The Stovebolt Site Forums The Shop Area The Electrical Bay 6v/12v conversion confusion

Where do I start, I purchased a 1951 GMC 451 back in July/August. I was told by the seller that he converted it over to a 12v system, but upon further inspection he installed a 12v battery, the negative battery cable is going to a bolt on the transmission, and the positive is going to the starter. From what I've read in "Tech Tips" the positive cable should be on the negative post and vice versa for the negative cable. Is that correct or am I missing something? Would the 6v electrical components/bulbs still work or would they all need to be replaced with 12v equivalents? I'm not opposed to keeping it all original, I'm just not sure where to begin. The wiring itself is a mess. I'm sure I'll need to rewire the whole thing at some point. I just want to get it operational at this point. Does anyone know of a vendor the makes a wiring harness for "Big Bolts" I've seen plenty for pickups. Thanks in advance for any and all help.

GMC originally was positive ground but if it was converted from 6 volt to 12 volt it was probably converted to the more common negative ground. No need to change it back.

If your truck has been "converted to 12v" the person who converted it probably intended to convert it from positive ground (that the GMC 6v system would be when stock) to negative ground at the same time. I also assume that you have a 12v alternator installed. In that case:

The battery cable from the - post on the battery (the - post is smaller than the + post) should connect to the engine block. I like to use the top stud that mounts the starter motor, but being bolted to the tranny case would probably work OK.

The battery cable from the + post on the battery should connect to the large stud on the starter switch. Both battery cables should be in good condition.

My '52 GMC 12v conversion uses 2/0 size battery cables connected to the stock 6v starter. It spins the engine real fast, but you need to be careful not to leave it engaged too long. I never spin mine more than 5 seconds without letting it rest.

The 6 volt lamps will burn out very quickly when exposed to 12v. They should all be replaced with 12v equivalents.

If you are still running points-type ignition you need either a stomp starter switch with a small lug on the side (my preference) or a coil marked something like "12v, no resistor needed".

The only real difference between small trucks and your 451 is the length of wires going to rear lighting. If you know which end of a soldering iron to hold and how to use shrink wrap making your own harness for these simple trucks isn't that hard to do. It must be easy because I did it on mine.

Thanks for the in site.

Bill, it still has the generator installed. Will that be an issue? The coil says 6v, is that an issue? Another question I have is the headlight switch. Would that need to be changed out to a 12v switch as well? None of the lights work, could that be a bad switch?

I'm still up in the air on converting it back to 6v. I've read pros and cons about the issue here on the forum. I guess it's personal preference. What's your take?

When you redid your harness, did you get your wire locally or the internet?

Thanks again

It sounds like the guy only did the job halfway. Is the generator a 12v generator or a 6v generator?

Wire can be purchased at the local parts store or the internet. Doesn't matter. 6v should be 12 gauge wire. I'd use 12 gauge for 12v too even though 14 or 16 ga can be used.

There are several companies that produce complete wiring harnesses but it would be pretty easy for you do do it yourself, one circuit at a time.

It is possible to use a 12v generator and external voltage regulator, but given the small electrical load that you plan to have it is much easier to convert to 12v using a GM 10si alternator that has an internal regulator. Some people like the "1 wire" variety, but I prefer the 3 wire type. Just 2 more wires to run and it gives you a "ALT" light and voltage sensing back under the dash instead of just at the output of the alternator. Do you know if your generator is 6v or 12v? How about the voltage regulator (should be mounted on the driver's side firewall)?

The 6v coil either needs to be replaced with a "12v, no ballast resistor required" coil or the starter switch needs to had a stud on the side (click here) to wire around a ballast resistor which you probably don't have either.

Do some troubleshooting before you replace the headlight switch. Make sure voltage is available on the "bat" connection. If not, fix that before replacing the switch. Classic Parts sells both 6V and 12V headlight switches for your truck. The big difference is the rheostat value for dimming the dash lights and the self-resetting circuit breaker for the headlights.

Saw Mill Would go with 12 volt neg ground later you'll need that for alt. In 30 trucks over 50 years I don't think I ever replaced a light switch. Once your on 12 volt with 12 v bulbs your light switch will work fine dash lights will dim just like before. If you take out your light switch knob (press the button to release it) then put the extention tube on your wd-40 deep into the switch and spray on your way out . This will lube the dry switch internally,most light knobs are shrunk-up where the stainless trim ring will rotate on the knob. Rotating the knob will help knock corrosion off of rheostat dash light dimmer (OK to spray it a little too) Mostly the light switches are pretty durable. Would repair the old wiring harness if at all possible it is double the size for 12 volt service as is ! Also only takes 3 wires to the rear of the truck with an interlocking turn signal switch to do tail,stop,turn signals.

It sounds like the 12v has the majority of the consensus. With that being said, what way would be best, old style fuse block or the newer modern blade fuse? Thanks again. I really appreciate it

You can buy really nice little modern blade type fuse blocks today for very little money. It's beginning to be hard to find the old style, and the old style fuses are not sold everywhere any longer either.

If you’re not trying to stay vintage, original, check eBay for universal long hot rod harness. Don’t try to use it for 6v as it is lighter gauge (12v uses lower amps) you can choose number of circuits and flasher for turn signals. 3wire alternator would be my pick. Add relays for headlights and horn. Big truck you may have to extend taillight wires.Get a wiring schematic and try to stay close to the original run locations keeping the old wire in place, removing as you go. These guys will help if you get stumped. These old trucks are pretty simple᠁compared to.

Saw Mill Think I would go with modern blade fuses,glass ones are harder to find and the fuse blocks (offshore equipment probably) are not as dependable. Had a pretty good looking older fuse block but got resistance in it and would overheat blow the fuse. If glass fuses get hot they'll blow. Blade fuses are easy to get and can jumper thru those holes when trouble shooting. However I still use glass fuses I guess because they havn't caused me much trouble yet. My turn signal (signal-stat ) is a junk yard find hook-up is certainly ugly enough under the hood.

I second the blade fuses, just got this fuse block for a project - it's really nice, especially for 20 bucks.

Frank