6-Volt, Negative ground???

pdora

pdora

Senior Member
I took delivery yesterday on a 1930 Tudor. As I began inspecting it I discovered that it is wired 6-Volt, negative ground!

Because I don't have access to the previous owner I would appreciate any comments from the Barn.

The lights (all incandescent) work OK; they don't care about polarity. The same goes for the starter motor and the electric windshield wiper.

The generator has been replaced with an alternator (pictured).

At rest the electrical system measures 6.2 volts. When running it measures 7.5 volts while charging. Thus, the alternator seems to be functioning properly.

Whoever did the conversion took care of the ammeter polarity (it goes down, not up when the lights are turned on).

I hope he saw to the coil polarity. It seems to run OK.

My surmise is that all he had available was a negative-ground alternator and proceeded accordingly.

Because I intend to convert to 12-volt negative ground in a few months, I am inclined to leave well enough alone.

Some questions:

1. Is this all that unusual?

2. How can I be sure the coil polarity is correct?

3. Are there any other considerations I have missed?

Thanks.
 

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that was done a lot because + ground equipment is hard to find in your local parts store, check the coil and see if it has + or - on it if it does the - goes to the dist, changing polarity doesnt hurt any thing just have to get the proper componts (alt) :rolleyes:
 
I bought a Model A a few years back that was 6 volt negitive ground. Worked fine. I switched it back to positive ground to keep it easy for me to remember things. Works fine both ways. I have a generator.

Neal
 
When I had my 31 pick up it had a negative ground. Everything worked and it ran good. It had a regular model A generator on it. I kept it that way for several years. i finally converted it back to positive ground inorder to be more "correct." My current "A" is a 31 rdstr and is positive ground. FWIW
 
I would say it is very unusual. I have never seen a 6V negative ground alternator, only 12 V negative ground. The units that the venders sell are all 12 V alternators converted to 6V positive ground. This should be an advantage if you change to 12V. if you change to 12V you will need a ballast resistor on the coil or change the coil to 12V . I use a coil with a 3.0 OHM rating. Bratton sells a coil polarity tester on page 74 of their new catalog part number 17140 price $22.75 . Its kinda pricey and most model A folks are TOO stingy. Im glad I bought mine about 20 years ago when they only cost three or four dollars. They do come in handy and I work on model A's pretty often.
 
Coil polarity can be checked in the following manner: It can be checked by holding the spark plug lead about a 1/4 in away from the spark plug terminal with the engine running. Insert the point of a wooden lead pencil between the spark plug and the lead. If the spark flairs and has a slight orange tinge on the spark plug side of the pencil, polarity is correct. If the spark flares on the lead side, the the coil connection should be reversed. (Motor Auto Engines & Electrical Systems , 1977)
Hope this helps?
Tom
 
Group,

You can also check the coil polarity per the Les Andrews book. He suggests that you use a multimeter with a high voltage scale.

I did, and found that the polarity was wrong. After the change, the engine seemed to have a better spark.

Marc
 
Do not change the battery polarity without having the alternator modified to Pos ground.

Just changing the battery leads will fry the regulator inside most Alternators.
 
neg ground cause less arc than pos ground . learned this in electronics . seems it is prefered to hook the ground up last . iv run a 6v pos grd alt for yrs .............. steve
 
From original poster

Thanks for the help. It appears that whoever did the conversion on my car knew what he was doing. Everything checks out.

I will leave it as is until I am ready for 12 volt conversion.
 
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BTW have a chart for all the bulbs in Model A both 6V/12V and their candle power if you would like send me your email address. Also go to the barrel drive or modern bendix as 12V on your starter will eventually break the bolts on the original style bendix
 
I thought the reason everyone (companies) converted over to negative ground was due to positive ground was more likely to cause chassis\frame corrosion?
 
My 29 Tudor is 6v negative ground. I have checked it with a meter and it is 6v. Someone told me it could not be 6V negative. Someone told me people restore them to 6V negative ground to add a CB radio or a radio. I don't know.
 
Ken/SC said:
My 29 Tudor is 6v negative ground. I have checked it with a meter and it is 6v. Someone told me it could not be 6V negative. Someone told me people restore them to 6V negative ground to add a CB radio or a radio. I don't know.
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If you are still running the generator all that has to be done to change to negative ground is turn the battery around and swap the cables then reverse the wires on the ammeter and coil. you will then need to polarize the generator by holding a wire across the poles of the cutout for one or two seconds and the deed is done. I have never seen a 6V negative ground alternator. The ones that the venders sell are 6V positive ground.
 
I realize this is an old thread, but I discovered that my Model A has a 6 volt negative ground setup as well. I am going to replace the generator with an alternator (I know I know), so I will have to put it back to positive ground. I assume that all that needs to be done is swapping the wires around on the coil and the ammeter. Would that be accurate? I am running LED tail lights which I had to run a separate ground for, and a electric wiper motor, and will also be installing a restored 1930's era Southwind heater (I use this car daily year round and live in Chicago where it gets pretty nasty during the winter). Thanks in advance for any advice.
 
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