6 volt negative ground

Pamodelaroadster

Pamodelaroadster

Member
I just got a 31 roadster and I noticed it is 6 volt negative ground. I’ve read this is sometimes the case, it runs, starts fairly good.
Should I just not worry about it and leave it alone?
 
Check the polarity of the coil and make sure it charges properly and ammeter deflects properly. If so, might as well leave it. Sometimes people don't realize the A is supposed to be positive grnd so just install the battery wrong, that's why I suggest making sure it is rewired for neg grnd.
 
The Model A will run fine on a negative ground. As Eagle said the only things are the coil polarity and charging. If the generator has been re polarized it will be fine. The vendors sell a device for checking the polarity of the spark, see https://www.snydersantiqueauto.com/product.htm?pid=976303&cat=41753

If the lights have been changed to LED's they will not work if they have the wrong polarity. That may be why the battery was changed. The incandescent bulbs will work either way, of course.
 
Thank you for the replies. It’s got a generator, it’s putting out about nine or 10 V when it’s running. The ammeter doesn’t move, and the coil is connected to the negative side after a isolation relay that has a key under the hood, I would think if it’s negative ground it should be connected to the positive side,of the coil?
I know that isolation relay is bad because it’s dropping .2 V just by turning it on but even cranking.
I don’t see how to post pictures maybe I can’t because I am new to this site. Thanks again for any replies
 
This is a little more complicated than originally posted. The generator should not be putting out that much voltage. It should only be 7 or 7.5 volts. The ammeter should be indicating 10 amps charge. It may indicate 10 amps discharge when actually charging if it is wired backwards. The isolation relay should not be needed and is normally not part of the wiring. It sounds like the coil is not wired correctly. Is the positive side going to the switch or to the distributor. Does it have plus and negative indicators on the coil or "Bat" and "Coil"? You may need to re wire the car. See https://www.mafca.com/downloads/Seminars/2018 Reno Electrical System Alex Janke.pdf

I would advise to get some help from someone who knows Model A's.

I am not sure but I think with only two posts you cannot post photos. A way around this is to post photos on another site and then post a link to that site.
 
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Does the ammeter work at all ? Just hit the brakes or turn on some lights, does it show a discharge ? [ it should]

The primary lead to the distributor should come from the negative lead on the coil. Points should wired same as the battery.

If the coils is wired incorrectly [ backwards] the engine will still run. The secondary voltage will just be about 30% less.
 
9 or 10 volts will overcharge the battery. Adjust the third brush to lower it to about 7V, or drive with the headlights on. With no deflection at the ammeter it's probably disconnected.
 
That PDF and all the comments are great. I checked out the wiring according to some diagrams and it’s all wired correctly for 6 V positive ground. The ammeter is wired up too. I won’t be able to do any more testing on it for a couple days. I’ll check some more things like the actual amp output with an inductive pick up, and all the connections then I may just switch the battery to positive ground and see how it works.
Thanks again, I’ll post more when I know more.
 
When I received my "A" the seller (deceased owners' son) was kind enough to put in a new 6V battery. Hooked it up Neg. Ground. Wouldn't start without a jump and fried the regulator and actually damaged the generator. Don't know how, but it did.
I tried re-polarizing but ended up replacing both. Chap
 
The ammeter itself should tell you what you need to know. It'll tell you the draw from lights which should be fairly accurate. Once the generator is working correctly the output can be set.
 
When you change the polarity of the battery, the generator may need to be repolarized. Search on the Barn for information on how to do this. Otherwise everything else should work OK. You may want to disconnect the generator and measure the voltage to see if the polarity is the same as the battery. Don't run the generator very long with it disconnected as the voltage can build up enough to be destructive. Running at an idle will let you know the polarity of the disconnected generator without it building up very much voltage. At idle it may be only a few volts.

You can use the coil polarity tester from Snyders to test the polarity of the coil.
 
99% of the time if the generator will motor, its fine. Just apply power to its input lead with the belt loose or off. The generator should spin slowly.

That should polarize it, but, a jumper lead across an original type cut-out terminals for a nano-second will polarize it. An aftermarket cut-out can/may be damaged doing this.


When turning on the lights [key off] the ammeter should discharge. If it shows a charge, its wired backwards. If it does nothing, something is wrong.
 
Well it’s charging 15 plus Amps on the meter not discharging., 10volts reved up. Everything is wired up correctly for 6 v positive ground except the battery is hooked up for negative ground. I tried to adjust the field brush in the generator and I kept looking for a set screw, there is none on my generator it appears to just be tension to keep it in the correct place, some pressure and I could get it to rotate, the adjustable brush was right next to the other brush and I believe that is maximum voltage/ current. The wires coming off the fields were very frayed and someone had covered some of it with electrical tape so I’m going to rebuild it, the front bearing is bad also. I’ll get the polarity tester for the coil. Someone along the line did not do this vehicle justice😕
 
Great news. Yes, the current is set very high. 10 volts is too high. Except for the very early generators, the third brush should just rotate and is held in place by friction. Rebuilding the generator sounds like a good idea. They are fairly simple. Lots of folks put in an alternator. You can get an alternator that looks like the stock generator if that is important, but they are expensive. When people brought their car in for service, the Ford dealers would measure the battery electrolyte and adjust the charging accordingly. A volt meter in addition to the ammeter will tell you if the battery is getting over charged or under charged. There are Youtube videos available on how to rebuild the generators. Overcharging the battery will ruin it in short order. You will need to adjust the third brush according to the type of driving you do, short hops or long distance, and whether you drive a lot at night or not.
 
If you don't like the looks of the inside of the generator, it can be easily rebuilt. The vendors all have rebuilt units for sale.

The moveable brush is just held in place by friction. These generators will easily put out 20 volts is left unregulated. Setting the output usually depends on how you drive, whether you use lights often or not as mentioned.

I don't understand your posts. The ammeter should show a discharge when the lights are on and the key is off. If it doesn't there is a problem.
 
I’m still confused also. The amp meter is showing charge when running, goes down when I turn on the lights, but the wiring including the coil is correct for 6 v positive ground. Generator output voltage is positive. Are some 6 volt coils different internally? I bought a new 6 volt coil with a tune up kit but haven’t installed it yet, still need to get a coil polarity tester, will order it today.
 
How is the coil marked ? If it shows + and -, it should be wired same as the battery. If battery + goes to grd then primary distributor lead should be +. I think I've already mentioned this.

Check coil you bought. It may be oil filled. If it is you might want to mount it with leads up. Check the primary resistance.
 
As Patrick stated, is the coil marked + and -? You probably don't have to install a new coil. The polarity tester will tell you if it wired correctly. The generator does not have a voltage regulator like a more modern car. You only control the current. So if you turn on a load, like the lights, the charging current will go down. The generator is another thing on the Model A that is manually controlled and not automatic. If the generator is set up for day driving and you should happen to do a long drive at night, you will have to get out and set up the current. This is a big reason why people go to alternators. Back in the day I would install a more modern voltage regulator in my Model A's. It takes some wiring changes and eliminates the use of the third brush.
 
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