6 Volt Positive DC conversion to 12 Volt Negative

Thread Starter

Gary Sandberg

Joined Sep 17, 2006
I am in process of putting a newer motor with 12 Volt Positive ground Generator in my 1949 Nash Statesman which is/was 6 Volt Positive Ground electrical system.

The various lights will be changed to 12 Volt light bulbs in lieu of the 6 Volt light bulbs, simple, RIGHT?

There is two 6 Volt motors, one for defroster and other for heater. I assume I will either have to reduce voltage to 6 volts and switch the ground conductor to the power supply and connect the tail of the power lead to ground to keep the motors running in same direction. CORRECT??

Alternatively, I could swap out the 6 Volt motors for 12 Volt negative ground motors

Lastly, and most tricky is the gas gausge, oil pressure gauge, and engine temperature gauges (all designed for 6 Volt Positive ground. All three gauges receive power from one conductor and are wired in parallel with the power then going to three sending untis which then go to ground.

It will be very difficult to get 12 volt sending units for all guages so I would like to cut the voltage to these three guages from 12 volts after the conversion down to 6 volts. Can someone help me with the sizing of the resister to acoumplish this voltage reduction?

Unlike the motors, I don't beleive the polarity switch will affect the guages. AM I correct in that ASSUMPTION?

I hope I have explained enough in detail to faciltate a response. Thank you in advance


Joined Apr 20, 2004

Sounds like quite a project. I guess the windshield wipers are vacuum powered, so you don't need to change any motors there.

As far as the gauges go, the oil pressure should be a bourdon tube type, so no power is used. The gas and temp gauges may be the same as modern ones. The sender in the gas tank is built like a potentiometer, so it will read backwards unless the source voltage is changed. Also, the temp gauge is a resistance that changes with coolant temperature - also going to work backwards.

The temp gauge is grounded to the engine block. Your best bet is to feed it with -6 volts, perhaps from a DC - DC converter. Current should be in the milliamp range. The gas sender may be the same. You might want to put an ohmmeter on it to see what the fixed resistance is.

The blower motors are going to be interesting. They will draw a lot of current - that's a drawback to 6 volt systems. If they are electrically isolated from the cases, you might be able to switch polarity by moving the power leads. Dropping voltage is another matter - it's hard to supply the current they want.

It almost makes you want to suggest running two 6 volt batteries in series so you can take power off one battery and avoid the level dropping. Then all you need to do is find two 6 volt automotive batteries....

Chris Wright

Joined Jul 26, 2006

I've got a '59 MGA that I will also be doing the switch from positive to Negative "earth", as the British say. (I never knew a car had wings, boots and bonnets until I got my MG.) Is the Nash statesman anything like the Nash Metro? I.E. full of BMC parts or is it mostly Detroit parts?

If your "new" generator is originally from a positive ground car, as you state, you will need to "flash" the generator and you will also need to switch the wires on the coil. Here is a page talking about switching polarity:

Do you have an electrical wiring diagram for the car? It may tell you some things you will need to know; Like, are there any diodes in the circuit or in any components? (Mine has one in the electric fuel pump, You've got an engine driven pump, right? Some relays have them too, but weren't very common back then.)

You will need to know the amperage or wattage of the the blower motors to figure either a dropping resistor or to get a total draw if a 6 volt regulated power supply is used. Start with how big is the fuse for one of the blowers and what else is on the same fuse, or look for a data plate on the motor. Not all early blower motors are polarity sensitive, if your motors have two black wires they may not be. If they have different colors they probably are. And if there is only one wire the case is grounded.

J. C. Whitney has two models of 12 to 6 volt converters for $19, but they don't state what the devices are, they just state that one has 6 and 8 amp terminals and the other has 10 and 20 amp terminals. Could be nothing more than a case with resistors in it. If you are handy, for $10 you can make a 6 volt regulated power supply to run your blowers and gauges, but you are going to need to know the total amperage or wattage of everything you will want to use with it.

What about the windshield wipers? Don't tell me they are vacuum operated!

The gauges I'm not so sure about. I would have thought that they used a standard d'Arsonval movement and would be polarity sensitive, but the gas gauge in the MG is happy with either polarity (the other two are mechanical). Here is an explanation of why: http://mgaguru.com/mgtech/electric/fg_03.htm (The theory of operation is toward the end). So it is going to depend on how your gauges are constructed.


Joined Apr 26, 2005
What about the windshield wipers? Don't tell me they are vacuum operated!
If his Nash was 10 years older than my '59 chevy pickup, they certainly can be vacuum operated. I wish I still had that pickup, but that was 34 years ago.
have you completed your project. (i just joined today)'
I have a 1951 Moskvich...positive ground. I am only thinking about going over to 12 and remaing positive ground.
however, your experience may convince me otherwise.
look for ward to hearing from you. john mcenaney


Joined Jul 17, 2007
Hi John,
This appears to be a very old thread, and the original poster never replied. You might try clicking on his name in this thread to bring up his profile, and see if he left provisions for members to E-mail him a note.

My understanding has been that the change from positive to negative ground results in a much better spark from the ignition system. It certainly makes it much easier to add a modern sound system!

I've converted a few antique British automobiles (1959 MGA Twin Cam and a 1960 Triumph TR3) from positive to negative earth/ground, which is actually quite easy to do. The conversion from 6v to 12v would be much more involved.