The Stovebolt Site Forums The Shop Area The Electrical Bay its a 6 volt, no its 12 volt, no its 6 -- OH NO ITS BOTH

I tore into the electrical system for my son's new '55 1st GMC today. Surprise!!! It has two 6 volt batteries. They are wired in series providing 12 volts to the starter, head/tail lights, alternator, and ignition system. The system is then half tapped (only one of the batteries used)to provide 6 volts to the cab, dash, instruments and heater. :rolleyes: I mean, this is really clever stuff, and I can't think of any theoretical reason it shouldn't work just fine. I expect its going to take me a full day to modify the standard wiring diagram to reflect the changes though -- and to confirm that he did it right.

It seems to work very well. Everything seems to operate just fine with the sole exception of the horn (any wire having anything to do with the horn is swinging in the breeze). Maybe he got that far and was finally stumped.

Has anyone ever done this before? Has anyone ever HEARD of this before? Can anybody think of any reason I shouldn't just finish the horn and leave it alone?

adrift in the Great Salt Lake
jerry

Sounds like an interesting set-up.I would like to know more.WHere are the batteries mounted?
Do you have any pictures?

Hardwareguy,

In the days of tar-top batteries, it wasn't unheard of to actually run a screw through the top into the internal connector between the middle two cells of a 12-volt battery. Voila, 6-volt tap for instruments with 12-volts for everything else.

Hardwareguy,

Got to thinking about where I heard about the tapped battery trick.

A trip to my 'library' reveals a nice article in Petersen's complete book of Engine Swapping No. 4 (published in 1975) about so-called 'dual systems'. One uses the tapped battery, another uses the dual 6-volt batteries, and still another uses a 'special electrical conversion relay system' that included directions and a self-tapping screw for making the 6-volt 'pick-up terminal' in the middle of a 12-volt battery.

Kinda depressing looking at that book, I remember buying it new and now it's 27 years old!

Mechainaic, don't have any pictures but the first battery is in the battery box and the second is mounted on the inner fender on the passenger side near the firewall. The ground cable connects to the terminal in the box and the other terminal connects to the battery in the engine compartment. Those are the only connections in the battery box. Everything else then connects to one or the other of the terminal on the battery depending on whether it needs 6v or 12v.

OlBlue, I'm going to look for that book, maybe in the public library? The book is surely out of print but who is the publisher anyway? I'ts really straightforward and I can't imagine that drawing it out is going to be much trouble but I'd love to do a little research before I get started on the electrical plan. The screw in the battery trick is wonderful. Kind of obvious (once it's said). I'm surprised someone doesn't make a "half tap battery" providing even a screw terminal access to 6 volts. Then again, maybe they do.

They do make 6/12 volt batteries; some of the vendors carry them though I can't recall exactly who off the top of my head. I can look if you're interested.

I'm not likely to be changing too must real soon but if you run across them I would appreciate an email. thanks, jerry

Are you guys thinking about the 6/12 batteries from www.antiqueautobattery.com ? I was thinking about these myself, leave the wiring as 6 volt and run some new accessories off the 12 volt portion of the battery.

My GMC had two batteries installed by PO. 6V under floor, 12V on the firewall. To start the truck, he connected the batteries with jumper cables.

Mb specifically, the "12/6" battery they show would be how my system is set up, only with two 6v batteries instead of one 12v with a half tap. The rig appears to have been "converted" to 12 volt but the accessories were left as 6 volt. I'll know more when I have time to draw it all out.