Six of one, and a half dozen more...

Six to Twelve Volt Conversion

This unFAQ was spawned by the members of the Old_Engine Fido conference, and the preliminary thoughts to converting a 1951 International boom truck to a 12 volt electrical system. The contents are based on the input of various conference members.

We are also assuming you have weighed the advantages/disadvantages of undertaking a 12 volt conversion on your vehicle.

At first glance, the change might seem to be a quick job, but once you think about it, there are many systems that need to be looked at. Lighting. Ignition. Starter. Instumentation. Charging system. We'll have a look at each system, and hopefully address any questions you might have on the subject.


Give it a little more juice and it will glow a lot brighter. At first, this might not seem too bad, just go in and change to the 12v bulb once it burns out. You won't have to wait long however.

An additional consideration at this stage, is to plan for dual rear lighting incorporating automatic signaling if your vehicle is not so equipped. But that is for another unFAQ.

Bulb sets that you will encounter:
-Brake lighting
-Signal/Marker lighting
-Courtesy ligting

At this stage it will be a good time to assess the current condition of your lighting system. Do you have sockets that should be cleaned? or replaced? Are the sockets replaceable? Will you need to be creative in your replacement to fit an appropriate 12 volt bulb? If unit replacement is the order of the day, a bit of leg work will be required to find what works best in your situation. Obviously a work truck can do with any lighting fixture you find, while a stock look-a-like will take some carefull investigation. An autowreckers can provide a wealth of possibilites, get to know your lighting fixture, meausure it, make diagrams, whatever it takes to match a donor fixture. And don't forget to look at the various imports! Some of them have provided near stock fit in older domestic iron.
And by all means, if you insist on taking your original fixture to an autowreckers have them sign it in, otherwise you might end up buying your "salvaged" part back when you try to leave the premises.

Fortuneatly for the convertor, lamp bases have remained fairly consistant throughout the years (with qualifiers of course). If you have the owners manual, or shop manual for your vehicle, theres a good chance that it lists the specs of each lamp. Take note of each (candle power (or wattage), original number, location used) lamp in your vehicle. Armed with these bits of information, grab a sample of each 6v lamp and head to your friendly neighbourhood auto supplier, look through their lamp catalog for matches to the base, and power ratings. Electronic supply catalogs of various vintage may also assist in identifying candle power and base type of your original lamps. When selecting a 12volt lamp take note of the bulb style, some styles, while having the same rating, can have a very different bulb on them and may not fit your lighting fixture. (eg. s-8 bulb, your typical 1156, and an s-11 which has a large rounded bulb, or an R-12 which looks like a squished wide flat top bulb.) [The letter indicates shape, the number indicates bulb diameter in eights of an inch].

We are working on a cross reference list for bulb. A rudimentary table can be found at the main unFAQ page. If you have any 6 volt bulb data, please e-mail the information to me so I can incorporate it into the table. (E-mail address can also be found on the main unFAQ page).


We can't stress enough to have an ignition system in top condition. Regardless of vehicle use, a poor ignition system results in a poorly operating engine, so if anything is questionable, replace it.
That said, various methods have been put to practice regarding the ignition system. For some low use equipment the original ignition system is run as-is with just the addition of a ballast resistor for the coil.

For units requiring extra duty a 12 volt coil is recommended. It provides a better turns ratio than an original 6 volt coil, as well it is suited for use in a 12 volt environment. If it is a standard coil found on most domestic vehicles you will need to utilize a ballast resistor (most OEM use coils are rated for 8volts). Some aftermarket performance coils operate with no ballast resistor, as well as a handfull of OEM coils. Obviously, using a true 12volt coil will eliminate the need for a ballast resistor. The original points are more than suitable for 12v operation. If you compare the points, the 6 volt units are more robustly constructed.

