Doubling-Up From 6 To 12 Volts In Your Vintage Ride

Tackling a car build can be a challenge, usually requiring the convergence of a number of automotive skill-sets. Most individuals are often able to cover a few – until they are faced with the dreaded issue of wiring. For many, it’s a bridge they will gladly pay someone else to cross.

If their project dates back to the mid-’50s, or earlier, they are faced with the additional decision of keeping it a 6-volt system or upgrading to 12-volts. For most, that choice should be a simple one to make, as the benefits of the switch – or upgrade as we like to view it – are clear. Unless you are performing an OE restoration where originality is desired, switching to a modern 12-volt system is practical and cost-effective.

To make that process even easier, we turned to Ron Francis Wiring (RFW) and its online resources. We’ve also cracked open the RFW catalog, which contains all the hardware and information you’ll need to get the job done. We’ll also educate you on some of the common mistakes to avoid and tackle the process as if you’re doing a fresh wiring installation. If you have an existing 6-volt system you’re converting, we’ll let you know what you can keep – and what you will need to replace.

Is More Better?

The most obvious question is why make the switch? If you’re dealing with an older car that has no wiring installed, you can go in either direction. If the original wiring is still in place, it’s going to be at least 65 years old with matching wire technology. As pointed out, unless originality is a requirement, the conversion makes sense. The most obvious benefits of a 12-volt upgrade include increased engine-cranking capacity; along with the use of modern lighting, and accessories like power windows, electric door locks, air conditioning, and radios.

One consideration you’ll need to ask yourself is what accessories will you be installing? The foundation for happiness with your car and its operation starts with a wiring kit that can handle those accessories. Before making a purchase, look at the various suppliers on the market, and do your homework. There are different kit levels available and the components you plan on installing will dictate the best choice.

Converting from a 6 to a 12-volt system can often involve scenarios that are less than ideal, like the pile of the original, cloth-wrapped harness that is at least 65 years old.

Ron Francis Wiring offers four different wiring kits to cover anyone’s build requirements. The Retro Series kit is its most basic offering and comes in either a 6-volt or 12-volt configuration. It is also the least expensive choice and will meet the basic needs for someone who doesn’t plan to add or upgrade any components. That doesn’t mean it’s watered down though. Designed with most of the terminals and connectors pre-installed, it comes with Hi-Temp, cross-link wires that are clearly printed with each function for easy installation. Keep in mind, this is a pre-wired kit, and wire length on a pre-wired kit should also factor into your purchase decision. As an example, the Retro Series kit comes with 25 feet of wire from the headlight switch to the rear taillights, and 22 feet to the headlights. Pre-wired kits should be mounted on the firewall, so you’ll be limited in the placement of the fuse block since wiring the vehicle begins at the fuse block and goes outward to each component.

The choices are varied when buying a wiring kit, but one of the key factors of that selection process will be options your vehicle will have installed. As an example, Ron Francis Wiring offers four distinct kits that take into consideration the overall installation, cost, and any planned accessories. With the switch to a 12-volt system, one of the key items to replace will be the alternator. It is highly recommended that you opt for a one-wire, internally-regulated unit like the one shown.

If you’re looking for a more robust kit where the bulk of the legwork has been done for you, RFW has you covered with its 6 to 12-volt conversion kits. Based on the Bare Bonz fuse panel, this kit is designed for whatever brand vehicle you’re working on. It is a basic kit that will cover the upgrade.

At the other end of the spectrum, you have the Ron Francis Express Series and Access 24/7 kits. These have a forward-thinking design that allows you to add accessories in the future. Unlike the Retro Series, these kits have an expanded fuse block including an electric fan relay, and have some mounting flexibility with regards to where they can be installed. Also, they are not pre-wired. With kits like these, you run the wires from the various components to the fuse block, which allows for a cleaner installation. These also have Hi-Temp cross-link wires that are clearly printed with each function for easy installation and can be tailored to your specific application.

The foundation for a replacement 6-volt harness (left) is usually very similar to its 12-volt counterpart (right). The two harnesses shown are from RFW and are based on their Retro Series wiring kits. Retro kits are pre-wired and are geared as a replacement for an existing harness where no additional options are being installed.

No matter which manufacturer’s harness you decide to use, there are a few things to keep in mind before making a purchase. For some individuals, the temptation is to find something cheap. Places like eBay are a prime source for kits that look like they will work at a fraction of the cost. Reputable kit makers will sell you a product that they’ve banked their reputation on, and part of that reputation is built on the technical support after the sale. Using the RFW kits as an example, each one comes with its own serial number, which will allow their technical support staff to aid you in the installation. If you ever decide to sell your vehicle, the new owner will still be able to get the same service. Avoid being penny wise and dollar foolish, and keep in mind, the bitterness of poor quality remains long after the sweetness of the low price is forgotten.

