6 or 12?


IHC in the early to mid-fifties.

Golden Jubilee
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Post Thu Sep 06, 2018 12:44 pm

6 or 12?

Hopefully tomorrow my new 56 S120 comes home. One of the first thing I want to address are the electrics in it. Right now it has a weak 6 volt battery in it I also see that someone has put a 6 volt coil in it and the truck starts very hard but does run this way. Doing some research some claim that 56 was the year IHC went to a 12 volt system on their trucks while other authorities on the subject say it was 57. Which is it and more to the point what is the correct system for my 56?
56 S120 4x4,

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Post Thu Sep 06, 2018 1:58 pm

Re: 6 or 12?

I'm not sure of the first year IH switched to 12v. For your pick-up though I'd check to see how many volts the generator is putting out? If you have a 12v system and it is hooked up to a 6v battery I imagine that wouldn't be too good.
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Post Thu Sep 06, 2018 4:19 pm

Re: 6 or 12?

6 volt positive ground was standard for the S series trucks. 12V was a paid upgrade.

My advice, if the 6 volt electrics work, consider sticking with it. The best upgrade you can do for the 6V is to buy new 00 (double aught) battery cables and install. You can buy premade ones on Ebay, I did. The price is decent and cheaper than I could buy parts/tools to make my own.

Keep in mind, if it has a 6 volt system in it, it was most likely delivered from the factory this way. Have you ever heard of anyone converting from 12V to 6V?
Last edited by 56-S-Fan on Fri Sep 07, 2018 12:51 am, edited 1 time in total.
56 S-120 4x4 started the whole mess.
S-132 dually longbed,
S-120 4x4 Travelall,
S144 Forestry truck,
S160 Holmes tow

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Post Thu Sep 06, 2018 5:31 pm

Re: 6 or 12?

56-S-Fan wrote: Have you ever heard of anyone converting from 12V to 6V?


Not really, but someone not knowledgeable about old trucks may have messed with it in the past without looking at what components it had. Time will tell on this one.

BTW if it does turn out that most of the pieces are 6 volt I'm going to keep it that way and thanks for the source on double ought wires!
56 S120 4x4,

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Post Thu Sep 06, 2018 6:53 pm

Re: 6 or 12?

I recommend Brillman for cables. They will make them up to whatever length you want 00 gauge, or on up and are helpful, friendly and reasonable in price.- Stickman 207
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Post Fri Sep 07, 2018 6:01 am

Re: 6 or 12?

Keep in mind that in order to get work done, it takes watts, electrically speaking.

Watts equal volts x amps.

So in order for, say, your 6V starter to crank your engine, it takes X number of watts/kw.[Say, around 1.2kw or so, if your starter is in good shape.] To crank the same load on 12V, it takes half the amps that 6V takes. THAT is why 12V cables are generally a smaller gauge than 6V cables.

Same principal applies for your other electrical loads.

Keep that in mind when working on your electrical system, and you won't go wrong.
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Post Fri Sep 07, 2018 8:04 pm

Re: 6 or 12?

I Still run 6 volt in my 1955 R120, but I run a one wire 6 volt negative ground alternator and a 950 Cold cracking amp tractor battery with the double aught battery cables and everyone thinks it is 12 volt when I start it. Also replaced the headlights with 6 volt halogen. Very pleased with the results. I also just installed a 6 volt electric fuel pump as I had a problem with vapor lock. I should be good to go anywhere!!

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Post Sun Sep 09, 2018 1:49 am

Re: 6 or 12?

I got to look things over today, put the battery (6v) on a trickle charger and started cleaning components up in hopes of finding ID tags or labels to find out what voltage the system is. The best I could do was identify the headlights as 6 volt, even the voltage regulator had no markings.

Now here's the latest! The battery was hooked up with the negative terminal to chassis ground and the positive to the starter. To drive the truck home I had to jump the battery and it ran ok for the 15 mile trip. I figured the charging system was alright because the truck must have been running off that not the battery. My question is shouldn't the positive terminal been hooked to ground and would it make any difference with the battery connected as negative ground?
56 S120 4x4,

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Post Sun Sep 09, 2018 9:06 am

Re: 6 or 12?

With no electronic components like an alternator with diodes, the truck doesn't care and the generator can be used on either polarity if it has been polarized to that polarity. So before you run the truck after switching, you need to re-polarize the generator ( do a search on the forum for how to do). Look at the coil if the + side of the coil is connected to the points, it is set up for + ground, if the - side is to the points it is set up for - ground.
Look at the starter and generator for tags, black tag = 6 volt, red tag= 12 volt orange tag = 24 or higher volt

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Post Sun Sep 09, 2018 6:41 pm

Re: 6 or 12?

cornbinder89 wrote:With no electronic components like an alternator with diodes, the truck doesn't care and the generator can be used on either polarity if it has been polarized to that polarity. So before you run the truck after switching, you need to re-polarize the generator ( do a search on the forum for how to do). Look at the coil if the + side of the coil is connected to the points, it is set up for + ground, if the - side is to the points it is set up for - ground.
Look at the starter and generator for tags, black tag = 6 volt, red tag= 12 volt orange tag = 24 or higher volt


Thanks for that information, It all makes sense to me and was a big help. I had a little time this morning to look things over and found that the coil was not wired right for negative ground so I changed that. If the battery holds a charge I will be getting it hooked back up in a day or two and see how it goes. If the battery is still flat then I'll have to wait until I get a few dollars ahead to buy one.
56 S120 4x4,
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