Converting 6V- to 12V+

I am sure this must have been discussed a thousand times on this site but maybe you will enjoy my picture anyway!

We have a Thompson refueler c1950. It is powered by an English Ford 10hp side valve engine and is presently 6V positive earth which we want to convert to 12V negative earth.



We have a 12V coil and ballast resistor, it will be no problem changing the few light globes, there is no windscreen wiper and I am sure the starter motor and horn will really enjoy the 12V.

This leaves only the dynamo (generator) to take care of.

Do I need to find a voltage regulator/cut-out or can I just put a diode in the main output lead of the dynamo and give drive to the field coil direct from the ignition circuit?

Welcome Mr John!

Just curious why you want to change it to 12v?

With only a 10hp engine, 6v will work just as well as long as everything is right. That looks like a pretty rare piece, would be a shame to loose it's originality.

But if you do want to change it, as for the charging system, most people do away with the generator and replace it with an internally regulated alternator.

If you want to keep the generator, it can be made to work on 12 volts if it has a regulator. The regulator would need to be changed out for a 12 volt. The original regulator possibly could be readjusted, but that is a meticulous procedure, a crap shoot at best.

If all it has is a cut-out relay, I don't believe that can be converted, not enough adjustment in the generator.

Back to the 6v system, are you having starting issues? Charging problems? Usually starting problems are caused by poor connections, or too small cables. Standard automotive cables are too small for 6v systems, They need size 0 or 00 cables, available at heavy truck/equipment suppliers. Also a worn starter will cause problems. If the starter is needing attention, converting to 12v will likely not solve the problem, and could finish off the starter. Another problem with 12v conversions, some 6v starters work well with 12v, but others do not like the extra shock of hitting the flywheel harder. It can break the drive, the housing, or flywheel gear teeth.

Another consideration is the starting ease of the engine. If the compression is down, the fuel system needs attention, or the ignition is weak, converting it will make little difference, and could quickly damage the starter from over heating it.

If the generator is not working, or the battery bad, those are easily fixed. If there is a problem with the battery discharging between uses, a battery tender will always give you a fully charged battery to begin with. Starting out with a fully charged battery will make up for a low amp generator and frequent starts/idling.

Let us know...

Hi Steve

Thanks for the welcome. We are an aviation museum in New Zealand with a variety of vehicles, tugs, forklift, truck etc, and we want to convert to 12V partly to avoid the high cost of 6V batteries and also to standardise our fleet where for various reasons we often have to use jumper cables.

Thanks for your comments and advice.