Old 6 volt systems, can you jump start with 12 volt?
 

A friend asked me to help him get his old ‘55 F500 moved off of his farm to display at his business for the holidays. Anyway I said sure, but when I got there it was a 6 volt system. Battery was totally dead as it hadn’t been run in 2 years. We ended up just pulling it as I didn’t want to risk frying his vehicle or my trucks electrical system.
In all my years I never had to deal with a 6 volt system that I can remember. Can a 6 volt system be jumped with a 12 volt or not. Googling came up with conflicting information.

NO. That would be a bad thing.

George

Yes it can. No it shouldn't be done.. I used to jump a 6 volt 1955 international truck I owned with 12 volts and never had a problem. I never did it with lights or radio on just the blower fan to absorb some voltage. I also did it fast like kissing your sister. Just enough to start it. The starter generator regulator and ign system can handle a short 12 volt burst, but not lights or radio. I was 18 years old in 1970 on my own with no parents and in my 1st year of college. I did what I had to do until I could scrape enough$ to buy a battery. Probably jumped it a dozen times. I ran it for 3 years with no problems before i sold it and even took it across country from Tucson Az. to Pittsburgh Pa. In Pittsburgh I worked at a coal wash plant in the country in the middle of nowhere. An 18 degree below 0 day after work it wouldn't start Cranked real slow. A buddy came out with a 12 volt car to jump me, but it still cranked to slow. I built a fire under the oil pan and after it got cooking it started right up with a jump. I don't know, but I don't see how a jump from 12v to 6 v could damage the 12 volt system. If anybody would know it would be George and I would appreciate it if he would enlighten me.

Best method.....when your 6V battery is a bit shy
 

Thanks guys. He asked me to work on it after the holidays, so it’ll be a leaning curve for me! Brakes went right to the floor, I had to slow down with the emergency brake!
Cool old truck though.

Few more

I got my first car in 1954, so I've used 12 volts many, many times to start 6v/8v cars/trucks/tractors. As stated, don't leave it connected while starter's not running.

There have been days when I've wanted to put a crank like that on the front of my Chevelle.

K

WOW...I didn't know it was such a common practice.

If I were to attempt it, I would be concerned about the following:

1) if the 6V battery was still in the circuit, and not totally dead, there may be a high inrush of current into the 6V battery....don't know if it would be high enuf to blow either battery but i'm not going to test it........

2) The engine should crank really well, but leave it connected only to crank and disconnect other circuits like lighting, etc. as mentioned. Obviously the ignition circuit will still be connected, but prolonged connection to 12V may damage it.

3) I don't recommend doing the above, but when caught between a rock and a hard place, do it but be cautious.

George

All OEM ignitions that weren't transistorized were 6-volts. 12 volt systems started as 12-volt and then switched a resistor into the ignition to reduce it down to 6. The purpose of this was to reduce the voltage through the points and keep them from burning. You could use a GM starter with an "R" connection or a Ford starter solenoid to switch a resistor into the circuit. I believe they called them a ballast resistor but that wasn't what it was, actually.

There's going to be a lot of hydrogen released when this is being done, be very mindful of creating a spark when hooking the cables up, and the last connection should be made to a ground away from the battery, same when unhooking, first disconnect should be a remote grounding point.

If you've never had a battery blowup from hydrogen explosion, you don't want to. I've personally had 2 explosion while working under the hood and witnessed carnage from just starting a car and there was an arc, blowing the case of the battery all to hell.

I would also disconnect from the 12v side first. That's where your arc will likely occur. The gases will likely be at the 6v battery.