Is it Pos or Neg?

67 BGT.
Bought a single 12v to drop in where, judging by the battery cables, 2 6v had come out.
There is a newer looking radio in the dash, not cecessarilly hooked up. Some kind of black box with a red, grey, and black wires is screwed to the front apron, but not hooked to anything. It has two diodes showing.
The first question is, how do you tell if your generator car is still positive grd, or has been converted to neg ground?
Second, is that black box a converter to change pos to neg for the radio?
TIA!

Look at the batteries that came out. If the "backside" of the circuit going to the frame ground was off of the negative terminals they were in use as a negative ground. The opposite would give you positive ground of course.

I have seen some pretty weird jury rigging in LBC wiring associated with the perceived problems of positive and negative ground radios and other accessories. Usually the best thing to do is yank all of the mess out and start over, making things as simple and clean as possible. Diodes would not be needed in any event, I suspect that the DPO didn't know what he was doing.

Jack

There were no batteries to come out.

Look on the bottom of the tach face an see if it says positive or negative. Go with that. Jack

Jack, you can switch an early tach to neg ground.

Yep, but it could give you a starting point. In real life if I were confronted with the issue I would just hook the system up as negative ground and pull all of that radio stuff out in order to wire up the simple circuits to runt the music box. If for some odd reason it fires, run over to Wally World and buy a new one. :-) Jack

That's a tough one Chris. Anything you do to try and test the generator is likely to polorize it the direction you hook it up and it could be either way right now. There may be some indication on the regulator as to which system you have. I think I'd take the generator and regulator to a local rebuilder and see if they can dope it out.

Tough one Chris. If they installed a 12 volt, more than likely they put it in the passenger side. That would more-than-likely have required a new ground to somewhere. Odd battery cables could be a giveaway on either side or the other. I know Im not much help here but just trying to help!!!!

Look at the coil and see if the white wire is on the positive side and black and wight on negative. If so it set up for negative ground.

I think. It dawned on me to check the coil. The white/black is on the pos terminal and the white is on the neg.
That would mean positive ground, right?

yep

Thanks Tom! You must have been telepathing to me! Now if I can figure out what the black box is for!

The black box is the telepath antenna...just needs a battery!...LOL

Chris the black box is a polarity box (it has another name, but I cant think of it now, not enough coffee). This was fairly popular back in the 60s for our pos grnd cars. I have one on my 67 Rdstr. It simply takes a - grnd circuit and swaps it to +. You have to be careful though as if you touch any part of the accessory that you have wired to the box to the frame, it blows the whole circuit and lets the smoke out of the black box. I just reread my last sentence, and it sounds like gobblygook, I know. But its not magic. Its just an oooollllddd technology for folks who didn't want to change the polarity on their cars just to put in an 8 track tape deck. I recognized it when I bought my 67 from the original owner, and I had one on my 67 Healey. It is a genuine PITA, but if its run correctly, it works fine. I wouldn't know where to find one today, so you have a real piece from a time capsule. If you decide to lose it, I'll take it since I know how it works. E-mail me, and I can try to give you a more in depth description of how it works.
Have fun