Wiring 12V ?

A man at a local tractor repair shop had drawn me a wiring diagram and said it would work fine like that. Well I wired the tractor as the diagram showed and it starts and it seems to run fine. I was wondering why he wired P1 and P2 of the alt. together.

In your diagram P2 is only connected to the BAT terminal when the switch is on. That's ok but P2 will not drain the battery if left permanently connected to BAT. P1, on the otherhand, will drain the battery if permanently connected to BAT and that is the reason for connecting it to the ACC terminal of the switch (or using a diode or marker lamp in the line) .

Also, I put the ignition switch and the lights on the other side of the ampmeter. That way the ampmeter is only reading what is going into the battery from the alternator. Also, the ampmeter will read negative if the tractor is not running and the switch is accidentally left on or the lights are on. When I turn off the engine, I always check to see if the ampmeter is showing zero.

Actually, it appears that the common wire from #1 and #2 is connected to the ACCY terminal on the ignition switch, and while running the wire from #2 to a point as close to the battery as possible will give closer voltage regulation, it will work adequately just the way it is. Having a resistor, "idiot light", or diode in series with the #1 wire will prevent internal voltage regulator damage under certain circumstances (failure of the rectifier in the alternator), but for typical use on an old tractor, the system will likely work fine for years the way it is.