Farmall Super C wiring diagram (6v to 12v)

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Farmall Super C wiring diagram (6v to 12v)

Tom Does anyone have a wiring diagram for a Farmall Super C showing how to convert from 6v to 12v using a GM alternator and 12v coil? Would I have to use a ballast resistor inbetween the ingnition switch and coil if using a 12v coil? Thanks!
John M. Greenlee I don't have a wiring diagram but a few pointers. You will have to make sure the new polarity is negative ground. You will have to make sure the coil is 12V as well as the lights. If the alternator has a built-in regulator you an eliminate the old one. The starter should be ok but be careful adn don't crnak too long each time.

Dick Moore

When converting a tractor from 6 to 12 volts, five basic areas of the electrical system must be considered:

•Charging System •Ignition System •Starter •Accessories •Battery

Charging System

There are many different methods for converting a charging system to 12 volts, however, the most popular approach is to use a Delco alternator with an internal regulator. These alternators are available in several different configurations with the most common being the model 10-SI which was used on General Motors vehicles from the 1970’s through the early 1980’s. Because there were literally millions of these alternators built they are readily available and have proven to be very reliable. The 10-SI was also built as a “one-wire” or marine alternator, but this model is less common and therefore more expensive. A one-wire alternator is slightly easier to install (one wire versus three), however, the standard automotive 10-SI offers better charging characteristics at low speed.

The price range for the 10-SI ranges from as low as $10 at a salvage yard up to $50 for a rebuilt unit from the auto parts store. The 10-SI was built in several different amp ratings, but any of them will work since a tractor’s electrical requirements are typically very low. Probably the most common 10-SI is the 63 amp model and many auto parts stores stock it as part number 7127. Another variable to consider with the 10-SI (or any alternator) is pulley diameter. The smaller the pulley, the faster the alternator will spin. For example, if the alternator pulley is one fourth the diameter of the crankshaft pulley, the alternator will turn 4 times the speed of the engine, if it is one third the diameter of the crankshaft pulley it will turn 3 times engine speed, and so on. A typical 10-SI (3-wire) will begin charging when the alternator is turning about 1200 rpm.

Mounting the alternator tends to be the most difficult part of any 12 volt conversion because a bracket usually needs to be fabricated. On some tractors the “bracket” will simply be a long 3/8” bolt but for others the bracket will be more involved. For many tractors, a hack saw, some strap metal, and a little common sense is all that will be required to mount the 10-SI. The most important consideration is safety, the bracket must be substantial enough to handle the weight of the alternator plus the tension from the belt without excessive flexing.

(Note: Wiring instructions for the 10-SI are included at the end of this article)

When using a standard Delco alternator, it is necessary to change from positive to negative ground. On a tractor this normally only affects the ammeter and coil. (Coil is discussed below.) The ammeter is affected because it senses the flow of electrical current and is sensitive to which direction the current is flowing, i.e. charging versus discharging. Reversing the wires on the ammeter is all that is required to make the ammeter read correctly.

Although not required, a voltmeter is strongly recommended for 12 volt conversions. The voltmeter can be used in place of the ammeter or in addition to it. A voltmeter offers several advantages over an ammeter. First, because the amp draw of a tractor’s electrical system is so low, it is often hard to tell if the alternator is charging because the ammeter needle is so close to zero. With a voltmeter, a voltage of 13.5 - 14.5 volts will register anytime the alternator is charging regardless of how low the amp draw may be. Secondly, the wiring for a voltmeter is much simpler because it can be connected to the ignition switch versus an ammeter which must be wired in series with the heavy 10 gauge wire from the alternator. This provides for a safer installation since all of the amperage from the alternator doesn’t have to pass through the dash. And finally, a voltmeter is more effective when troubleshooting the electrical system as it can help identify problems such as poor connections, a weak battery, dragging starter, or a faulty alternator.

