Magneto ring failure?

[Chris Bamford]

A local friend has just a few miles on his '26 Coupe and experienced 'sudden-onset' rough running on magneto the other day. The car runs much as before when on battery but very poorly on mag. Previously, switching to mag from battery perked it up as one would expect. Al says the wiring checks out OK and the car is running a new Anderson timer.

He suspects the magneto coil ring. His best coil ring when assembling the engine had three damaged coils (as below) which he replaced with coils salvaged and rewound from another ring.

If the ring has failed it will need obviously replacement. But for now and for diagnostics, our question is this: If one of those three rewound coils has shorted to ground, will the remaining six, seven or eight coils that are not shorted out result in this rough running on magneto?

[DanTreace]

First easy check is for metallic debris on the mag pickup at the hogshead. Just a bit of fuzzy debris that is always mixed with metallic particles of normal engine wear will ground the mag intermittently.

Second is to check the switch contacts for mag.

Third is to do mag output check with light bulb, or meter, to determine output. Poor reading there could be coil ring issues.

[BobShirleyAtlantaTx]

It will only generate from the grounded coil to the mag button. Thus you will only have electricity for two of the coils if I’m thinking correctly. So if the mag is indeed grounded at 6 o’clock the motor run on 4 and 3 ??

[Bill Robinson]

Back in 2004, I had the same problem not long after a rebuild. The rebuilder asked me to remove the hogshead and look for something suspicious. Well I found it. There was a small piece of tie-wire wedged in between one of the newly wrapped coil rings and the mag ring. And, it looks like the wire was probably up against the hogshead, too. The members of our club, The Tennessee T's, call this a "Booger in the Pan".

[AndreFordT]

Here is an other one, freshly rebuild, stand for 200 miles and than failed.
Reason was debris and band material.

Andre
Belgium

[Chris Bamford]

Thank you all who have responded. Dan, I agree those are simple and smart things to check before condemning the coil ring.

What I had hoped for at this point, and have yet to hear, is confirmation that that a T can still run on magneto with half the coil ring shorted to ground.

My hunch is yes, but poorly. Bob Shirley postulates only half the cylinders will fire, but with friend Al’s car, all cylinders are still firing (half-heartedly) on mag.

So: Will it run on half a mag ring? And if so, on all cylinders or just a couple?

Inquiring minds want to know...

[speedytinc]

It will run poorly on all cylinders. If it put out 20v before shorting 8 coils, it is likely to put out 10v now. That isnt enough for a good magneto ignition. Why not try an in car recharge? The problem might be a bit of metal shorting a coil to ground. The recharge process might clear it. Its a very simple procedure. What do you have to lose?
I do my best work when i have nothing to lose.
One patch to keep driving, would be to run ignition on a separate segragatted 12v battery. even at a dead loss will run a couple days between charges.
She will typically run on 12v dc as good or better than mag.
A fellow club member ran a 13 on a 9.6v makita battery. 1 battery went 4 or 5 hours.

[Chris Bamford]

Thank you Speedy, that’s what I suspected.

I agree there is more to check before condemning the coil ring, but this is one diagnostic and got my curiosity riled up.

My Dad ran his ‘12 roadster jalopy on a 12-vt battery for years... 2-3 recharges each season was all it took.

[RajoRacer]

If one is going to run on DC voltage, you had just as well install a distributor - original coils are designed to operate on AC magneto output !

[speedytinc]

In this case, 12v battery is intended as a fix to keep running the car untill a proper field coil fix can be done.

[Dan Hatch]

Get yourself a good volt ohm meter and read resistance from the button to ground. You should see about .5 ohm. Less you have a short to ground more and you have open coil.
Check the button like Dan T said at same time. Good luck. Dan H

[speedytinc]

RajoRacer.
I dont advocate running a 6v t starter on 12v. thats partly why i say run a seperate 12v battery for ignition only.
I assume some guys dont have a spare t to stay in the fun.

Are you implying or saying running a stock t ignition on 12v dc will do damage or be a problem ??

I am charging a 12V battery to fire my coils off magneto voltage on my 14. (obviously converted to dc.)
mag puts out 35v without battery in charge, 33v when in charge.
Car runs equally good in either bat or mag setting. I run on battery for the advantage of an infinite advance/retard range as opposed to 3 or 4 advance/retard positions.

I have not heard of or experienced any issues yet.
Am i doing something damage to something??

[R.V.Anderson]

The mag will NOT run with half the windings grounded. It won't even run with less than half of the windings grounded. It happened to my '14 about 10 minutes after its first start-up. A broken cotter pin grounded the coil at about the 7 o'clock position, reducing the 17.8 volts that I had been getting at idle down to 2 volts, not enough to fire the coils.

[RajoRacer]

The coil points will be short lived running on DC compared to AC as was the original intention.

[AndreFordT]

My opinion: It will not run right with a grounded coil even if there are 15 coils left to produce the tension because of the current drop in the loop.
The place where the short is, there will always be a resistant and will drop the current out put of the magneto. (Spark smoke and heat)
The coils need between 1.3 and 1.5Amp to spark right and are not only working on tension but they need current too.
On my HCCT a good coils start to spark 1/4" long at 1.8Vac and 1.3Amp. That also the reason why with a good magneto and right coils you should be able to start the engine by hand cranking, 2Vac to 3Vac but more than 1.3Amp out put.

Just my opinion.
In the attachment some other photos of schorted magneto loops.

Andre
Belgium

[BobShirleyAtlantaTx]

As I think more about this, I believe I’m wrong in thinking only 2 of the coils would fire. The ac currant would be continuous but would decrease in portion the number field coils in service. I’m thinking the voltage would be about 1/2 normal output.