This is on a Series 3 V12 engine, front left thermostat housing. The temp switch (?) is in the harness, black to ground lug on one terminal, green/orange into the harness on the other. What does this switch do for the car?
I'm converting to EFI; I don't think I need it any longer, but not knowing what it is I'm not sure how I came to that conclusion.

 

I am not familiar with the V12 engines. But judging by the location, it is the temperature sender for the temperature gauge in the dash board. If you have a volt meter, check the continuity of the green/orange wire to hopefully a similar wire at the gauge. Your new EFI system may need that input.
Good luck and let us know how it turns out.
Bill.

 

Right. That's the thing, the normal culprits are accounted for. The gauge temp sensor is on the RH water manifold, the fan temp switch is at the bottom of the radiator. The EFI temp sensor is on the XJS donor manifold that replaced this one. What else on a carburetor engine needs to know the temperature? The switch is closed on the bench, and is marked to suggest that it removes a ground at 70C from some system. What would that be?

 

I can't imagine what control in a carburated engine would need to know the temperature other than a cooling fan auto On/off switch. It may be that the ground removal at 70C sends a signal for the auto On switch to turn on the cooling fans.
Bill.

 

All right. Solved.
Jeremy at SNG Barrett knew immediately, in spite of the omission of the switch on the SNG diagrams. It is part of the emission control system, fitted only to late US/CAN exports. I operates a vacuum valve that modifies the operation of the distributer vacuum retard above 70C. It was so late in the production run that it did not even make it into the service manuals and is not on the wiring diagrams. And best of all I don't need it in the EFI conversion.. And it is NLA in any case. I'll clean this one, test it and make available for a purist.
Well done Jeremy.