Choosing a good quality voltmeter is as important as having one to begin with. For accuracy and reliability, heavy duty gauges such as those made by Datcon and Stewart Warner are a good choice for tractors and other heavy equipment.

Ignition System

The coil is the only component of the ignition system affected when converting from 6 to 12 volts. Like the ammeter, the wires on the coil must be reversed. This means the negative (-) terminal of the coil is the one that connects to the distributor, while the positive (+) terminal connects to the ignition switch. Also, for the system to operate properly, the coil must also have the correct resistance value for use with 12 volts. Since there are several ways to accomplish this, there seems to be considerable confusion surrounding coil selection. Most of the confusion comes in when referring to “6 volt” coils versus “12 volt “ coils and the accompanying ballast resistor that is sometimes used. The important point to remember is:

The total resistance of the coil plus any external resistor (if used) should be 2.7 to 3.3 ohms for a 12 volt system. This level of resistance will maintain the correct voltage and amperage in the ignition circuit. Coil resistance is measured between the plus (+) and minus (-) terminals.

Most original equipment 6 volt coils are about 1.2 ohms, while most original equipment 12 volt coils are about 1.5 ohms. In order to get to the 2.7 to 3.3 ohm requirement, a ballast resistor such as a Napa ICR13 (Approx. 1.8 ohm) can be added to these type coils. Another approach is to eliminate the ballast resistor altogether by using a replacement coil with 3.0 ohms of resistance. Two commonly available 3 ohm coils are the Napa IC14 and the Pertronix High Voltage 40511.

If the tractor is equipped with a magneto, the basic ignition system itself will be unaffected by the change from 6 to 12 volts. Some tractors with magneto ignition, however, will need a new ignition switch for controlling the power to the alternator and other accessories. Most auto parts stores stock universal ignition switches such as the Napa KSI-32 or Borg-Warner CS-101 which have terminals for grounding the magneto and controlling 12 volt accessories.


The starter will be unaffected by the change from positive to negative ground (it will rotate the same direction) and generally will tolerate the increased voltage, assuming the starter is in good working condition. Like other parts on an older tractor, starters were designed with considerable safety factor and as a result they can be run on 12 volts without damaging the windings. A common practice is to use the starter as is, and rewind it for 12 volts when and if it ever needs major repair. As with any vehicle, it’s best to avoid sustained cranking and the starter should be given ample time to cool between starting attempts if excessive cranking is required. When converting to 12 volts, it’s a good idea to check the starter drive and flywheel ring gear to make sure they are in good shape since the additional “kick” of 12 volts will put more stress on these components. Also, if the starter is dragging, now is the time to repair it because the additional voltage will only aggravate the situation.


The most common (and usually the only) accessory on a tractor is the lights. In most cases the 6 volt bulbs can be replaced equivalent 12 volt bulbs. In those instances where a 12 volt replacement is not available, a new holder or lamp assembly will be required.


Changing the battery is so obvious it is seldom mentioned when discussing a voltage conversion. When selecting a 12 volt battery, the physical size and electrical capacity of the battery should be considered. A typical Group 1 six volt battery measures 8-7/8” long x 6-7/8” wide x 8” tall, so most battery trays have ample room for a 12 volt battery. The popular Group 26 (8-1/4 x 6-3/4 x 7-1/2) battery will easily fit most applications as it is somewhat smaller than the Group 1 dimensions. A Group 25, will provide an even better fit since its dimensions are almost identical to the Group 1 six volt. Also available is the Group 35, which is identical to the Group 25 except the posts are reversed.

When considering battery capacity it is easy to get caught up in the “more is better” approach and base the selection strictly on the CCA (cold cranking amps) rating. CCA ratings of 500-800 amps are seldom needed in vintage tractor applications and furthermore high a CCA rating doesn’t necessarily mean the battery is well constructed. Unfortunately, most battery manufacturers don’t publish details of the their plate and grid construction, so it’s hard to compare brands. However, for tractors that see infrequent use, a “commercial” or “heavy duty” rated battery will hold up better over the long haul than a typical discount store automotive battery. Most of the leading name brand manufacturers (Delco, Interstate, Diehard, etc.) offer heavy duty batteries that are well suited for tractor use.