The Nuts And Volts Of Converting

If you have original 6-volt wiring and you opt to retain it, our advice is to make sure all the wires are still in good shape. The biggest issue will be insulation that has crumbled or worn through in some spots. It will be at least 65 years old and you will be limited in what you can do with it. A modern wiring kit is a right option. Let’s walk through the changes you will need to make based on the components that you’re planning on running or already have in place. You have a number of options.

Converting to 12-volts means you’ll obviously swap out the battery, however, when choosing one, you’ll need to figure out what post orientation will work best with your mounting location. The 6-volt starter can stay in place, but the solenoid will need to be changed. There is a trade-off in retaining the 6-volt starter, as cranking capacity will be greatly improved but its life expectancy will be slightly diminished.

Wiring kit makers have crafted their products for ease of installation which comes down to separating the major components into wire subsets that get labeled and bagged. All wires are color-coded and printed with their specific application. Where applicable, plugs are either attached or supplied within each bag. The RFW kits take full advantage of numbered, labeled, and color-coded mounting points. Insulated fork terminals are used to make the connections on the non-wired kits. Each Ron Francis kit comes with its own unique serial number. Other items, like a Quick Response code (QR code) on the side of the panel allows for the wiring diagram to be displayed on your phone or tablet with a scan.

If you’re running a points distributor, it can be retained. The coil must be swapped along with a ballast resistor, which should be brand-specific for the vehicle you’re converting. If you’re working on a Ford, you’ll need to install a ballast resistor for use on a Ford. You will also need to install an alternator since 6-volt systems used generators. You can install a 12-volt generator, however, we recommend an internally-regulated GM-style alternator as the best option. If you plan on using halogen headlights, you will have to use an alternator, as generators cannot sustain their amperage requirements. If your car has 6-volt horns, most will not work in a 12-volt environment, so the only option is to replace them.

For the interior requirements and components, any of the higher-end wiring kits will have enough circuits and relays to handle the broader range of options found in today’s modern interiors, and all will be supported. If you have a 6-volt wired car, you will be able to keep the existing 6-volt gauges and clock; however, it won’t be a simple plug and play option. You will need to install an electronic voltage reducer in conjunction with the original sender. All the top wiring vendors sell suitable reducers and if you look at the RFW catalog, there are a number of different ones offered. These will also be needed if you have a 6-volt heater and wiper. Our recommendation on the wiper motor is to simply replace it with a 12-volt unit.

Some of the higher-end luxury cars from the mid-’50s had a number of creature comforts like power windows and electric power seats. On 6-volt cars equipped with these kinds of options, the window regulators and seat motors will still work – you just need to get used to them working much faster. If you have an existing 6-volt radio, the best advice we can give you is to simply replace it. Negative-ground radios CAN be used with a voltage reducer, but the question would be why go to the trouble? Interior switches can be retained, however, we recommend that you replace them, as they will be as old as the wiring. You can also retain the stock ignition switch but a relay will need to be added if you are running, or are planning in the future, of adding accessories.

One thing that will need to be completely replaced will be all the existing lighting in the car. All interior bulbs will need to be swapped with 12-volt units. This applies from the front to the back of the vehicle, as all lights will need to be replaced. If your vehicle has turn-signal indicators, the 6-volt flasher must also be replaced with a 12-volt unit.

Having a good ground is fundamental to a healthy wiring environment. This is especially critical in a car with a fiberglass body. The examples shown are the solutions offered by RFW. Retaining the original 6-volt gauges will require the use of a voltage reducer. The RFW VR-1 reducer is an ideal unit for someone that only wants to keep their original gauges, while their VR-4 is the right choice for someone who plans on keeping the 6-volt gauges, heater, and wiper motors. The two other options, the VR-10 delivers up to 25-amps continuous, while the VR-11 is also similar, but has an adjustment for a 6 to 9-volt output.

The last thing to point out as you make these changes, be mindful of having things properly grounded. You can never have enough grounding. We recommend using ground straps or a grounding kit as a means to get this accomplished when adding accessories. We also suggest from a financial aspect, the cost of the additional parts required should be taken into consideration, which should help guide you in the right direction. As stated previously, our recommendation is to start fresh with a new wiring kit which eliminates the need for band-aid items like voltage reducers. Also worth pointing out for anyone tackling a project like this, each vehicle will present its own set of unique challenges requiring slightly different solutions. Ron Francis Wiring has a toll-free helpline to guide you through any unique challenges your wiring upgrade may throw at you.

Article Sources

About the author

John Machaqueiro

John has been immersed in the car hobby since his teenage years. A fan of endurance racing, and muscle cars, those interests have evolved into a journalism career that has allowed him to photograph and write about them.
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