Need some big help.

[hicrop10]

I have mentioned this before and got some great response.But I’m still lost.First question will be how do I know if my truck is positive or negative ground?Secord question what the hell m I doing wrong.I have provided some diagrams of how I have my truck wired up.When the truck is running or not the whole thing is hot.By that I mean touching my test light against anything it lights up(steering column,frame,oil filter and so forth).Any help would be greatly appreciated.When it is running it runs great.I have a rebuilt generator for it and a new voltage regulator,I don’t want to damage them.

[hicrop10]

My number is 215-669-1252 and I’m in Philadelphia Pa.

[Geoff Weeks]

Looks like it is wired for neg ground, but I can't say for sure.

Once again, look to the alternator (if there is one) or the voltage regulator for a generator (they are sometimes marked as to polarity).

If not marked, and there are no electronics, then chose one polarity and run with it. Neg ground makes the most sense today.

[hicrop10]

How do I choose polarity?Mine has a voltage regulator and a generator.

[Geoff Weeks]

Connect the chosen polarity battery to the frame of the truck. with the rest of the batteries hooked up, go to the regulator, jump the battery terminal (B) on the regulator to the armature (A) for a second or two, and that is it. You have polarized the generator and it should start charging the correct way for the way the batteries are hooked.

[hicrop10]

Tks will try in the morning 

[mattb73lt]

These trucks were positive ground from the factory. The two I have both were set up to run twelve volt starters with negative ground. The B42 had no series parallel switch originally and I deleted it from the B73 before I ever ran it. Two things I had to do when setting them up was reverse the leads to the ammeter so it read correctly under charge, if not it'll read a discharge when the generator or alternator comes on line. One way to test for this, is before running the truck, is to put a load on the system by turning the headlights on and checking the ammeter for a discharge or just momentarily giving it a direct short to ground downstream of the ammeter. If the polarity is wrong it'll show a charge.The other was to when I had the generator rebuilt I asked to have it set up for negative ground. 

Regarding the voltage regulator, almost every one I had new out of the box, had instructions on them to "Flash the Field". They are not polarity sensitive until the field side is momentarily energized only for a second or two. All the instruments and senders don't care about the polarity and will work normally with either polarity, with the exception of the ammeter.

Looking at your diagram, it looks like it's set up for negative ground. The lead from Battery A is going to the ground side of the starter. I have a wiring diagram in an original Mack maintenance manual I used to wire and convert the B73 to the 12V starter and switch to an alternator. The new harness I had was set up for the S/P switch and a generator and I wanted to delete it properly and switch to an alternator. Let me dig that out and see if I can post it here to help you chase the wires to the switch. You can hook the batteries up to the S/P switch but leave the starter disconnected. Using a voltmeter, test the various smaller terminals for 12 volts, nothing should read to the starter lead. This would be testing the S/P switch in the "run" position. Using a jumper or pressing the starter switch with the ignition switch on should give you 24 volts at the starter on the voltmeter. That would test the S/P switch in the "start" position.

To be clear, you're not changing your polarity so if the batteries are hooked up wrong the ammeter will be your initial clue they need to be reversed by putting that load on or creating a momentary short. You have a new S/P switch installed so it should work if wired correctly. Once you get it running I would still check that both batteries are charging by receiving around 14.5 volts by checking them with a voltmeter individually. That's one of the issues a S/P switch will give you, is that one set of batteries is not charging.

P.S., I have a brand new 12V AC Delco voltage regulator still in the box if you want it? I bought it years ago and have no need for it as I have alternators on everything, now.