The rule that makes 12-volts work so well is the one that says if you double the voltage, the amperage required to power that same accessory is reduced by half. For example, the original 6-volt motor requiring ten amps of electrical current would only need five amps of electrical current to run at the same speed. So with this, the total load for the Model A with the lights on running at night and trying to blow the horn would only be half as much with a 12-volt system as with a 6-volt system.
Six-volt electrical systems use heavier gauge wiring to deliver the higher amperage draw. Therefore the wiring does not have to be changed when converting to 12-volts. Also the windings in the starter and horn are also heavier due to the amps used in a six-volt system.
If you are still using a six-volt generator it can be converted to 12-volts. You will need to have the field coils (magnets) replaced. These are the magnets that are attached to the inside of the generator case. This is a job that can be handled by any qualified alternator/starter repair shop. You will then need a 12-volt regulator. Just ask for a 1956 or newer 12-volt regulator to match your application.
Alternators were first introduced in the early 1960’s. By the 1970’s alternators were being built with solid-state internal regulators. This improved the reliability of vehicle charging systems. The biggest difference between an alternator and a generator is that an alternator is able to produce a strong electrical output at idle and low rpms. This makes the headlights bright all the time, and provides a way to keep the battery fully charged at all times.
An alternator generates electricity just the opposite way a generator does. Inside an alternator, the armature remains stationary while the field coils rotate. That makes the alternator able to spin up to 10,000 rpms before suffering any internal damage. Therefore a smaller pulley can be used on an alternator to keep the rpms up while idling.
The best alternator to buy is a General Motors 10SI series alternator. You will want to have the alternator made into a single wire alternator. These alternators are also available through your Model A vendor.
You will have to change all your light bulbs to a 12-volt lamp. This will make the lamps you use a standard lamp available at your local automotive store. This convenience will help eliminate the amount of parts that you will need to carry.
Your Model A was built with a positive ground system. This means that the negative (-) terminal of your battery is connected to the starter terminal and the positive (+) side of the battery is connected to ground. In order to use the above modern alternator on 12-volts, you will need to reverse the polarity. When this is done, you will also need to reverse the wires on the amp gauge in the dash and the coil.
The ignition coil will either need a resistive ballast added to the incoming power or my recommendation is to buy a coil with an internal resistor. Internal resisted ignition coils are much more reliable and will eliminate a common trouble spot, the external ballast resistor. One coil that you can get is a NAPA Echlin PT# IC64. Remember that this coil needs no external resistor.
Last but not least, you will need a 12-volt battery. Make sure to measure your battery box, so you can get the right dimensions for your new purchase. I would recommend that you find one with the cold cranking amps close to the six-volt battery. Happy driving.
This tech tip was published in the March/April 2002 “A” Quail Call.