A good electrical system depends on clean sound connections, free of grease, dirt, or corrosion. The condition of connections to ground is just as important as the hot wire connection because the current must return to the battery via the ground connection. It is good practice to use a grounding strap between the body and frame, as well as the engine to frame, as both the body and engine are on rubber mounts. The battery should be grounded directly to the engine.
Use #14 stranded copper wire (fine strand) with vinyl insulation for general hookup of lights, accessories, primary ignition, etc. Use #8 stranded copper wire (fine strand) for connecting the alternator or generator regulator to the ammeter, and from the ammeter to the battery. If a generator is being used connect the A (armature) terminal on the generator to the A terminal on the regulator with #8 stranded copper wire as well. Terminal ends should be used wherever possible and all connections should be properly soldered.
Wiring should be taped into looms using vinyl electrical tape or plastic looms can be used from late GM cars. This is corrugated plastic tubing split lengthwise that allows fast, neat wiring with the wires readily accessible without having to untape.
On the diagram, the broken lines from the boxes marked alternator and generator designate the charging circuit connections to the main system depending on which is being used. An alternator is recommended, and if used, should be used with the proper alternator regulator. Wiring diagram of alternator regulator is for Delco system with voltage regulator separate from alternator.
The ignition switch on the diagram is the four terminal type that incorporates a start switch. These are a common replacement type sold in auto parts stores. Headlight switch is also the common replacement type. IGR ignition resistor is the standard replacement type that delivers about 7.5 volts to the coil.
The 6 volt gas gauges on older Jeeps can be used with the 12 volt system by obtaining a 12 to 6 volt dropping resistor and connecting the hot wire to the gas gauge dash unit at the 6 volt terminal of the dropping resistor. The 6 volt gas gauge dash unit and tank unit will be damaged if connected directly to the 12 volt system.
The ammeter is a current measuring device and will work on any direct current voltage. An ammeter that was originally used on 6 volts will work perfectly well on 12 volts. The current rating of the ammeter should be compatible with alternator output. That is, if using a 60 amp alternator, use a 60 amp ammeter. A 30 amp ammeter can be used with the 37 amp Delcotron alternator or a 30 or 40 amp generator.
If electric oil and temperature gauges are being used, they should be connected per the diagram, surrounded by the broken line. Mechanical oil and temperature gauges are preferred for accuracy and reliability.
This is the most common electrical "swap" on early Jeeps and is usually done in conjunction with an engine swap as all conversion engines require 12 volts for starting.
There are several dual relays and other switching tricks that have been used with two 6 volt batteries to get the necessary 12 volts for cranking but these are more trouble than they are worth and the second battery is a problem to mount and maintain.
By converting the entire Jeep to 12 volt, a simple system that uses easy to obtain lamps and other parts is achieved. One of the most common questions we are asked is – "is 6 volt wiring large enough to handle 12 volts?" Quite simply, it's twice as large as it has to be. When you double the voltage, the amperage is cut in half, so 6 volt wiring is more than adequate but – most vehicles that had 6 volt systems used the old style rubber and fabric insulation that is probably so brittle that it will break instead of bend. Also, most of these older vehicles will have had their wiring modified by any number of previous owners and can be a real mess to sort out. Evaluate the particular vehicle and consider a complete rewire job using new wire and standard parts store switches and controls, rather than trying to patch an old system. Take the time to do it right and the electrical system will be trouble free for many years.
To actually convert from 6 to 12 volts start by changing all the bulbs and sealed beam headlights. Mount the 12 volt battery using a negative ground to the engine. Because the engine and body are mounted on rubber (theoretically at least) a braided jumper cable (sometimes called a grounding strap) should be used between the engine and frame, and between the frame and body to insure a good ground. The positive battery cable should go to the starter solenoid (or relay, if not part of the starter, such as on Ford engines).
For charging the battery we advise using an alternator rather than a generator due to the higher output at low RPM and lower maintenance requirements. If you have a choice, use the internal regulator type alternator for the utmost in simplicity of wiring.
