12 volt
 

I just bought a 1930 Town car. The car has been converted to 12 volt. How do I know if it is negative or positive ground? Will it work either way or blow the fuse? The battery was not in the car.

Re: 12 volt
 

Ck the battery, which post goes to ground?? Most likely the negative post.
Paul in CT

Re: 12 volt
 

Another way - The battery posts are different sizes/diameters, and the cable clamp openings are sized to match the posts.

Re: 12 volt
 

Battery cable post clamp size is the best clue. Coil polarity is another. Ammeter wire color: if the wires on each firewall post match, it's positive ground; if reversed, negative.

Everything will work either polarity. As has been said, probably negative ground.

Best to set it up to the polarity you want, then polarize the generator to the battery before starting.

Re: 12 volt
 

I've seen this expressed before, and assume it is true but do not understand why the starter does not run backwards and be a problem, since it is a DC motor.



Can you/someone please explain?



Thanks

Re: 12 volt
 

Loosen the belt and bypass the cutout, the gen will motor. If it's a diode cutout without bypass it will motor only with the wrong polarity battery. Best way to determine polarity, but requires a diode.

Re: 12 volt
 

Most 12 V conversions ditch the gen and replace with a one wire 12V altnerator.

Re: 12 volt
 

Battery was not in the car! 50 - 50 chance I would try neg ground it will not blow the fuse one way or the other, only if you have a short

Re: 12 volt
 

Being at 12 volt it's more than likely negative ground, especially if it has an alternator.

Re: 12 volt
 

Does car have alternator or generator ?

If alternator then you might find the answer on it somewhere. Usually 12v alternators are neg grd as said.

If generator then its a crap shoot but it'll work either way.

The battery cable clamp ends is a good indicator as said.

Re: 12 volt
 

My guess is that it is a 90% chance that it is negative ground. All modern electronics expects 12 volt negative ground so people usually make it that way.

Charlie Stephens

Re: 12 volt
 

A person can install a twelve volt battery and a Three OHM coil and they will be running twelve volt positive ground . I run my 31 roadster 12 volt positive ground . I use the Fun Projects 12 volt voltage regulator that takes the place of the cut out . None of the wire connections have to be changed . I adjust the generator for a ten amp maximum charge . A one wire 12 volt alternator could be used in place of the generator but it would probably require changing to negative ground . Changing to negative ground would require reversing battery cable connections , coil connections and the ammeter wires will need to be reversed or it will show discharge when it would actually be charging .

Re: 12 volt
 

What drives any electric motor is magnetic attraction and repulsion. Like poles repel and opposites attract. There are two major parts to a starter motor - the armature and the field windings. If you reverse the polarity of both, it runs in the same direction. Reverse only one of them, it will run in the reverse direction.
The field windings set up a magnetic field and the armature sits in the middle of it. Another magnetic field is created by the current flowing through the armature. That causes one side ot eh armature to be attracted to the field, the other is repelled by it. The direction of this field changes as the current reverses direction. That is achieved by the way the commutator is connected to the armature windings. That is, as the armature turns, the brushes are connected to different windings by the commutator. When the timing of the changes is right, the armature spins.
This clip probably explains it better than I did.
https://www.youtube.com/watch?v=CWulQ1ZSE3c
When I was in secondary school, each year we had to make something for display to parents on open night which demonstrated something we had learned that year in our science class. When was 14, I thought I was clever making an electric motor, only to be well and truly trumped by a couple of friends who teamed up to make a device which carried their voices on a beam of light. That might not sound overly impressive now, but in 1962, it was more than most parents could comprehend.

Re: 12 volt
 

The key reason for the starter to always turn in the same direction is that the field winding is in series with the stator. When the direction of current changes both the field and the stator changes so the direction is the same.

If the field winding was in parallel with the stator, then reversal would occur with a change in the polarity.

Marc

Re: 12 volt
 

Yup.

Permanent magnet motors also reverse with current reversal.

Re: 12 volt
 

Thanks guys. Badpuppy pointed out why my confusion. I was trying to remember schooling never used from 45 years ago, and I was stuck on permanent magnet motor info. The explanations above refreshed my memory. Thanks again.

Re: 12 volt
 

If has a 12V battery installed, most likely changed to negative ground as others have stated if you have an alternator. The generator will work either way if polarized correctly.
You can make the change to negative ground, and by swapping the cable connections on the batter, changing the coil connections, and amp meter. Starter, horn and lights do not care which way the current flows. BTW when taking the laser optics course for degree in EE in 1970 demonstrated how a laser radar would work.

Re: 12 volt
 

Can you even get a positive grounded alternator? I believe all the alternators I've ever worked on had the ground to the case (none floated), and who would buy special diodes to reverse polarity? I'd bet a 4 pack of Heady Topper that if it's an alternator then it's negative ground.

Re: 12 volt
 

There are pos grd alternators.

12 volt
 

https://brillman.com/product/6-volt-...SABEgKPevD_BwE

This is probably not for a Model A, but it came up on a quick search. My Model A has a 6 volt positive ground alternator on it.


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