Hi Mike - if you don't already have a battery disconnect switch on there, it might be a good idea.
I like this style:
Top Post Battery Master Cut-Off Switch | Princess Auto
(Canadian prices are usually 2x USA ones, even with our dollar at par or better.)
For an old diesel, you won't need the bypass fuse - gives you a fuse holder to use somewhere else.
The green knob has a threaded stud on the bottom, that makes the electrical contact when tight. Give it a turn or 2 loose, and the battery is totally disconnected. Unscrew the knob and remove it, and somebody would have to remove this fitting and reconnect the battery before they could start the tractor.
On many older tractors, add-ons often get wired directly to the battery, as opposed to the ignition switch.
Until you get all the wiring sorted out, this would help you avoid draining the battery, or letting the smoke out of the wires !
Always important to remember - electrics run on smoke - once you let the smoke out of the wires, things don't work anymore !
Rgds, D.
