TD5 oil warning light conundrum

[oldapprentice]

I have a 2003 Defender TD5, 92k miles, runs sweet as a nut.

This week the oil light came on, and stayed on. Checked the oil, full. No nasty engine noises.

I was suspicious that this wasn't actually an oil pressure problem, so connected a pressure gauge (which was last calibrated July this year) and started the engine. On idle, cold, was 45 psi. On increasing the revs a bit the pressure went up to 50, 55 then 60 psi as the revs increased. I didn't take it any further than that as the pressure looks ok to me and compares well with the spec in the workshop manual. So, the oil pressure was ok......

I bought a new oil pressure switch (correct LR/Lucas part) and fitted it, to try to eliminate that from the equation. This did not solve the problem, as:

- the oil warning light came on when the ignition was switched on (engine not started) but went off a couple of seconds later.

- I pulled off the connector from the oil pressure switch and tried again. Same result; the oil warning light came on when the ignition was switched on (engine not started) but went off a couple of seconds later.

I checked the continuity of the wire that runs from the connector behind the instrument panel to the connector on the oil pressure switch and it buzzed out ok.

So, I have an engine with bags of oil pressure but an oil warning light that is pretty useless.

Has anyone else had this problem? Would be very pleased to receive your advice as I am now a bit stumped.

[crwoody]

It suggests to me a short to earth somewhere along the wire from the sender to lamp, perhaps one that's aggravated by the movement of the engine when it's running?

It's a very simple circuit with a 12v feed to the lamp, a wire from the lamp to the switch then the switch to earth via the engine block, so if the lamp is coming on when it shouldn't it can only be a very few things.

Have you tried checking the white/brown wire resistance to earth with both ends disconnected - engine running/stopped?

Maybe also to check the pressure switch connector resistance to earth with the wire disconnected, again with engine running/stopped.

The only other thing I could think of here is some kind of blockage in the oil feed to the pressure switch but I would have thought that any problem here would have been cleared by removing/replacing the switch and pressure testing.

[oldapprentice]

It suggests to me a short to earth somewhere along the wire from the sender to lamp, perhaps one that's aggravated by the movement of the engine when it's running?

It's a very simple circuit with a 12v feed to the lamp, a wire from the lamp to the switch then the switch to earth via the engine block, so if the lamp is coming on when it shouldn't it can only be a very few things.

Have you tried checking the white/brown wire resistance to earth with both ends disconnected - engine running/stopped?

Maybe also to check the pressure switch connector resistance to earth with the wire disconnected, again with engine running/stopped.

The only other thing I could think of here is some kind of blockage in the oil feed to the pressure switch but I would have thought that any problem here would have been cleared by removing/replacing the switch and pressure testing.

Thank you crwoody for your advice and the diagram. I'll go and check (when it stops raining)