People often call us asking questions about how to rewire their tractor for a 12 volt conversion. We have a variety of answers depending on what kit they purchased. Some kits are so complete that they provide a new wiring harness, while other tractors don't have kits at all, just individual mounts, an alternator, and a ballast resistor. The fact is, since we can't see the tractor and how it's currently wired, it's hard to be of help beyond what the instruction sheets say. Well even if there is no kit with instruction sheet and harness, converting to a 12 volt alternator on most old tractors is actually quite simple, and if you look at your wiring, you can figure it out. If your machine doesn't have a kit, buying the individual parts and making your own wiring changes is very likely self explanatory. I'll explain why.
Old tractors had no fluff, like modern contraptions, in their wiring. Thankfully, most were made before the days of computers, sensors, even true voltage regulators, some didn't even have an ignition as we think of it, just a magneto that was totally discrete from the tractor's wiring. Take the following wiring diagram as an example. This is for a Allis Chalmers CA, but frankly, applies just fine to every old Farmall, AC, Case, JD and many others. Your wiring diagram may be a lot more complex due to having a voltage regulator as many later machines had, but, when you convert, that's coming out and will never be used again.
Before I go on, just a couple of cautions on the actual work, if you don't follow them, it might best to have a pro help you. Remove the battery before you do anything, disconnect it, pull it out, set it in a safe place. The second is, when you are working around batteries, remember that they can explode when you get a dead short (like dropping a wrench on them). They make safety shields that cover your face just like the kind you use when you run a chain saw. Well worth the investment to have one in the shop to wear when working around batteries.
Here is the diagram we are going to use, let's simplify it right off the bat. See the wire notated as 2, it runs to the "F" on the generator and to some sort of resistor to control that generator F terminal. Just cross it out. Totally unnecessary with the one wire alternators we sell. What was that wire? Well, in the "old" days, the only voltage regulation was to make the generator start to charge more when the lights were on, otherwise, it was preset to put out a consistent low voltage of just enough to maintain or usually had a setting to recharge the battery after starting and heavy drain. That wire went to the field coil of the generator, to bump the charging up depending on how much ground the light switch said to give it. The alternator has no use for that, it has it's own internal regulator.
Then lets delete some more since that makes it simpler. Take the generator and cutout shown and white them out. Yep, gone. In their place goes the alternator, and that one wire going to the ammeter, now is the one wire coming from the alternator. At this point, let's think about the ammeter. First, you can't run to the same side, since with the 12 volt conversion, we are going to use negative ground rather than positive ground that all the old tractors used. So the drawing needs to change to swap connections from one side of the ammeter to the other. If you don't swap the two sides, the ammeter is going to show discharge when you are charging, and charge when you are discharging. The second thing about the ammeter is that it's probably rated for 30 amps. This was plenty sufficient for the old 6 volt generator system, it likely could not produce enough amperage to blow that ammeter up. Not true with the alternator. It can produce 60 amps and more depending on conditions. I think I'll break the paragraph here, because it deserves it's own.
Take a step back and think what happens when your old wires, which have been rubbing against metal surfaces for the last 60 years finally break through. If that is a hot wire, that is, something from the hot terminal of the battery or generator, it creates a dead short and attempts to draw an infinite amperage. The alternator is going to ramp to it's maximum and beyond, attempting to meet the draw. As long as nothing blows in the path, it's just keeps ramping up. Well that little 30 amp ammeter is first inline. It's going to fry, and it might even catch fire. This is not something I like to think about, since often the instruments are located about where the gas tank is. This deserves some protection because frankly, you don't really want your alternator to keep doing it's job in that scenario. If you'd prefer not to upgrade to a 60 amp ammeter, the way to protect it, is to put a fuse between the alternator and ammeter. Even if you did upgrade ammeter, it's still a good idea to put some protection into the system. Any auto parts store will have a modern spade fuse loop with #10 wire that you can wire in easily. Then put a 30 amp fuse in the fuse block. Now the alternator can't go to it's maximum and your ammeter will be protected. Just to be complete, though this article is only about the alternator, the battery can do the same thing, ramp up to fairly high amperages in a shorted situation, so putting something fusible on the battery is something to consider, since if the fuse blows, the battery will try to provide the amperage (yes, I've been accused of worrying too much).
