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Contributed Article
Recommendations for 12 Volt Conversions

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.

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 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.

Article Copyright 1999 Genesee Products

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