Most 12 volt system coils and point type distributors operate on about 7.5 volts. In factory installations this is obtained by a special resistance wire to the coil or a ballast resistor. Don't consider anything but the ballast resistor to get 7.5 volts – the resistance wire method is not reliable over a period of time as its resistance increases with age.
The Jeep should now start, run, and charge on 12 volts. This leaves us with the gauges and accessories. Most older Jeeps used mechanical oil and temperature gauges and these require no power.
This leaves the gas gauge and ammeter. First the ammeter (actually we advise using a voltmeter rather than an ammeter as you will never be surprised by a dead battery and they are simpler to hook up). This gauge, as its name implies, measures amps which is the term for current flow. Because of this, it will work on any direct current voltage. The ammeter must, however, be capable of measuring the total amp output of the alternator. A 30 amp ammeter will work with a 37 amp alternator, such as found on some GM engines. Most GM alternators put out 50 to 60 amps and this will burn out a 30 amp ammeter. You will have to buy a 60 amp ammeter to use with the high output alternators. Once again, we suggest a voltmeter instead, for reasons already stated.
Finally we get to the gas gauge. This is a two part unit, the dash gauge and the tank unit. These will only work properly on the voltage for which they were designed. If you put 12 volts into a 6 volt gauge, it will burn out one, or sometimes both units. This can be solved by a voltage dropping resistor. These are commonly sold in auto parts stores under the trade name Volt-a-drop. If you can't find one of these, use another ignition ballast resistor and run the gauge on 7.5 volts. It won't be completely accurate but it will work. Don't use the same resistor you use for the ignition as it could affect the ignition by "robbing" voltage from it. The best solution for the gas gauge is to buy a new 12 volt Stewart Warner® dash unit and matching tank unit.
What about the accessories? Perhaps a heater or electric wipers? Once again, the Volt-a-Drop solves the problem but this time you will need the larger size for the greater amperage draw required by these types of accessories.
Refer to the wiring diagram for connection of the previously discussed components.
If possible, have your 6 volt starter wound for 12 volts by an electric motor specialist, or switch to a 12 volt version of the starter motor. It is possible to run a 6 volt starter on a 12 volt system, but the amperage draw is considerably more and increased wire size would be indicated.
This conversion is usually done to simplify a military electrical system and make lamp and component replacement less expensive with easier to obtain parts.
Even though we are doubling the amperage by cutting the voltage in half, the military wiring will work on 12 volts. The biggest problem you will have here will be with old dried out, hard insulation and "tying into" the wiring because of the odd military waterproof connectors.
We have tried using a 12 volt system with two 12 volt batteries, wired in series for 24 volts so they both charged, and tapped off one to get 12 volts for starting. It worked but the dual batteries were a pain and when you need a regulator or generator in 24 volt, be prepared for some expense.
As in the 6 to 12 volt swap, we advise the use of an internal regulator type alternator. If using a GM engine, this will bolt up with stock brackets. Otherwise, a special bracket will have to be made for the engine and a wide pulley made up for the alternator to match the early style wide belt, or in some cases, dual narrow belts.
If doing an engine conversion, the engine being used will have a 12 volt starter, relay, etc. If simply converting a stock M38A1 to 12 volt, you will have to get the 12 volt civilian starter, distributor, and ignition coil.
Gauges are a little more of a problem on 24 to 12 volt swaps. The military "charging indicator" is a combination volt and ammeter. The charging indicator will be useless on 12 volt. The fuel gauge will be operating on half its normal voltage which will make it about 80% incorrect. Once again, we advise using a voltmeter and a new dash and tank unit for the gas gauge.
All bulbs and the sealed beam headlights shold be replaced with 12 volt units. The 12 volt headlights won't be compatible with the military connectors but these can be cut off and standard three-prong headlight plugs used instead. Check your wiring to see that you don't "cross" wires and get the high and low beam wires mixed. You could have one high and one low beam, or both low beam hooked to the high beam wires. The high beam indicator should (obviously) indicate high beam and the civilian connectors wired as such.
This diagram and information is based on the use of GM parts.