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I’m getting started on the 1954 GMC 450, Fire Truck conversion to a truck.
I ran the truck about 70 miles last summer to get it home, and with a new 6 volt battery connected negative ground, it ran fine.
At the end of the summer, I was having the battery go dead, and thinking that there were wires here and there shorting to ground, I went with that, and disconnected the battery cable and tried charging the battery. I could only ever get a partial charge, not enough to crank over the engine, then I got to worrying that the truck might be positive ground!
I find conflicting information about this , online.
Could one of the group help me with determining this?
Can a positive ground truck run if connected with the ground wire negative?
It seems this has been discussed a lot, but I still am in the dark about it.
Thanks for any help on this !
Charley

Well, you demonstrated that it will run. Apparently the charging circuit is not working. Somewhere on this site is a tech tip about converting a 6v + ground GMC to 6v - ground. I'm looking for that tip now. As I recall it's simple, not much to it really. This same discussion comes up occasionally on the old tractor forums.

so if you were to polarize it for negative ground, and the battery is installed with a negative ground, it should charge no matter which way it was set up before. The difference is that the amp meter would read backwards if it were a positive ground setup.

Firetrucks have a lot of electrical loads that are always hot, and seem to run the battery down since there are lots if hidden, dead loads. I always put a knife switch or similar on the battery ground, solved a lot of dead battery issues.

Ed

Thanks fellows, for the help . I will carry on with trying it. As I said , it ran beautifully, so I suppose that means the polarity is correct?
The coil can be connected either way, and still function? As for all the electrical stuff, I am disconnecting it all, and starting from scratch wiring wise. I just need it to run, right now.

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Wow that is a beautiful truck. It must of had great care for many years!

Has not lived in the 'rust belt.'

Ed

If this was not suggested above, do the following.

Disconnect all your ground cables/straps (you may break a few bolts).
Clean-until-shiney the connection-points and the cable-ends.
Then, apply an antioxidant conductive grease to all mating surfaces.
If this does not help/solve your problem, you will at least know you have good grounds.

Thanks, Tim. I will certainly get those ground connections in good shape.
Can you tell me, with the coil plus side wire going to the distributor, and the minus side going to the starter main post connection, does this indicate a positive ground set-up on this truck?

Wait to hear from someone else, Charley

I'll see if I can find a 1954 big GMC wiring diagram.

Your New Design (47-55) Z-Series GMC would have come from the factory with a positive-ground electrical-system.

Where do the the leads from your battery connect?

If the (+) post of the coil primary is on the distributor side, the truck was positive ground at some time in its past. That's the correct polarity for an early 1950's GMC. Ditto on the suggestion to clean and tighten all the battery cable connections and grounds. It only takes a tiny bit of rust or corrosion to severely restrict the current flow, and 6V systems can't stand any voltage drop at all, either in connections or cables. All your cables, hot and ground, need to be a minimum of #1 gauge, and 0 or 00 is better. I like to make my battery cables from arc welding cable. It has smaller wire strands than battery cable, thousands of them, and less resistance. Run a ground cable from the + battery post all the way to the bellhousing or a starter attaching bolt to eliminate unnecessary connections and ground straps between the battery, frame and body to the starter. Smaller-gauge grounds can be installed from the engine block and body to the frame. The starter has the highest current draw, so it makes sense to give it the best ground path possible, Lights, ignition, generator, and other accessories can make do with smaller-gauge grounds. To polarize the generator, touch a lumper wire to the BAT and ARM terminals of the voltage regulator for half a second or so, once you have the battery installed th4e right way and all the generator/regulator wiring connected correctly. Good luck!
Jerry

Thanks guys! Now we’re on the right track.
As I have said, the truck ran great connected negative ground.
Is it possible that simply connecting the battery that way would automatically polarize the generator?

Maybe, but not likely. A generator armature core is made up of dozens of layers of thin steel plate, and the plate is "mild steel" which is not capable of retaining much magnetism. The momentary connection made between the Bat and ARM connectors sends what's basically a dead short from the battery to ground through the armature windings. This short surge of heavy current creates what's called "residual magnetism" in the armature. If a polarized generator is spun up without any wires connected to it, a small voltage (usually 1/2 volt or so) can be measured at the armature terminal. This small voltage is what kicks off the charging cycle by allowing current to flow in the field coils, controlled by the regulator.

If the armature has no residual magnetism it won't charge at all, or if it's been polarized the wrong way, it can actually recharge a totally dead battery to the wrong polarity. This can severely damage a battery. A quick flash with a jumper lead at the regulator wires assures the polarity will be correct.
Jerry

A lot of food for thought. Thanks all of you