The condensor can easily be replaced by any condensor found on later production detroit iron. Any decent autoparts store will have these on display, so a "donor" vehicle type is not required. Following are some "Donor" vehicles, and AC Delco P/N's for the insistant counter guy;

A ballast resistor can be utilized from any Single ballast resistor Chrysler product. A single ballast resistor being identifiable having only two prongs. You want to wire the ballast resistor in series with the ignition feed to the coil. Be certain to mount this resistor away from wiring harness' and other meltable components. This resistor DOES get hot. And don't forget to pack a spare in the glove box. Again, this part is usually found on the shelves and no "donor" vehicle type is required. But just in case;

For some more condensor, ballast resistor, and coil varients, see 'AC Delco Illustrated Electrical Catalog'.


The starter is perhaps the easiest of the lot, it will survive quite nicely on a diet of 12 volts. Believe it or not, 6 volt starters are designed to handle a bundle more current than their 12 volt counterparts. However, the solenoid should be replaced with its 12 volt counterpart. In a pinch, the 6 volt solenoid can be used--but be warned that it may overheat severely and melt down (with the possible effect of locking your starter on, or causing a fire). Barring any outside factors the 6 volt starter will provide many reliable starts.

Typical starter advice applies: Have clean terminals. Be certain battery cables are in good condition. Under no circumstances replace the starter cables with a smaller guage cable.


Time for decisions again! Do you want to retain your original quages? Or do you want to upgrade your monitoring devices? Do you even have guages? Do you want guages?
If upgrading, do you want to retrofit later model "factory" guages from some donor vehicle? Or do you want to look to the aftermarket?

Original Instrumentation
If your vehicle is only equipped with idiot lights, you can simply change the bulbs and be done with it.
If you have guages, you can utilize a dropping resistor in the feed line to the instrument cluster (same warning as with the ignition ballast, mount it away from anything meltable).

Here are a couple possible pre-made devices that may make things easier;

A point of interest for those who wish to retain a 6 volt system (or, utilize their original 6v guages in a 12v environment), AND upgrade their guages somewhat, look to the Ford Mustang II (197?-1978). The Mustang II utilized 6 volt guages! Mounted to the back of a Mustang II instrument cluster is a voltage limiter (from what I gather, it operates like mehcanical voltage regulators using points.) and in the Mustang II, each guage has an 8.5 ohm resistor for each circuit. Check out a wiring diagram for the Mustang II for reference and ideas to incorporate into your system.

Alternately you can look to later model 12 volt instrument clusters for donor guages/sending units. Again, you'll need to scare up wiring diagrams for the donor vehicle to properly wire in your new guages.

Aftermarket Guages
If you don't mind a custom look, have a rats nest of bad wiring under the dash, or after a quick and easy way to wire up guages the aftermarket is the way to go. It will also allow you to remove all old guage wiring and start from scratch. It might sound daunting, but given the straightforwardness of wiring guages, and easy hookup of aftermarket units, its an easy task. The only drawback to aftermarket guages is cost.
The basics wiring of these guages is a ground wire (sometimes through the case), instrument lamp, power wire, and finally a wire to the sending unit.

The instrument cluster will probably be the most time consuming part of the conversion if other than original guages are used. Take your time, check everything, and you will have a reliable and good looking instrument cluster.

Charging System

It seems obvious, but it has to be said. You will need to install a 12 volt battery. Measure your battery compartment and head off to the AC Delco dealer. Take a look in the "Passenger Car, Truck & Industrial Battery" catalog. It will provide the dimensions for all the available batteries. Select the largest battery that will fit.
When your taking your measurments, don't forget to check hood clearance! Not so much an issue with old vehicles, but if you have a snug hood, a top-post battery might get welded to the hood. I prefer side-post batteries, so consider those. Battery cables with any configuration of length and terminals are available from the same place you purchase your battery.

By far the easiest, and most common unit out there to consider using in your 12 volt system is the General Motors 10SI unit. It has an internal regulator, and requires only 3 external connections. The 10SI also gives you the option of using a 1 wire regulator (self-exciting regulator) which requires only 1 external wire.

