[67Elcamino]

Can you use a 12 volt battery to start an engine over that has a 6 volt setup. Actually the car is a 6 volt. I believe I read that you can turn the engine over with a 12 volt battery. Also the car is a positive ground car. 1940 Desoto. Is there anything I need to watch for? Im not planning on using the car with a 12 volt batttery just trying to get the engine to start. I do plan on eventually converting it to a 12 volt

[SSedan64]

Yes, it will work, Starter will spin up quick.
You'll need to use a Ballast Resistor to protect the Coil & Points. It will drop the Voltage down to about 9V+.

[rdupree]

Not sure how you are trying to do this, but DON'T use a 12volt battery to jump start a 6 volt battery. Bad things can happen. You can install the 12v battery as positive ground in place of the 6 volt unit. Leave the lights and radio turned off while you have the 12 v battery installed.

[timothale]

If you do a conversion to 12 volts you can keep the 6 volt starter. We used to put in 6 volts starters in 12 cars to get them to turn over and start at 40 degrees below. You just don.t crank very long or you can burn it up. And the starter doesn't matter if it is pos or neg ground

[67Elcamino]

Originally Posted by SSedan64

Yes, it will work, Starter will spin up quick.
You'll need to use a Ballast Resistor to protect the Coil & Points. It will drop the Voltage down to about 9V+.

Where exactly would the ballast resistir go?

[Cape Cod Bob]

Why are you wanting to use a 12 v . Why not just get a 6 volt until you convert.
Cars ran great on 6 and still do.

[Oldguy48]

If 6V electrical systems were "Great", then why did the auto manufacturers switch to 12V ? I had a number of cars with 6V systems in them years ago, and had trouble with each and every one when trying to start them when the weather got cold. And those cars were properly maintained. Never again...ever !!!
And to answer your original question, I jumpstarted my 1948 Plymouth (6V) from a 12V battery numerous times when it refused to start in cold temperatures. Usually started right up, and didn't seem to suffer any ill effects.

[vicrod]

Also, consider the other 6 volt devices in the car like the fuel gauge. 12 volts will fry it. Disconnect it.
Also, disconnect the generator wiring.
The oil pressure and water temp. gauges are mechanical and the ammeter is Ok.
Turn off all other 6 volt equipment.
The wire leading to the coil should have a ballast resister. This will get very hot but helps to limit the current going to the coil. This resister and/or the coil may burn out with 12 volts applied. Possibly replace the coil with a 12 volt but a ballast resister is still required.
Remove the spark plugs and spray some kind of rust penetrating oil stuff into the cylinders. Try to turn over the engine with a wrench before trying to start it in case it is rusted.
Then a pre-lube oil pump should be used to pump oil throughout the engine.
Then spin over the engine with no spark plugs to make sure the engine develops oil pressure.
Fresh gas needs to be flushed through the fuel system. The carb may need to be cleaned.
Check for spark and fuel.
Then put in the spark plugs and_________________________.

vicrod

[EOD Guy]

Jumped started many a 6 volt vehicle with a 12v battery.... I turned off anything that turned off, hook up the jumpers and started it...... now I had a buddy next to the battery to undo the jumper cables as soon as it started...... I don't recall ever burning up anything.

[ogre]

same here
jumped many a 6v w/12v
never had a problem
but most were industrial machines

[vicrod]

If the 6 volt battery is still connected - in theory it will load down the 12 volt battery and help to regulate the voltage to 6 volt system for a short time.
Be very careful when connecting a 12 volt battery to a 6 volt battery of unknown charge state. A large current will flow to the 6 volt battery resulting in gassing of both batteries. Any spark can ignite these gasses causing an explosion. This is true even when a 12 volt to 12 volt connection is made.

vicrod

[67Elcamino]

Thank you I accomplished what I wanted. To turn the engine over. I havent got it started yet but I need to get me e 6 volt battery. The car came with 2 batteries in the trunk. Both 6 volt but I noticed that they where completely dry inside. if these batteries where laying around for 10-15 years are they gone? can I fill them with distilled water and charge? How can I test them?

[vicrod]

IIRC back then batteries were sold without the electrolite (acid). The acid was added just before charging and placing the battery in service.
The condition of these batteries is unknown. The water may have evaporated etc.
They are worth the price of the lead however.
A new battery is the way to go. And keep it charged.

vicrod

[67Elcamino]

Is there any way of knowing if the battery is any good without acid or water? its completely dry. when looking for a new 6 volt battery can I use any 6 volt battery? like a battery of an RV or motorcycle etc..? Im sorry if these questions seem basic but I dont really know electrical

[timothale]

Most motorcycle batterise probably won't have enough Cold Cranking Amps, 40 years ago batteries were built differently, they were put together with melted tar, there were small shops that would drain out the acid, melt out the tar, take out the plate assembley, wash it off , clean out the sludge in the bottom and if the plates and separators were still good put it back together and melt tar back in. and sell you a rebuilt battery.. they usually would last 3 years max. recycle the old ones and invest in a new 6 V