Electrical ProbeDear Ole Mechanic;

I have an old pickup that I am fixing up.  It has a 6-volt electrical system.  Can it be changed over to 12-volts?  If so, what do I need to do?

Shorty Circuit

Dear Shorty;

Sure, it can be changed over, but like most redesign projects involving cars and trucks, it will take at least twice as long and cost at least twice as much as you originally think it will.  Also, you will need to be--or need to get--very comfortable with electricity--Ohms Law and the volts, ohms and amps that make it all work.

That said, I will give you some good news and then some bad news.  If the existing 6-volt wiring is in good shape without any cracks or bare spots in the insulation, it will handle 12-volts with no problem.  The bad news is that not all of it will be needed and some new wires will be needed.  You should grab a couple of different wires in the engine compartment and bend them a few times to see if the insulation cracks or comes off.  If it does, then you will probably need to replace most of the wiring.  Back to the good news, if the starter is in fairly good shapen then it will not need to be replaced  as most 6-volt starters work great on-12 volts.  This is true even if the 6-volt system is a positive ground. 

When you go to a 12-volt system, it should be changed to a negative ground.  Changing the ground will effect some components but wires and bulbs could care less.  I will note the affected items as I go.  What will need to be replaced is the generator.  Either a 12-volt generator with a regulator or an alternator will need to be used.  Because of its simplicity and longer, more trouble free operation, I would recommend an alternator.  If a GM alternator from the 1980’s or 90’s is used, then a separate regulator is not needed, as they have internal regulators.  Most of them can be set up so that only two wires are needed for them to operate correctly and one-wire alternators are available (in chrome if you want to pay for it).  Of course, an alternator will need a new mounting bracket.  Build it strong and be very careful to get the belt lined up right.  All the bulbs will need to be replaced; fortunately, bulb sockets have been standard sizes for many years.  Replacement sockets for those badly corroded or rusted ones are as close as the nearest parts store or automotive mail-order catalog. Don’t forget the instrument panel or gauge lights and interior lights.  Most switches will handle 12-volts with no problem if they are in reasonable shape.  The heater or defroster motor may need to be replaced, but you should try it with 12-volts, as it might work for short periods if the truck was originally a negative ground.  If it was a positive ground, it might turn backwards.  Testing is the only way to find out and this applies to any other electrical motors as well,except for the previously-mentioned starter. 

The ignition is rather simple to convert, since the points and condenser do not care if they are positive or negative ground.  The coil is a different story, as it must be hooked up to match the ground and it is voltage sensitive.  Either replace the 6-volt coil with a 12-volt one or put in a ballast resistor in the power-feed wire to the coil.  One for a 1957 Chevy car works fine and they are readily available.  A 6-volt ignition system will work on 12-volts for a short while, but the points will pit and burn quickly and sooner or later the coil will burn out--probably sooner.

A note here, if you are working on a old Ford, they already have a ballast resistor that drops 6-volts down to about 4-volts for the ignition system.  Do not eliminate this resistor but do put in another ballast resistor to drop 12-volts to 6-volts.  Old Fords are the only vehicles that I know of that have a resistor in a 6-volt system.

Electrical Voltage TesterNow for the single largest problem, because I have saved the worst for the last.  That is the dash gauges.  Some older trucks had mechanical oil-pressure and engine-temperature gauges and their readings would not be affected by a change in the electrical system.  Any electrical gages will need to be replaced or isolated and provided 6-volts for correct operation.  This can get even trickier if your truck has what is called a constant-voltage regulator built into the instrument cluster.  It drops the original 6-volts down to about 3-volts for the gauges.  Isolating the gauges from 12-volts and providing 6-volts can get tricky, as the dash is usually grounded to the battery through the body and frame.  If the power-feed wires to the gauges can be isolated, then a resistor can be put in the wire to drop the voltage down to the 6 or 3 volts the gauges need.  If the truck was originally a positive ground, the gauges may read backwards with a negative ground.  All you can do is try it.  Don't ignore this instrument voltage-requirement, as it involves the gas-tank sending-unit and you do not want a short and sparks in the gas tank.  The simplest solution to this problem is to install new 12-volt gauges.  Some antique-looking, new 12-volt gages are available but only for the most popular trucks like the old Fords.  If you are dealing with an old Studebaker or Hudson truck, then you may need to make a new dash or instrument cluster.  If you go with new gauges, then be sure to get the correct sending units for them.  To simplify things, tackle one system at a time and test it to make sure it is right before moving on to the next system.

Easy?  You didn’t ask about “Easy, you just asked if it could be done.  Don’t get your wires crossed. 

Herr Professor Nuzanbolts