How to reverse the polarity in a 12v DB?

[dbtravis]

Took the 1920 DB touring out yesterday and got stuck with a dead battery. Apparently the starter/generator unit is no longer generating. I'll get to fixing or adjusting this later. However, for now I am curious about the proper installation of the battery. It says in the book, and I'm fairly familiar with connecting the POSITIVE terminal to ground. This is what I have been driving around with. However, the ammeter is always right at zero (now I know why). However, anytime I turn on the headlights or honk the horn the ammeter jumps to the charge side. This doesn't seem intuitive to me - shouldn't it swing to discharge when there is a current draw... especially if there is no incoming amps from the starter/generator unit?? So - I did a little reading and I decided that I might try to switch the installation of the battery. The previous owner installed a "modern" radio back in the 1970's. I don't think it works anymore but maybe he did something if that required a negative ground. I switched the battery around to negative ground, the car started just fine and it showed a slight discharge when it was running. Now when I turn on the headlights it acts as I would expect and shows a strong discharge. Still no juice coming from the starter/generator.

So... my question is if there is any reason NOT to run the car with a negative ground? Is there anything that I can or should do to correctly polarize the starter/generator or maybe the reverse current cut-out (which I believe is in the starter pedal??) ??? All things the same, I'd prefer to run it as proper with a positive ground - but are there any adverse effects to running a negative ground that I should be aware of? At this point I am just trying to get the car to work and charge properly so I'll take whatever ground is best for the way I am set up.

Thanks for any help on this issue.

- Travis

[dwollam]

A '20 SHOULD be positive ground although the generator could care less which way it is set. DB's went to positive ground in '23. Believe that is also when they went to the GA generator.Check the fuse on the generator and also could be a problem in the starter box. There is an adjustment on the back end of the generator for the third brush to regulate voltage. Brushes and or brush springs could be bad. Double check the fuse. Mine looked good and sometimes charged, sometimes not. had a funky fuse. I have run the same generator both positive and negative depending on which way I wired the ammeter. Also look and see if the commutator is clean. Be careful what u clean it with, Do not use sandpaper.

Dave

[nearchoclatetown]

Dave, not to correct you , but I think you mean not to use emory cloth. Sand paper is what the book says to use. My understanding of pos, vs. neg groung is the change was made when the horn button was taken from the door to the steering column.

[Guest DodgeKCL]

You can't change the battery polarity without changing the ignition coil polarity. The coil is always wound so that the coil high voltage is negative. This is because the centre 'hot' electrode emits a far better spark when it emits the electrons and not the 'cold' grounded electrode, the bent one on the spark plug's side. Your car will start to show a hard starting condition if the coil is hooked up backwards because the spark will be greatly reduced in strength. I'm not familiar with your year of Dodge but most coils that come with a car from the factory are only 'one wire' and can't be switched over to the other polarity. 'Universal' coils (with a + and - terminal on them) can be hooked up for either polarity but they came out much later in the 50s. I would stay with positive ground and troubleshoot your problems from there.----- If the ammeter shows a 'charge' when a load is turned on when the engine is not running then the meter has been hooked up backwards. Perhaps the guy who installed the radio did it? It's only a matter of reversing the wires on the back of the meter.-----You can't change a generator from one polarity to the other without 'polarizing' the field and armature windings. You can destroy the generator if you don't do it. I'm surprised you haven't had a lot of smoke and flame by now. But I see there is a fuse in the generator's windings and this may be what has saved you up until now. If indeed the fuse is blown, then you need to go back to positive ground and replace the fuse and start from there. Just before starting the engine again however I would touch a lead from the battery over to the generator's output wire and make sure it's set to positive ground. Just touch it for a second that's all that's needed. You may have to pull the fuse while you do this. Hope this helps.

[dbtravis]

Thanks for the advice. Sounds like I'll stick with the positive ground unless anyone thinks I should do differently. The horn button is on the left side of the steering column if that matters. I can switch the wires on the back of my ammeter to show the correct direction of charge.

One question... when I "polarize" the generator, you did not mention if I should touch a wire on the output of the generator to the positive (grounded) or negative (ungrounded) side of the battery? Because this sounds fairly important I don't want to mess it up. Also, just to be 100% sure, am I suppossed to put the wire to the post on the generator that has the wire up to the starter pedal? If there's a risk of burning something up I want to make sure I get it right on the first time!

Thanks so much for the help everyone!

