That does sound like the switch has failed in the open position. The light coming on when the wire is shorted to ground confirms the rest of the simple circuit is working.

No, the light does not come on with the engine not running.  It will come on if I short the sensor wire to ground.

Thanks, I'll give them a shout.

 

An open circuit with the engine off would definitely be a problem.  A "normal" switch will be closed until its opening pressure is reached.

The sending unit you found may work and even with shipping it looks to be less expensive than where I told you to look.  I would email Somerford and ask them if they can confirm that sender will work with the gauge you have (send them all the numbers on the gauge face).  My past experience is that they have been good about responding to correspondence.

Except for the Minis equipped with an Oil Pressure gauge, most had just the "idiot" warning light. The pressure switch for those was just an On/Off variety. Engine running would produce an open circuit (pressure pushes open the switch) to turn the light off. Engine not running (or loss of pressure) and the switch closes and light comes on. Light flickers at marginal pressure as the switch is toggling open/closed/open/closed with pressure fluctuations.

Yours is likely this kind of pressure switch.

Does the oil light comes on when just turning on the ignition (engine not running)?

Thanks for the info..., so this one should work.  

I'm thinking of going the mechanical gauge route with the T-piece for the pressure switch for the warning light.

As for my current oil pressure switch, I think it's NFG.  When I measure its resistance it indicates an open circuit.  

Measure the resistance of the sending unit with the engine off and then running. The sender you have could be just for an idiot light.

The only British cars I have worked on that had electric pressure gauges were some of the MGBs.  Most MGB owners I know who have them would rather have the mechanical oil pressure gauge because the replacement sending unit is quite expensive. 

See:
//www.gaugeguys.com/Smiths/classic.htm
Half way down the page you will find the current version of the electric oil pressure gauge.  Then they direct you to the sending units (accessories) at the link below:
//www.gaugeguys.com/Smiths/accessories.htm
700 kPA is just over 100 PSI so you would need to select a 100 PSI sending unit and for the Mini... one with 1/8" threads (so probably PTR1000-6-10 @ $82). 

Perhaps if the seller knows what car your gauge was originally designed for you could find a used sending unit for less somewhere.

I and am wondering if anyone could help me with an electrical oil pressure gauge I won on Ebay.  Here's the link - //cgi.ebay.co.uk/ws/eBayISAPI.dll?ViewItem&item=350465814780&ssPageName=STRK:MEWNX:IT#ht_500wt_922

Any idea how I can find out what pressure sender I would need to get to make this work?

I hooked it up the the one presently in the engine but it doesn't deflect at all.  It will deflect to full range when grounded.  Is there anyway I can check to see if my current oil pressure switch is functioning?