the light goes off. Removing the wire off the sender should turn the light off unless you have a stray -ve (earth) on the cct to the light. 66 and on use a graduated sender which send varying voltage say from 0-12V to the oil pressure gauge to measure the pressure. Your setup measure pressure or no pressure.  Senders are cheap $20 Oil pressure   No oil pressure 

[vinverify01]

I searched online and found this.

The oil sending unit is responsible for monitoring the oil pressure in the engine. If it's not functioning correctly, it can cause the oil light to stay on. The unit may be sending a false signal, indicating low oil pressure when it's not actually the case. You can look for 65-70 Oil Pressure Sending Units in Sydney Mustang Parts, cost round $56.

[jiffy]

This

Easy to test
Turn the ignition to on
Remove the lead from the sender light should go off
Short it to the block light will go on
If as above replace sender or no oil pressure 🙁