Starting a 6 volt positive ground car with a 12 volt source

[John Barry [CLC17027]]

If I understand things correctly, it's feasible to jump-start a 6 volt car using a 12 volt source. However, in the instance of a 1940 LaSalle, I'm constantly aware of the fact that it has a positive ground. In this case, I assume that the black (-) cables should be connected to the matching battery terminals on each car. I assume further that the 12 volt source should have the red (+) cable attached to its battery terminal. Finally, the remaining red (+) cable should be connected to ground on the 6 volt car, correct?

It almost goes without saying that all lights, etc. on the 6 volt car should be off, and that the 12 volt source should be disconnected as soon as possible once the 6 volt engine has started.

Comments/corrections are needed. Thanks.

[Cadillac Jack 82]

You have to be very quick about it but yes its possible.

[John Barry [CLC17027]]

That doesn't sound all that safe to me: kind of in the "feasible, but..." category. Thanks.

[Michael Petti]

When I was a kid, my friend had a 1941 Studebaker Commander with 6-volt system. We regularly jumped it from 12-volt batteries because that is what was in most cars in the 60's. Be sure to turn everything off before jumping and disconnect quick. Don't crank the starter too long either. If engine doesn't catch right away, you could damage the starter. We did not damage ours, but we were careful.

[John Barry [CLC17027]]

Thanks: that just reinforced my conclusion.

[John Barry [CLC17027]]

Something of a follow-up to my jump-starting question: I've used Google to look for a 6 volt jump starter pack but haven't found one--or for that matter, one that allows either 6 or 12 volts to be selected. Maybe I haven't searched correctly. Does anyone know of such a device; that is, a 6 volt jumper pack or a dual (6 volt / 12 volt) pack? Thanks.

[Jay Friedman]

I've done it quite a few times over the years.  As mentioned, make sure lights and all other accessories are turned off.  However, the generator, the voltage regulator, the coil and other electrical components that can't be "turned off" could be harmed by 12 volts, so disconnecting the 12 battery quickly is important.  In my experience, however, since it is a DC motor 12 volts won't hurt the starter itself. It will just turn faster, though 12 volts could hurt the starter solenoid.

[Jeff Rosansky CLC #28373]

I don't have a 6v, but have thought about this before.
Would it be wise to insert a switch to isolate the dash gauges when jumping? Easier than pulling a fuse I would think.

[Jay Friedman]

I don't have a 6v, but have thought about this before.
Would it be wise to insert a switch to isolate the dash gauges when jumping? Easier than pulling a fuse I would think.

I've never experienced dash gauge problems after jumping a 6v car with 12v.

[TJ Hopland]

I have done it many many times over the years but usually on more simple machines like tractors so they don't have as much lighting and things like radios to be concerned with.  I have done it with cars and no bad things happened that I know of but like I said my experience is mostly with less complex machines. 

I believe its been covered but you ALWAYS match up the batteries + +  and - -.  Doesn't matter which is ground just make sure if its car vs car that they are not touching metal to metal in some way other than the jumper cables.   If you are a hook up the ground last person and to the engine/chassis that is fine you can still do that as long as you know which is which.  There is some science behind that and its the running rig you want to do that with but the next paragraph addresses the running part so much less risk in this situation. 

What I was always told by the 'old timers' that were doing this on a daily basis back in the transition period when all this stuff was still in daily use is don't have the 'live' rig running.  A 12v rig running especially in the alternator era is going to be in the 13-14v range.  Not running its gonna be more like 12.5 with no load so a lot closer to the 7's that the 6 volt rig would be expecting when running.  Ya its still higher but its an easy way to loose a couple volts to close the gap.  You then add in some crappy connections and possibly crappy cables you would be lucky to actually be able to transfer more than 6 volts especially when you hit the starter.