After converting to 12 volts, 4 gauge battery cables typically provide ample capacity for tractors up to about 40 HP, unless the cables are over 6 feet long. For long cables and tractors above 40 HP, 2 gauge cables are usually adequate. In reality, poor connections are more often the cause of excessive voltage drop than undersize cables. And of course, a little preventative maintenance at the connections (both ends of both cables) is all it takes to avoid a problem in this area.

An excellent glossary of battery terms and other general information on batteries is available at:

A final thought………..

As with other aspects of the tractor hobby, there is no one “right” way to do a 12 volt conversion. The intention of this article was not to be a cookbook approach, but rather a general reference for that very frequently asked question of “How do I convert my ________ to 12 volts?” Hopefully the information presented here will help you decide what’s best for your specific situation.

10-SI Wiring Instructions:

The 10-SI alternator has three terminals, however only two of them get connected to the tractor. (Thus, the 10-SI is sometimes referred to as a two-wire or three-wire alternator)

•The “Bat” terminal should be connected to the ammeter with a 10 gauge wire. If a voltmeter is used instead of an ammeter, the 10 gauge wire can be run directly to the stud on the starter switch or solenoid where the battery cable connects. •The number “1” terminal should be connected to the ignition switch with 16 or 18 gauge wire so the alternator only receives power when the tractor is “on”. A one amp / 50 volt diode should be installed between the ignition switch and the “1” terminal to prevent backward current flow through the alternator when the ignition switch is “off”. •The number “2” terminal should be connected directly to the “Bat” terminal (16 or 18 gauge wire) on the back of the alternator.

Note: Genesee Products offers several products to help with your 12 volt conversion, including a wiring kit for the 10-SI alternator. For more information on these products, please visit their web site or contact them at

Article Copyright 1999 Genesee Products
Reproduced with permission from Genesee Products

james schnake
email wiring diagram forfarmall super c

james schnake i donot have the wiring diagram i am looking forone for the farmall c
Brooke Rookerson
same as apove

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george snavely I bought my dad's '51 super c. tractor barely started....very slow cranking speed. So I had the
alternator checked...bad. So I put in a new alternator/regulator and a new battery. Same
problem...I removed old wiring from the alternator
through the ignition switch, fuse, and light switch. I should have made a wiring diagram but failed to do so. Am wondering if there could be
an additional electrical problem i.e. starter winding or coil? thanks. GS

Jeff Wykrent George,

I am having the same problem with the cranking speed of my 52' super c. Over the weekend I put new wire on and noticed a small difference. I checked voltage from the 6v generator and it is putting out 7.6v before the regulator...after the regulator it the voltage is 6.78v.

Does anyone know the correct specs for voltage output? I am assuming that 6.78v is good to charge the batt.

Also, I have noticed that my Amp meter reads that the tractor is charging some of the time and then discharging at other times regardless of the RPM's of the engine. Would this mean I have a short somewhere?

Also if anyone has a wiring diagram for a C / super C PLEASE e-mail it to me.


TBecker Jeff, did you ever receive a wiring diagram for a C International? If so, would you please send a copy to me, as I am having some problems with the wiring on my C.
Thank you.

James Zaczkowski I am overhauling my 1952 Farmall Super C and need the manual with pictures and diagrams for engine and clutch and throwout bearing. Does anyone have this online or can you send me copies?
al clarke we have a 4 prong switch
alternator system

al clarke we have a 4 prong switch
alternator system
the two wires that atach to the alternator-the white is hot to the touch.
as to the 4 prongs on the new switchiwhich wire goes to acc
to bat
to sw I assume from switch

Alvin Long Hi I have my dads Super C I have bought a 6 volt alternator and would like to replace the harness with one for this setup,but I dont wish to go to 12 volt
Thank you if it works would you like a donation ?

richard i have a jacobsen 12 volt, 4 cl. has any one have a wiring diagram, for the iguition. i think the motor is a ford. thanks for help
J. Elking ..... No one seems to know for sure----Is the Farmall super A ----positive ground or negative ground ? Thanks to anyone that can help me answer this question. JJE
Nathaniel Graeser The Farmall A is 6 volt positive ground.
Max Davis where do you locate the serial # on this Tractor? And how can I tell if it is a 1951 or 1954?
Jeff Is this the same setup for a Farmall 200?