[Guest DodgeKCL]

I'm not familiar with your electrical system per se but the action is to momentarily touch the 'high' side of the battery to the generator to magnetize the metal in the armature and field in the direction of the battery polarity. So yes you short the negative (high side) of the battery to the generator output for just a second. It's better if you unhook everything from the generator and just touch the gen. output wire to the battery neg. by it's self using a jumper wire. This will magnetize or 'polarize' the gen. to the neg. battery supply. We tend to call them 'positive ground' but they also can be called 'negative supply systems'. These old DC generators can be set to either polarity by this 'polarizing' so you have to make sure your generator is going to charge in the right direction when the engine starts.----------

I didn't want to confuse the issue by putting this in the main body of my answer. What happens when the generator is polarized in the opposite direction, it aids rather than opposes the battery current and the moment the engine starts turning the generator over the battery finds a short circuit to ground and tries to discharge every electron it has right through the generator to ground. And the faster you rev the engine the more current flows through the generator to ground or in reality, the other side of the battery. Imagine the 200 or so amps that the battery can supply to the starter motor going through the generator!! Or at least trying to go through it. It will destroy the generator in a few seconds. Usually eating the commutator segments and brushes to a molten blob. When the generator is polarized in the correct polarity it opposes the battery current (voltage) and no reverse current flows. The only current that flows over into the battery is the result of the battery post voltage falling below the generator's output voltage. And only a few amps flow to keep the battery 'charged up'. These are the 'charging amps' we see on the ammeter. There is only one caveat and that is that once the generator slows down or the engine stops, then a gizmo called a 'cutout' comes into play and disconnects the battery from it's generator. This is because a stopped generator is just a bundle of wire and the battery will slowly discharge through to ground and kill it's charge. Usually over night. But the points in these cutouts have been known to stick. It's one of the reasons why you should install a battery disconnect switch and turn it off when you leave your car for an extended period of time I do. Robert

[dwollam]

Here is what I get for typing late at night! <img src="http://forums.aaca.org/images/graemlins/smile.gif" alt="" /> I meant to say a 20 should be NEGATIVE ground and yes they went to positive ground when the horn button was moved from the drivers door to the center of the steering wheel. Also I have never had to polarize a G or GA 12 volt starter/generator and I have indeed run them both positive and negative ground with no ill effects.

Dave

[nearchoclatetown]

Dave, I only have one finger that knows how to type and sometimes it doesn't listen to what I want to say. My limited understanding of electricity says that polariseing is for the regulator and since a DB doesn't have one I agree it shouldn't be a problem. I also have switched from pos to neg without any ill effects.

[Guest DodgeKCL]

Voltage regulators are not polarized one way or the other. They can be used on either polarity but the point material was applied to the moving points in a fashion that was polarity sensitive. If you use a neg. ground regulator on a pos. ground system it will work but it will not last very long. The only part of the electrical system that needed polarizing was the generator. You may get lucky and have touched the battery high side to the generator at some time during installation and it got repolarized to the correct direction but if the generator is not polarized in the correct direction for your system, you're looking for a world of hurt. These old DC generators MUST be polarized before use.--- On reading the above reply I realized that the mistake in thinking the regulator has to be polarized is because mnay of the instructions for polarizing has the owner/mechanic shorting out the BAT terminal to the F (field) terminal at the regulator. But this is only for convenience sake. The generator could be buried on the side of the motor but the regulator is always up top on the firewall or fender. But shorting the terminals on the regulator does not polarize the regualtor,it polarizes the generator. I prefer to remove the wires from the generator and bring over a 'hot' jumper lead and touch the field terminal directly on the generator. But I have seen a reverse polarized generator that has been run for a few minutes on an engine. It ain't a pretty sight. The cause was a shop assuming the genrator would be used on a more common negative system instead of our old positive ground ones. This is good advice boys and girls. I wouldn't dismiss it out of hand so fast. As a senior electonics engineer for 40 years I made a good living at it both in the U.S. and Canada.

[Guest imported_RAH]

Guys, Guys, For Travis, Yes the early DB with 12v can be connected whichever way you wish. Obviously the ammeter will need to be connected properly for whichever condition. Yes, the '20 would have originally been connected as Neg. ground. As Dave stated the switch to Pos. was roughly when the horn button moved from the door to the steering wheel. I believe Robert (KCL) is confusing the later 6v. two unit info. with the North East 12v single unit starter/generator. The first time you step on the starter switch that unit will 'polarize' itself according to the battery conections and the Model G, or GA could care less which way they are connected. As to the later 6v generators the quickest means of 'polarizing' after a rebuild or whatever is to simply remove the cover on the cutout relay and hold down the relay for a couple seconds. Presto you will be set to charge assuming the other wire connections and fuse, etc. are correct.

I hope this clarifies things. Rodger "Dodger" Hartley

[dbtravis]

Thanks, Rodger! And, thanks to everyone else for the interesting discussion... I certainly learned a lot about electrical systems! :-)

I'll connect it up with negative ground. The ammeter wires are already set to that direction anyhow. I have a rebuilt starter/generator unit, so I will put that one in there to see if it works (supposedly was rebuilt about 20 years ago and stored in a box ever since). Hopefully this will get me the extra juice I need so I don't always have to connect the battery charger after every trip!

I REALLY appreciate your help (everyone) thanks so much!

- Travis