On my B, early on I would pull the throttle all the way down to the first notch to let it die. I did find though that the battery would be dead the next day. Surprised me because I thought all the ignition switch did was ground the coil to kill the spark. But, switch off and the battery stays up. 

Then I got to looking at electrical schematics for B and C tractors. I found a difference in one small little detail. On some prints the switch was labeled "magneto switch", on some it was labeled "ignition or magneto switch". Some prints show a circuit ran from the negative side of the amp meter to the switch and some didn't have this circuit. 

I guess the reason I'm really asking this is my tractor died yesterday. No spark. I haven't started looking into it yet but I figure the switch is just bad. Looks to be original. But the original issue I had of the tractor battery draining if I let the switch on has me wondering why that is? 

I have the generator with the relay on top

To my knowledge, bone stock. Still 6 volt positive ground. I recently had the original generator rebuilt and got it back up and running. Hasn't been modified. Original electric starter. Year built is 1942

 You still don't identify what year this is, you are your assuming all years are the same. Your B tractor id is right there by the Shifter, It's not hard to miss, look it up and it will tell you what year it is and stock.

http://www.tractordata.com/farm-tractors/000/0/0/3-allis-chalmers-b.html" rel="nofollow - http://www.tractordata.com/farm-tractors/000/0/0/3-allis-chalmers-b.html  

This might help you find the info you seek.

Mine is a modified 1938 with a torque tube and flywheel for electric start (I figure 6 volt) positive ground, but I'm using 12 volt neg ground.

 

No question is a stupid question this is how we learn

And I might not have answered your question.

Keep Asking If Need Be This how I learned.

I'm going to get this tractor up on the driveway tomorrow and dig in.

The B that was equipped with a Magneto had either a single push/pull grounding switch, or a dual terminal grounding switch. The real early B's had the single self grounding switch. Later in the B/C production, the two terminal switch was introduced as a replacement. One side of the switch went to the Mag, the other side to ground.

Now as the B progressed through the years, Battery ignition came on board. (Distributor) Those particular Tractors came with an ON/OFF ignition switch.  Turn the key ON hit the Starter rod and fire it up. To shut it, turn the key off and cut current to the Distributor's coil.

As for the charging end of the system, the 6V Gennie put out about 7-10 amps max depending on where the third brush was located. As Rick mentioned above, the cutout relay sat on top of the Gennie. All this did was sever the Battery from the Generator at low idle or when the engine was shut off so the Gennie didn't act as a dynamo and drain the Battery while the Tractor sat. That relay has nothing to do with charging.

The 3 position Headlight/Charging switch was your manual control for turning the lights on as well as increasing the ground (field) to the Generator to increase Gennie output. The 3 position switched worked this way. All the way in, low charge (3 amps) through the resistor that's located at the bottom of the switch. Second position out bypasses the resistor and now full fields the Gennie as well as turning on the Headlights and work lamp if equipped. Third position out Lights OFF and full charge remains. This was designed by Delco Remy to work this way in case the Battery was low from sitting and the Battery can be fast charged without the lights being a draw on the system. THE 3 POSITION SWITCH IS A SELF GROUNDING SWITCH AND MUST HAVE A GOOD GROUND IN THE INSTRUMENT PANEL. IF IT DOESN'T IT WILL NOT CHARGE. Visit our website. There's a lot of good reading there on the 6V & 12V wiring systems.  HTH

Steve@B&B

bb-customcircuits.com