Physically mounting your alternator will require a bit of ingenuity on your part, spacers, brackets may need to be utilized to line things up properly. Various bracketry can be had from a variety of GM vehicles. An inline six equipped vehicle with low-mount alternator will have a simple block mount pivot you may be able to utilize in a v-8 installation. Trucks and cars have another assortment of brackets. Check a wide range of years and engine type vehicles for something you can use in your situation.
The aftermarket is another source for universal bracketry--check a rod magazine(catering to real Rods that is) for bracket suppliers.
Another source for new retro-fit brackets and spacers is AC Delco. See the "Specifications & Selection Guide: Charging Systems, Cranking Motors, Batteries & Cables." You will find a number of different related topics in this catalog.

See the 10SI unFAQ on how to wire the alternator.

Other Items

Windshield Wiper Motor
A few things about wiper motors, the 6 volt unit is obviously 6 volts, and if exposed to 12 volts regularily it will burn out. Theres word that the wiper motor speed is current controlled. If that is the case, determine the current the wiper motor draws in 6 volt configuration and then calculate the resistance required in the feed line to the wiper motor.

The overlying suggestion is to install a 12 volt wiper motor. An entire unit, or retrofit just the motor.

You can never have enough grounds. If your just reading this for interest sakes, or if you are planning electrical work, grounding should not be overlooked. Weird things can happen when you have poor grounds. Check existing ground straps/cables. Alternator case (typically through its mounting bracket) to the battery. Battery to body. Body to frame. Instrument cluster to body or frame. Firewall to engine/frame. If you are into lots of electrical accessories, upgrading or adding ground straps is a wise idea (firewall to frame, truck bed to frame, etc.). Its nice to feed all your accessories with a 12 or 14 guage wire, but if you ground everything to the firewall/body, you might have an effective ground that is only a fraction of the required guage to handle the current if you rely on stock ground straps.

Switches & Wiring
The original wiring and switches will live comfortably in a 12 volt environment.

The one item that will getcha is the activation voltage of the coil. Feeding it a higher than rated voltage could compromise its life. It may run hot. It might be fine. Some relay coils will be content on 12 volts, others may burn out rapidly. If you want it all done right, or there is any question to the integrity of the relay, its best to replace the relay with 12 volt units. You may not even have any relays. In any case, check your wiring diagrams, and assess whats being switched with a relay. How long is the relay activated? What would happen if the relay stuck on? What if the coil overheated? Just a few questions that may decide to upgrade the relay. Of course, the easy answer is to just add a ballast resistor to the coils feed. But that adds clutter and an extra item to fail IMHO.

Final Thoughts

If you decide to convert from 6 to 12 volts, be neat, try to stick with wire color codes (so you'll have it easier in coming years should something go awry). Stick on wire markers don't after a few months, and if you have a bundle of same colored wires coming out of a harness you will be spending unnecessary time figuring out whos who.

Document any changes you make to circuits.

If replacing wires, consider larger (larger cross sectional area, smaller guage number) wire depending on circuit use.

Do not use your electrical wiring to hang things.
Leave sufficient slack in your wiring so flexing or contraction stresses do not compromise wire crimps or connections.

Double check everything and dress wires appropriately.

This information is only intended as an overview and may not include all the necessary information, data, or facts.
Every vehicle is unique, and research for your particular vehicle is recomended.

Just a handfull of contributing Old_Engine members (in no particular order);
Elvis Hargrove (moderator), Ian Woofenden, Jim Dunmyer, Roy Witt, Sean Dunbar, Matt McCarthy, Jim Dawson, Ben Carpenter, Joe Rowehl, Carl Brown, Gerry Calhoun, Craig Healy, Russ Woofenden, Alex Bilan, and the thoughts provoked by the late Cloyce Osborne.

Be Seeing You! ;)

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This Page Created By Alexander M. Bilan (
All Rights Reserved.
June 13, 1997
Revision 2.0 November 29,1997