D plug wiring

[jg123]

Always just used positive and negative for a d plug but putting another socket in the cab and wondering what the 3rd wire is for, the tractor is using 3 with the other sockets. Looking at this diagram is it 12v to activate a relay in the control box and some boxes require it?

 

[spark plug]

The 3rd wire is a switched live from a relay, I would imagine a switched ignition live. But without seeing the rest of the wireing diagram I’m not sure as it doesn’t show what is switching the relay.

 

[jg123]

Ye just checked now, 3rd pin in socket is switched live from ignition, the other is permanent live. Guessing lots of boxes don't need it as iv not wired it in the past but I'll stick it in just incase

 

[Gerbert]

On our john deeres the big one is switched, had a massey ferguson on demo and it had permanent live.