Model A with 12v coil

[TexWillis]

I drove about 750 miles to a car show in Saltillo, Mexico in June. After filling up with gas in Monterey, I started having firing issues. Before making it to Saltillo, I stopped a few times, and eventually just replaced the distributer and carburetor. I could only get up to about 30mph or so. Before leaving, this mechanis in Saltillo brought over some test equipment and jumper wires, and said that my coil was wired backwards. Up to that point, I hadn't touched the coil. We swapped the wires, and lo and behold, it made the 750 miles back running like a fine-tuned sewing machine.

Has anybody ever heard of a coil changing polarity? What could make that happen? Thanks!

[Mike V. Florida]

Never head if such a thing.

[Patrick L.]

I've never heard of a coil deciding to up and change its polarity, but, many have been installed improperly over the years. An engine will run with reversed polarity.
That said, I assume you're running on 12v. Does the coil have an internal or external resistor ?

[TexWillis]

12v coil with an internal resistor.

[Purdy Swoft]

Coils are now listed in OHM rather than volts . An internally resisted 12 volt coil is rated at 3.0 ohm . A six volt coil is rated at 1.5 ohm and can be run on 12 volts if an external ballast resister is used . I've never heard of a coil changing polarity . A coil will still run either way that it is connected . If the polarity is reversed at the coil the engine will be slow to start and won't perform at optimum level .

[TexWillis]

Sponsored Links (Register now to hide all advertisements)

Thanks. It sure had me baffled, and I'm still scratching my head!

[Mike V. Florida]

Originally Posted by Patrick L.

I've never heard of a coil deciding to up and change its polarity, but, many have been installed improperly over the years. An engine will run with reversed polarity.
That said, I assume you're running on 12v. Does the coil have an internal or external resistor ?

Your right they can't they are made one way.

[Joe K]

If the polarity is reversed at the coil the engine will be slow to start and won't perform at optimum level .

Plus one on that. Some of the aftermarket coils don't have the coil polarity correct on the bottom (or are not marked at all) and require the right wire to be crossed over to the left terminal, and the left wire to be - well you understand.

There is a test, possibly a test device to tell if the wiring is correct. Gosh, some of this stuff is starting to slip away...

Anyone?

Joe K

[Patrick L.]

Originally Posted by Joe K

Plus one on that. Some of the aftermarket coils don't have the coil polarity correct on the bottom (or are not marked at all) and require the right wire to be crossed over to the left terminal, and the left wire to be - well you understand.

There is a test, possibly a test device to tell if the wiring is correct. Gosh, some of this stuff is starting to slip away...

Anyone?

Joe K

Not quite sure what you're asking. But, I like coils that are just marked + or - . Then just start out hooking it up the same as the battery[ the points are considered ground]. It'll be right 99.99% of the time.
If you want to check it then you can do the pencil check [ I tend to have hard time seeing the flare direction though], a DVOM, or one of those polarity checking do-hickey's.
You could also get one of those super-duper coils and not worry about it as they'll put out more juice than a Model A should ever need.

[Mike V. Florida]

Tom Wesenberg Tip:
"To check coil polarity hold a pencil lead in the spark path with the plug wire about a third of an inch away from the distributor terminal. You’ll see a FLARE on one side of the pencil lead and this flare should be on the spark plug side of the lead." Tom suggests to hold the pencil in the middle or even slide the pencil into a plastic tube to avoid an electrical shock.

If the flare is on the plug wire side. Switch the two wires on the coil.

[Y-Blockhead]

Originally Posted by TexWillis

Has anybody ever heard of a coil changing polarity?

If you look at the way a coil is made that is impossible. It can, as stated above, be connected backwards.

Perhaps when the car was changed from 6v POSITIVE ground to 12v NEGATIVE ground the coil polarity was not changed.

[Mike V. Florida]

Please give us a little more history on the last work you did on the engine.

When did you tune it up last?
Did you replace the points?
What do your plugs look like?

With everything just right it would be hard to tell if the coil is backwards by running the engine, but as things wear, dimensions/gaps change then the polarity would show up as a problem. Just thinking.

[tbirdtbird]

Tex, I suspect the coil was hooked up reversed for a long time. I think it is weakening.
IT is true that with the correct polarity it will make some difference in performance. USually not enough to notice. BUt if the coil is getting weak, then that could make all the difference.

I don't prefer the internally resisted coils because that only adds to the heat buildup inside the coil; thus premature failure. You do a LOT of driving so that means a LOT of heat! FSI is not able to source an American made 12V coil meaning with an internal resistor

I am getting American made 6V coils from FSI ignition in Oklahoma and then I add an external resistor since i am running 12V

You going on the Labor Day tour? Hope to see you there! If you are going I'd at least carry a spare coil if not a spare plus change out the one that is on there

DAVe W MAroon 180A

[Pat Martone]

FYI - I think, but cannot be sure, that I was having trouble with a 12v coil (with internal resistor) getting too hot and causing problems. I ended up with a bad 12v epoxy-filled coil (Flame Thrower) after about a year of use and sent it back to Pertronix for a free replacement, then in about a year, the replacement coil may have begun acting up too. Therefore, I've recently switched to a 6V coil and added the external-mounted ceramic insulated resistor (provided by several Model A suppliers), and my problems seem to have gone away. Time will tell if this was a good change for me.

[TexWillis]

A Mike V.,
1) I tuned it last in March before leaving out for the MAFCA Spring Tour in Texas. It did fine there and back, On the State Tour, then on into Mexico. Ran like a sewing machine up until this incident.

2) I replaced the points and condenser about 1,000 miles before this happened.

3) Plugs looked fine after this incident.We pulled them off when we got to Saltillo, Mexico.

[TexWillis]

@ DAVe,
No, I cancelled my hotels. We have been sitting with Chana's dad on Hospice, and he passed away early Monday AM. We need to help her mom with making arrangements (Thursday, 2:30PM at Restland, Dallas), and maybe sit with her through this weekend.

We'll have plenty of opportunities for Labor Day tours, but I felt that we needed to sit this one out. Have a good trip!

[Purdy Swoft]

Originally Posted by Patrick L.

Not quite sure what you're asking. But, I like coils that are just marked + or - . Then just start out hooking it up the same as the battery[ the points are considered ground]. It'll be right 99.99% of the time.
If you want to check it then you can do the pencil check [ I tend to have hard time seeing the flare direction though], a DVOM, or one of those polarity checking do-hickey's.
You could also get one of those super-duper coils and not worry about it as they'll put out more juice than a Model A should ever need.

Coil polarity will be controlled by the way that the battery is connected , positive or negitive . I don't bother with the pencil trick . I would also have problems seeing the direction of the flare . I run one of the super duper coils, love it, have no problems with it but sure as hell don't want to get shocked by it. Years ago I bought a device from Bratton that tells if the coil is connected backwards. Brattons coil tester is listed on page 77 of the current catalog Part# 17140 price $23.50 . They've really went up since I got mine. I consider this little device a valuable tool !!! I check every coil that I install with it . Even though I run the super duper Pertronix Flamethrower epoxy 3.0 OHM coil , I want it to be connected correctly so I get full power from it.

I figure that the coil in question was just hooked up backwards in the first place.