John Deere 40S Electrical Problem

If I remember correctly the switch positions are:
1. off
2. engine run
3. engine run and front lights
4. engine run and all light
5. lights only.

If the switch is good you should not have to add a switch. Positive ground doesn't matter to a light. If it never had lights the pigtail may have been cut off by the last owner. The lead to the front lights was in the engine wiring harness and came out of the harness in front by the coil. If it's been rewired, if may not have been added back. The rear lights was a seperate wiring harness and may not have ever been on the tractor, if it never had lights. It would be easy to add to both the front and rear leads.

You will likely have to remove the dash to get to the back of the switch to track down the wire color or to add the wiring for the lights. Use a volt meter to tell which terminals of the switch are hot in those position. I think the early 40's had a switch on the rear light to turn the built in tail light on or off. In later models it was in the dash switch.

This sounds like a short in the switch or wiring to that pole. Power to the switch comes from the ignition/battery circuit. If a pole is shorted, it will kill (short) the power to the ignition. Depending on the previous wiring, it should have blown the fuse.

I"m not sure about the 40S but on my 420C, the switch controls the headlights, rear light and rear flood.

Wiring for the lights should not be any different for + or - ground systems. The bracket/mount for the light is positive and the light lead will go to a switched circuit for positive ground systems.

For negative ground systems, the bracket/mount is negative and the lead will still go to a switched circuit.