On Sunday, 15 April 2018 12:56:29 UTC+1, JimK wrote:
Graeme Wrote in message:

On to the next little problem - oil pressure warning light always on. I
fitted a new pressure switch a while ago, which made no difference so
checked today, and found that the light is on (with ignition) even when
the wire to the sender is disconnected. Inspected the loom, with no
obvious damage, which suggests a short to earth somewhere in the rat's
nest behind the speedo. Bugger.

Being a late (68) model, there are two yellow oil warning lights, one
for low pressure and the other showing clogged filter. I have never
seen the clogged filter one lit in 22 years, so tested it by shorting
the connection to earth, and it works.

Well, the lamp does.
--
Jim K


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The oil pressure warning light is connected and"live" when the ignition is on.
The oil pressure switch just earths the other leg of the circuit. It opens when oil pressure rises.
So if you disconnect the wire to the oil pressure switch, the lamp must go out.
Unless the wiring is mis-wired or there is some strange fault.
You need to determine which side of the bulb is earth and replace that wire which runs between that (earth) side of the bulb and the oil pressure switch. & see what happens.
This wants looking at urgently.