Trouble shooting a friends car. Basically looks like a new 6 volt battery BUT battery dead (on tester) Battery positive hooked to negative ground strap. Charging and testing the battery tonight.
I will hook negative terminal too negative car body ground strap. Question. What problems could this have created? Demagnetize magnets? Other problems to check for him? Maybe a Model A member as they have positive to ground?
Would not think it would have affected the mag magnets. I think your concerns would be with the generator and the cut out. If the generator is not damaged, you might have to polarize the generator. Could have damaged the battery if it was totally dead.
To kill the magnets David, you would have to apply battery current to the magneto terminal on the hogshead either directly or via MAG terminal on the ignition switch or the magneto terminal on the coilbox (depending on year of car). Getting battery current on the hogshead terminal via any path is bad. BTDT.
I would slowly charge the battery in the correct polarity, then polarize the generator, and see what happens.
If it is on a non-starter/non-generator car it doesn't matter. The coils are normally not run on battery very long. We went for years before I read where the T was negative ground while the A was positive ground. Our 1915 and 1918 started fine that way from about 1948 to 1970ish.
The battery was not originally ever connected to the magneto circuit. So unless the ignition switch is defective; or the wiring is defective; or it is wired wrong; you should have two completely separate circuits. The battery should not receive any current from the magneto nor should the magneto send any current back to the battery.
However, there are modern systems that will allow the magneto to charge the battery for cars that do not have a generator. I believe those systems are sensitive to polarity (I think they use a diode -- but perhaps something else). When installed properly in a Model T that also has proper factory style wiring (i.e negative ground etc.) they work fine.
If you are talking about a starter/generator equipped car, and if it has any of the modern voltage regulators or even just a diode so it works only as a cut out, they also are sensitive to polarity. See the warning about using the correct polarity at: http://www.funprojects.com/pdf/5055R6-052111.pdf
Please clarify if the car does or does not have a factory starter/generator. Also is it wired like a stock Model T? Also, did it work properly and then the quit working? And if so -- did the owner do any thing such as change out the battery, wiring, etc?
Note if a previous owner set the T up to run with a positive ground and it worked then you will need to un do those changes. If the owner only replaced the battery and installed it incorrectly then charging the battery, installing it correctly, should solve most of the problems. Note if any modern electrical components had been installed installing the battery backwards could cause some parts to be damaged. So it would be very helpful to have a more information about the background of the car. I.e. new owner and it has never been run by him or same owner last 30 years and it just started doing this?
Good news as long as the owner does not let the battery voltage flow back into the magneto -- it should not be able to cause the mag to have trouble. But if it has a generator and you are running it without a load on the generator -- from memory (not as good as a written note for folks like me) -- that can cause the generator to die quickly.
I'm sure you will get it sorted out.
Mine was that way when I got it. I switched it back to negative ground and all is good. I had to flash the generator to get it to charge but no problems yet.
If it has the original type cutout it does not matter which ground is used ( - or + ). If it has a diode type or modern Model A regulator then voltage direction does matter. Unless you know what cut out has been used I would leave it as is, + ground. The electrical system does not care. The magneto as stated above will not be effected.
Won't hurt the magneto. You will have to flash the generator field, and swap the terminals on the ammeter so it reads correctly.
If you have a diode for the cutout, it would cause the battery current to flow into the generator when the engine is not running. That would quickly discharge the battery. This is true if the diode is placed for a negative ground. It is possible to run everything with positive ground if the generator and cutout are set for positive ground. No problem with the magneto.