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Several frequently asked questions deal with electrical systems.  I'll try to deal with some of the more popular ones here.

Wiring Diagram

Many people have asked for a wiring diagram. I don't have one here (copyright restrictions prevent me from just scanning it in and posting it, and it is too complex for me to have gotten around to redrawing yet), but the Operator's Manual (which you can get from Binder Books) has wiring diagrams for both magneto and battery ignition versions of the tractor. You really shouldn't be without an operator's manual anyway - it covers all the essential maintenance tasks, and is incredibly useful.

6 Volt or 12 Volt?

The most frequently asked question is "how to I convert my Farmall H to a 12 volt electrical system?". My answer to this question is always "Why?".  These tractors were designed to start perfectly well on 6 volts, and will today if you have proper cables and a good battery. The first thing to check is battery cables.  The reason manufacturers went to 12 volts is not that it starts better, but that it will work fine with thinner cables which cost less. Many Hs through the years had their cables replaced with thin 12 volt cables. If you have these, you need to throw them away and replace them with big 2 gauge or better cables. Make sure that your ground cable (remember, the H has a positive ground system) goes directly to one of the starter mounting bolts and makes good connections on both ends. Then make sure that the cable from the battery to the starter switch and on to the starter is also good. If your starter switch gets hot when you press it, consider replacing it too. If you have good cables and a good battery, it'll start right up. You should be able to get proper cables at Case-IH or Tractor Supply or similar places. If all else fails, get some 2 gauge welding cable and make your own, it isn't hard.

Besides there being no good reason to do it, a 12 volt conversion will reduce the value of your tractor to a collector - it won't be original any more.  If you go with an alternator, you'll have to fabricate a new mounting bracket because it won't fit under the hood any more (whatever you do, don't cut the hood!). So it will be glaringly obvious that it isn't original, and worse, the alternator will be sticking out to the side where it isn't protected from rain. Besides the H, we also have a Farmall A, a Super A, and a Farmall 200. All four start reliably on 6 volts!

If you still feel like you just have to convert it, then you can read this article at Yesterday's Tractors on Converting from 6 Volt to 12 Volt.

Repolarizing Your Generator

If your tractor has been stored without a battery for a while, or generally has charging problems, you may need to repolarize your generator. You need to do this anytime the generator or regulator has been removed or the leads disconnected. This is a really easy procedure, and requires only a short piece of wire. First, make sure that the positive terminal of the battery is grounded. Then, momentarily connect a jumper between the "BAT" terminal of the regulator and the "A" terminal of the generator.  Hold the wire on one end and just brush it across the other - you should see a spark. If you do see a spark, you're done. It is that easy.  Remember, don't connect the wire, just momentarily touch it.  And make sure you have the right terminals at both ends (they should be labeled). If you do the wrong ones you're likely to fry your regulator.


1999-2002 T.W. Cook - All Rights Reserved. Operating and repairing antique tractors is an inherently dangerous activity. In particular, Farmall H's predate many modern safety enhancements such as roll-over protection systems. Please, be aware of the proper safety procedures and take all possible precautions - be careful out there!

Some words, model names, and designations, such as "Farmall", mentioned herein are the property of the trademark holder, and are used for identification purposes only. This is not an official website.

This page last updated: September 27, 2003