Rover V8 oil pressure switch

[HealeyV8]

I have a SD1 3.5 V8 installed in my Healey. Because the car has a capillary Smiths oil pressure gauge I have this plumbed in.
I recently replaced the housing and this one also has the single prong oil pressure switch. I thought as it's there I would install a warning light.
I assumed the male prong on the switch would be earth to the light bulb. So testing I ran a cable from switch to earth on bulb and connected a live straight off the battery positive thinking the lbulb would light as the engine was off, so no oil pressure. But it doesn't. Am I doing this wrong or do I have a defective oil pressure switch?

[GreenV8S]

A normal oil pressure warning switch with a single terminal will connect the terminal to the engine when there is no oil pressure. Assuming the engine itself is grounded and you have 12V connected to the other side of the lamp, this would complete the circuit and light the lamp.

As a sanity check have you taken the wire off the switch and connected it directly to the engine and confirmed the lamp comes on?

[finlo]

Not sure if related to your problem but SD1's fuel pump circuit goes through the oil pressure switch thus extra connections, had to short mine out to keep mobile with a tired engine.

[HealeyV8]

Thanks guys, must be faulty switch then. There is no existing wiring, this engine is a custom install. The original pump housing didn’t have a switch just a mechanical pressure line to smiths gauge.
The replacement housing had the single prong switch so I thought i’d Test it and fit my own warning light. Before drilling hole in dash for light.
So I connect a wire direct to pos battery to a bulb and then a wire to switch to other side of bulb.
Doesn’t light up but does if I take off prong and connect to engine.
Was just checking my sanity that engine didn’t need to be running.