There are many tasks that must be mastered when restoring a pre-'50s bomba. But few elicit as much uneasiness as the electrical system. Whether the plan is to keep the 6-volt system or modernize it by converting to a 12-volt system, even the most competent mechanic may shudder at the thought of rewiring a car or installing a new wiring harness.
So, to help untangle the task of rewiring, we followed OG experts Eddie and Michael Tovar as they updated a '49 Chevy in their backyard. There are actually two operations covered in this article: converting from 6 to 12 volts and a total rewire. (Editor's note--Michael plans to repaint the car soon, so they only "mocked up" the American Autowire circuit/fuse panel and wiring installation for our camera right now. That's why the actual steps of running the wiring throughout the car aren't shown. However, watch for a complete "how-to" article on installing the kit when the car comes out of the paint booth.)
Before rewiring a car, you should have a basic understanding of how automotive electrical systems work. They're similar to plumbing in a house, but with some important differences. Electricity flows through wires much like water flows through pipes. Switches are like faucets or valves. They can stop the flow of current through wires, just as valves can stop the flow of water in plumbing. (Current is measured in amperes, amps for short.)
Voltage is like pressure (the positive side of your battery being high pressure and the negative side being low pressure). Higher voltage allows the same amount of current to flow through a smaller wire, just as higher pressure can push more water through a pipe. That's why the wires for 12-volt systems can be smaller in diameter than those in 6-volt systems.
The big difference between plumbing an and electrical system is that plumbing only goes one way, but an electrical system must be a loop, or circuit. A circuit must also have resistance, in the form of a light bulb or accessory. If there is no resistance in the loop, it becomes a short circuit that can destroy the system and the battery and possibly cause a fire (resistance is measured in ohms).
A typical automotive circuit would be one for lighting. Current flows through a wire from the positive pole of the battery (on some earlier cars the polarity is reversed), through a switch, and then to a light socket and bulb. From there it goes through the bulb's filament (which provides resistance) then back through the car's sheet metal and frame, through the battery's ground strap, and into the negative pole of the battery.
The first step is to change your headlights, taillights, dash lights and horn relay to 12-volt. You'll also need to install an ignition ballast resistor (from Fifth Ave. Antique) between the ignition and the coil, which must be changed to a 12-volt coil. A RUNTZ voltage reducer must be wired to the gas gauge (also from Fifth Ave.) and of course a 12-volt battery has to be installed (an Optima Yellow Top was chosen for this project).
A big advantage of converting to a 12-volt system is you'll be able to run a one-wire alternator instead of the generator. A Competition Specialties Inc. (CSI) GM Performance alternator and a custom straight-six mounting bracket, both from Coppa Automotive, were used in this case. (See the diagram for under-hood wiring details.)
This car has good wiring so it won't be rewired until it's repainted. However, if the wiring is bad--which is more often the case--the best plan is to rewire the whole car from scratch with a pre-wired modular fuse block and wiring harness kit because the pre-'50s cars (and trucks) had no fuses. One bad thing about early electrical systems was the wire itself, which was covered with cloth insulation. Through the years, cloth would deteriorate and the bare wires would touch each other, causing the whole wiring harness to short out and catch fire.
By installing a new wiring harness kit with plastic insulated wiring and fuses, that won't happen. New fuse blocks and wire harnesses with the correct OE colors and connectors are available through mail order for just about any car made in the last sixty years. The Tovars will be installing an American Autowire Factory Fit kit in the '49. Another advantage with these kits is that the fuse blocks have extra circuits so you can add accessories such as air conditioning, power windows and a modern sound system. (Depending on how many accessories you plan to add, a 10- or 12-circuit kit will be big enough.) Now, follow the photos as we show you how to do it in a day the Tovar way.
Parts List
modular fuse panel & new wiring harness
12-volt battery
12-volt coil
ignition ballast resistor
one-wire alternator
alternator bracket
alternator belt
12-volt headlights
12-volt dash and taillight bulbs
gas gauge voltage reducer
Here's everything you'll need to convert a classic to a modern 12-volt system and rewire it with a modular fuse panel kit.
1. First disconnect and remove the old battery.
2. Next install the new 12-volt headlights.
3. Now install the new 12-volt taillight bulbs.
4. Mount the ignition ballast resistor next.
5. Next cut wires the appropriate length and attach the connectors.
6. Attach the ignition and coil wire to the ballast.
7. Tie-wrap the wiring every 6 to 8 inches to keep it clean.
8. Install the new 12-volt coil and attach the wires (per wiring diagram).
9. Remove the old generator.
10. Unbolt and remove the pulleys from the generator and the alternator.
11. Install the generator pulley on the alternator. This will enable you to keep the original pulleys and belts on the engine.
12. Install the custom alternator bracket and alternator. | 
13. Now install the new belt and connect the alternator (per wiring diagram). |
14. Eddie (hardhat) and Michael (cone head) Tovar tackle the task of mocking up the new American Autowire modular circuit/fuse block and wiring harness. | 
15. With the wiring attached to the fuse block a convenient location under the dash is located for mounting the block. |
16. The new wiring kit even includes a new ignition, headlight switch and more. | 
17. The new fuse block will be mounted as shown and the wiring tie wrapped and routed to the proper locations.17. The new fuse block will be mounted as shown and the wiring tie wrapped and routed to the proper locations. |
18. Finally the new Optima yellow top battery is installed and connected. |  |