Now let's figure out what to do with the battery. You have to find a battery in 12 volt, that will fit in the case. Then you have to swap which pole goes to ground. We want the minus to go to ground in this system and the plus to be the "hot". That means the plus will now connect to the solenoid or starter switch. Finding the right size battery is easy, you just haul the old one into the auto parts store and let them do the figuring. The only issue there, is that since you want the poles reversed, if you are going to use the same battery cables, you need to think about which side of the battery those poles are going to be on. I personally prefer to change the cables out at that time, then I don't care, because I'm going to cut sizes to route correctly for each.
That's it... if you have a magneto. But if you have a distributor ignition, there is one more step. The 6 volt coil will work, but you need to swap the minus and plus sides, and cut the voltage down from the 12 volts it's now receiving. The wire running from the ignition switch to the coil, will need to be cut and a ballast resistor put in line. Ballast resistors are common and cheap and come in typical sizes like 1.5 to 3.0 ohms. You have to determine the amount of resistance you need for your system to know which to purchase. Given that your are going to have about 12 volts coming in from the ignition, you have to put one or more resistors inline to achieve the voltage the old coil expects. Generally, that means you want to cut the voltage coming from the ignition switch to 6 or 7 volts, because that is what the coil used to be getting. You do this by taking your VOM and measuring the voltage after the resistor is hot. The reason you want to do that, is because resistors change their readings when they heat up, and to get a good reading, the ballast resistor needs to heat up so you'll know your actual running voltage. If you want to check the entire system, measure the voltage on the other side of the coil going to the points. It should be down to 3.5 to 4.0 volts due to the coil having some resistance. (NOTE: in the diagram below, AC didn't show the coil because the diagram covered both distributor and magneto systems, no ballast resistor is required if it's a magneto, there is no voltage to the magneto and the resistor would only lessen the connection to ground from the kill switch).
Taking all the above into account, here is how I scribbled up my copy of that wiring diagram, try it yourself on your wiring diagram, it will start to make a lot of sense and soon, you'll be designing your own tractor's conversion.
1. The alternator wire will be a number 10 wire run to the ammeter. Must reverse the poles on the ammeter. Before hooking it up, use a spade fuse inline between them.
2. If it's a distributor ignition (not a magneto system), wire a ballast resistor between the ignition switch and the coil, remembering to reverse the coil connections (swapping plus and minus). Try a 1.5 or 3.0 ohm resistor and measure the voltage on the far side of the coil. You are probably looking for between 3.5 and 4.0 volts. If you measure the voltage on the near side, it should be about 6 volts after going through the ballast resistors.
3. Install a 12 volt battery that can fit in the battery box, run the negative to ground and the positive the starter switch/solenoid.
4. Replace light bulbs with 12 volt equivalents. Some may be special sizes of which we carry several, but others are standard bulbs and can be purchased at any NAPA. The 6 volt ones will burn out almost instantly when turned on.
5. The resistor light switch would have had a wire running to the generator. This wire is now unused. It should be insulated so it can't short to anything, tie-wrapped up and put out of the way in case you ever reverse the process, or just completely removed.
6. The six volt starter can continue to be used as is, but, it will burn out if the tractor isn't in top tune and is hard starting. You never want to crank and crank a 6 volt starter run at 12 volts, keep the tractor in tune. If the tractor is tuned well, it should fire up immediately and the starter will not be put under undo stress and heat up.
Now there are all kinds of other circuits involved as tractors became more complex in the 50s. Fortunately, by the 60s and worse 70s, when the floodgate of complexity opened, the tractors were already 12 volts, and those machines don't need updates. Something you run into with 50s tractors, is a more complex voltage regulator, which, while they are removed with the alternator, you have to trace out the right wire to use to get back to the ammeter (hint, this will often be connected to the B terminal on the regulator). Other circuits like accessories, are down stream from the conversion work, and you probably need do nothing with them if they are rated for 12 volts.
One exception I can think of, is that you may find a lighting circuit directly run from the old voltage regulator, and you'll have to decide where tap into your 12 volts to feed those lights. Many systems just tap off the plus side of the ammeter to give a hot line to the light switch. Just a reminder, the lights should be fused separately with an appropriately sized fuse smaller than the main fuse you put on the alternator line. The reason for that, is that exposed light connections can easily get ripped off while you are using the tractor, and it's best to have separate fusing, so you can still drive the tractor back to the shop when it happens. Almost always, you'll already have a lighting fuse in the circuit, but once you've converted to 12 volts, you can swap out the fuse for one rated for lower amperage. This is because as the voltage goes from 6 to 12, the amperage goes down for the same lighting load (this keeps Ohm's Law happy). If you had a 20 amp lighting fuse with 6 volt, you might try a 15 amp fuse in it's place as there is no reason to hold off blowing the fuse if there is a short.