Changing H to 12v

Wed May 24, 2006 2:36 pm

A good amount of the wiring is in need of replacing on my 1951 Farmall H. While I'm at it, how hard would it be to change to 12v? The 6 volt system is original, but it's not charging. If it is too complicated, i might just replace the wiring and see if it charges the battery. I'm just wondering, what is involved in changing it over? Thanks.

Wed May 24, 2006 2:54 pm

You will need to replace the generator with a 12 volt generator or an alternator. You will need a 12 volt coil or will have to add a ballast resistor to the ignition system unless you are running with a magneto. You will also have to replace all light bulbs with 12 volt bulbs.
The starter will be OK on 12 volts as long as you don't crank it too long.

Wed May 24, 2006 9:24 pm

You will also need a 12 volt battery.

I have changed over 2 Allis Chalmers WDs to 12 volt. Reason. It was much cheaper to purchase a single wire alternator ($36-) than to have the generator repaired ($60-) and purchase another regulator ($55-).

On the Allis' I put the pulley from the generator on the alternator. The Allis generator bracket worked for the alternators. I did have to use a stack of washers to line up the belt pulleys.

I did change the amp gauges ($12-). The Allis amp gauges only went to 20 amps. Alterntors will put out considerably more amperage than that.

It's an easy conversion.


Thu May 25, 2006 11:23 am

If you really step back and look, there isn't that much wiring to an H.

Dad and I re-did my Super H back shortly after I got it in from JP. It was "converted" to 12V by the previous owner, which meant basically that somebody scabbed a 12V generator up under the hood and ripped all the wiring to shreds! :shock:

I got a Hitachi alternator from Advance Auto, and it fit up under there nice and easy-like with some spacer blocks. Used the how-to document for wiring up a Hitachi alternator on a Cub, and "Bob M's" wiring diagrams that are listed often on the ytmag website. Idiot light in place of the fuse in the switch box (really cool), and it works great!

Sun Jun 04, 2006 9:28 am

Howdy all,
I've got a '55 Farmall-U which already has the 12V conversion from the previous owner. On a hot day after about an hour the engine dies. After about a 1/2 hr. cool off it starts right up again and the process repeats. Suspecting its still a 6V or maybe a bad 12V ignition coil and is overheating. Bought a new 12V coil and want to replace the Ballast resistor also.
Anyone know what value, type or kind of 12V Ballast resistor i should use?

Any help appreciated,

Sun Jun 04, 2006 2:17 pm

If the coil is actually a 12 volt coil, no ballast resistor is needed, it is built into the coil. The reason for the ballast resister was to allow the use of a 6 volt coil on a 12 volt system. also payattention to the polarity of the coil. If it is a neg. ground system (which most 12 volts are), the - goes toward the points

Sun Jun 04, 2006 6:56 pm

On the ballist resistor. Just ask the experienced person in the autoparts store. Chrysler vehicles had a seperate ceramic resistor and most likely that is what the parts person will hand you.

If you have a choice in Ohms, pick the one with the higher value (number). I'm thinking it's 3.2 ohms. Ideally the ohms (resistance) in the ballist resistor should be equal to the coil ohms.


Sun Jun 04, 2006 8:46 pm

Thanks for the replies,
Took a good look at the coil today and it sure looks like the original 6V to me. It has an odd shape and doen't look like a modern coil and its painted red the same color as the tractor.
Reason i'm asking about the resistor, i picked up a new automotive 12V coil at Farm & Fleet a few days ago and it states right on the coil to use an external resistor. Could be some coils don't need the resistor? i don't know, i'm confused as usual!
And to add to the confusion, been Googlen resistors and coils and what i read so far is the ignition switch has to support a ballast resistor, during cranking it shorts or bypasses the resistor for a hotter spark and when the engine starts and the key is let up, it allows the resistor in the coil circuit to prevent it from overheating.
Eugene, John, i think i read somewhere too that the resistor should equal the resistance of the coil. As a last resort, maybe i'll just try and run w/o the resistor and see what happens!

So now a 'New Question', did the '55 era Farmalls use ballast resistors for the original 6V coils? Is there a link to electrical diagrams somewhere in this forum?


Sun Jun 04, 2006 10:01 pm

Ken: There is a discussion going on about electrical wiring at the above listed site. There are two wiring diagrams included in the discussion.

You will need the ballist resistor with the 6 volt coil and the 12 volt coil if it is so marked. The purpose of the ballist resistor is to proportion the voltage between the coil and the ballist resistor. The ballist resistor acts as a voltage drop. It's a math problem, it cuts down the voltage from 12 volts to around 6 volts at the points. You need the ballist resistor to keep from burning up the points.

The description for wiring the ballist resistor (ignition circuit) you have been reading on the web is for automobiles and tractors that start with a key switch.

Yes some 12 volts coils include a resistor inside the coil.


Mon Jun 05, 2006 8:33 am

Coil packaging will be marked as "needs ballast resistor" or "no ballast resistor required." Pay attention to the packaging, and do as the packaging recommends.

IMHO, take the 12V coil back and get one that doesn't require a ballast resistor. In this day and age, there's no sense in screwing around when you can just get the right coil in the first place. Mine came from TSC.

Mon Jun 05, 2006 8:58 am

before i posted here i didn't even know coils came with internal ballast resistors. Gonna take your advice and see if Farm&Fleet has one-hope i still got the receipt!
thanks for that thread link! all kinds of info there! Also found the alternator wiring discussion interesting there. Good to know whoever did the 12V conversion on this tractor put the diode in the alternator's field circuit and so far looks like they knew what they were doing.

This is a great discussion group!

Mon Jun 05, 2006 9:04 am

Ken - those diagrams are from Bob Melville who you can usually find over on the ytmag site. Bob is quite knowledgeable in this are and has posted several wiring diagrams for various configurations and a couple of troubleshooting flowcharts on a website. They are available to everyone. ... 065733&f=0

Another great resource available on the net!

Mon Jun 05, 2006 10:48 pm

Bigdog, thanks for that great link - the info keeps gettin better!
Decided to ring out my Farmall and see if theres a ballast resister under the hood somewhere and if there, whether its bypassed during cranking. This tractor fires up instantly even after sitting for months. I'm convinced one of the reasons for this is the hot spark from the 6V coil being fed by 12V. The downside i'm geussing is an overheated coil and the reason the engine stalls after running a while.

That 4 terminal solenoid in circuit 6 or 7 with the aux. contacts is interesting! Gonna check into the mentioned ballastless coils and a last resort..maybe a ballast resistor with an added relay to shunt the resistor during cranking.