Jeff After reading my question, I realized I may have been a little vague. What I was basically wondering was would the procedures and parts for the conversion be the same for a Farmall 200?

Mike positive ground
steve positive ground .. i have one

Joe Meherg A friend of mine has a super a farmall it run find idling and it only firing on #1 & 4 we think it's the timing, can anyone help us. Thank you.
Randy R I have a Super C that was converted to 12v by the previous owner and it is not charging the battery. I'm sure it's in the wiring or connections but could use some advise/guidance overall. With battery adequately charged it starts and runs great, obviously when the battery is too low it doesn't start. I am going to clean all the connections, etc but I am wondering if he has the alternator wired incorrectly. It is a motorcraft alternator, appears to be internally regulated. for connections on the back, one is wired as a ground, one is the large wire that goes to the starter/battery, one is marked FLD (field?) and the last one is marked STA. Can someone tell me what should be connected to the FLD and STA terminals?


Bill R. MAYBE this might save some confusion with the grounding of a 6-volt vs. a 12-volt system.

The local Case/IH dealer advised that "most"
6-volt systems on tractors are positively (+) ground. "Most" 12-volt systems are negatively

In summary the following analysis may be helpful?

Six has an "x" in it. Postiive ground has a "+".

SO, each has a 'cross'...six and the positive sign. (If it's a 12-volt, the word 'twelve' lacks the cross, then it's a negative ground.)

Seems to work for me anyhow, good luck with it.

Vanessa Svehlak I am looking for a wiring diagram for a Super C. If u have one can u email it to me please!


Robert Mowbray I want to convert a Farmal A from 6v to 12v and install a altenator . This is a positive ground system . Does anybody have instructions to do this
Thanks Robert

Coy Thrasher Jr. I am looking for the wiring diagram for converting my C FARMALL over to 12V also could someone please email it to please and thanks in advance.
terry becker Please could someone send me the wiring diagram for changing my c farmall tractor from a 6volt to a 12volt system. Thank you for your help.

Elmer how to rewire to 12 volt alternator
Wes Dexter I thank all of you for the interesting forum. I have a Farmall Super C that my son and I have been all thru and restored. Left it 6 volt but I am having problems with it not starting well. It seems like the battery runs down fast like I am not recharging. How can I check the system to know if it is charging? Where did you guys get a wiring diagram?
Thanks, Wes
Leslie, Mi

bob harshberger I have a farmall A, it runs off a 6 volt magneto for the engine. tring to convert starting and charging to 12 volt. I have the one wire delco alt. 12 battery mounted on tractor along with lights.tractor starts and runs fine but need some help on the wiring from alt. to battery and the stwitche(on/off) and gauge(+/-)
John Lozowsky I need to re-wire this tractor completely and need help in obtaining the information to do it properly. It now has an alternator and a 12 volt system. Anything you can do for this 78 year old will be greatly appreciated
cyrush I can supply harness diagram for B250.

Please note these tractors were never built with an alternator )Generator only)

Local auto electrician will be able to advise depending on terminations of alternator used on ypur m/c.

Send me your e mail address so i can forward the documents (4 pages) PDF.

farm fucker fuck you all and george

farm fucker fuck you all and george

Dea R. Thayn I'm looking for a diagram showing the wiring order for a Super C and M tractors. thanks
don stremick could you p lease send a wiring diagram of a b250 international tractor

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