6V/12V, Grounding Polarity, and Generator/Cut-Out

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Model T Ford Forum: Forum 2016: 6V/12V, Grounding Polarity, and Generator/Cut-Out
Top of pagePrevious messageNext messageBottom of page Link to this message  By Joe LaMacchia, St. Louis, MO. on Sunday, July 03, 2016 - 06:01 pm:

When I restored my '23 Touring from the ground up, all manuals & wiring diagrams showed a battery negative to chassis ground. I run a 12V battery, negative ground, 12V lights of course, original generator, and a Lang's cut-out (P.N. 5055, diode style). Lang's states "cut-outs are negative ground only". Touring has run like a champ for years, never an issue.
I recently purchased a barn fresh original '24 Tudor Sedan. It has a 6V battery, positive ground set up. The previous owner, who is a Model A guy and admittedly unfamiliar with Model T's, probably installed the battery with a positive ground. I got the Sedan purring like a kitten except for 0 amp gauge reading. Being without a meter right now, I did a quick generator/cut-out swap with my Touring to see what works and what doesn't. I found that the Sedan generator is not producing, but that the cut-out works just fine. With the Touring generator/cut-out on the Sedan, I did get some funny amp readings and a few sparks from the generator wire when I attached to the cut-out post.
My issues......
Model T's are negative ground, correct?
Some old timers always say "positive ground", kid!
Are generators polarized to the polarity of the car?
Can I switch the polarity of the Sedan back to negative ground without any complications?
Would that affect the generator/cut-out?
Lang's states "cut-outs are negative ground only".

I should be able to use a standard cut-out with a 6V or 12V system without damage, correct?
(Been running that set up on Touring for years)

Thanks in advance, I know there are some really knowledgeable electrical guys here!


Top of pagePrevious messageNext messageBottom of page Link to this message  By Royce in Dallas TX on Sunday, July 03, 2016 - 06:21 pm:

Swap the battery cables. The cutout is polarity sensitive so it might start charging.

Standard cutouts are OK if they work, which they often don't.


Top of pagePrevious messageNext messageBottom of page Link to this message  By John F. Regan on Sunday, July 03, 2016 - 06:43 pm:

IF you have the generator polarized for positive ground and have a negative ground cutout mounted on it then you run the risk of destroying the generator rather quickly since when you spin up the motor and thus the generator, the generator is running then with zero load on it and that can cause it to self destruct rather quickly. Ignore my warning at your peril. Generator gets its field current from a "tapped down" point off the armature via the third brush. Thus with no load the output generator voltage rises and then the field current rises which causes further rise of the generator voltage and this process is called a thermal runaway and results in the quick burnout of the generator. STOP spinning the generator until you are sure you have it negative ground polarized and then also have that diode type 5055 cutout installed on the generator and the battery wire then also connected. You need ALL of that on the car hooked up correctly before you spin up the generator.


Top of pagePrevious messageNext messageBottom of page Link to this message  By Joe LaMacchia, St. Louis, MO. on Sunday, July 03, 2016 - 11:42 pm:

Thanks Royce & John. I did switch the battery polarity back to a negative ground and I will heed all of your other generator/cut-out suggestions.
Generator may have been burned up before I even purchased the car. Previous owner did have it running on positive ground.


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