MTFCA Forum

Hello everyone.
I have an odd situation I'm hoping I can get some help with.
I have a 24 coupe.
There is an occasional spark at the firrewall and carb end of the carb adjuster rod.
Of important note, I have just completely rewired the vehicle EXCEPT for the coil box itself. I also do not yet have a cover on my coil box, I have to purchase one. The coil box is in the cab on the firewall. Could I need a rebuild of the coil box itself? Does it really matter?
Thanks in advance!

Sounds as though you have a carbon track happening somewhere in the coilbox.

RajoRacer wrote: ↑

Tue Nov 07, 2023 7:43 pm

Sounds as though you have a carbon track happening somewhere in the coilbox.

Thank you! That was my suspicion, but I wanted to bounce it off of more knowledgeable folks than myself lol

I would finish your rewire job with the most important part of the Model T electrical system which is the coil box!
It’s the brain for your car.
If it hasn’t been rebuilt before I would rebuild it.
It may carbon tracking, a short in the wiring close to the firewall or etc. causing the arching. you’re seeing.
After nearly 100 years the box needs to be rebuilt.

John kuehn wrote: ↑

Tue Nov 07, 2023 7:51 pm

I would finish your rewire job with the most important part of the Model T electrical system which is the coil box!
It’s the brain for your car.
If it hasn’t been rebuilt before I would rebuild it.
It may carbon tracking, a short in the wiring close to the firewall or etc. causing the arching. you’re seeing.
After nearly 100 years the box needs to be rebuilt.

That will be my next step! I wanted to do it but thought I'd get away lucky and lazy! I'll do that next. Thank you!

If the spark occurs only when cranking the engine with an electric starter, it could be a ground issue.

I had the exact same problem, thought it was the coil box too. Turned out to be the ammeter. The insulators for the ammeter were left off by mistake and it was causing arching at the carb adjustment rod. Found it unexpectedly when the ignition switch panel was removed from the dash for an unrelated issue and left dangling by the wires and the arching went away. Put the ignition switch panel back on the dash and the arching returned. Took the ammeter off the ignition switch and arching stopped again. Oops, discoverd one of the ammeter insulators was missing. Put on new insulators and problem solved. The arching was so strong I was sure it was carbon tracking on the coil box, but not so.

Humblej wrote: ↑

Tue Nov 07, 2023 8:06 pm

I had the exact same problem, thought it was the coil box too. Turned out to be the ammeter. The insulators for the ammeter were left off by mistake and it was causing arching at the carb adjustment rod. Found it unexpectedly when the ignition switch panel was removed from the dash for an unrelated issue and left dangling by the wires and the arching went away. Put the ignition switch panel back on the dash and the arching returned. Took the ammeter off the ignition switch and arching stopped again. Oops, discoverd one of the ammeter insulators was missing. Put on new insulators and problem solved. The arching was so strong I was sure it was carbon tracking on the coil box, but not so.

Could you explain the insulators to me more? I'd like to check this as well. And I'm sorry but I'm not quite sure what you mean.

TXGOAT2 wrote: ↑

Tue Nov 07, 2023 8:05 pm

If the spark occurs only when cranking the engine with an electric starter, it could be a ground issue.

its happening while its running. even on mag.

The easy way to check if the ammeter is causing the arching is to take the ignition panel off the dash and let it hang by the wire harness so it does not touch metal. Use a test light from the ignition switch metal panel to the metal dash and see if you have a stray voltage. A bad ammeter insulator will cause the arching with the switch in the off, bat, or mag position, running or not running as long as the battery is connected.

Tim
Your problem sounds to me like a carbon track has deveioped in the wood of the coilbox and the high tension spark is shourting to adjacent metal parts to ground.
When a coilbox wood gets wet the car usually wont start. The natural inclination is to keep trying to get the car going, but that only causes more carbon tracks, exacerbating the trouble.
This problem was very common in the Model T era becasue cars were usually parked outside.
Usually touching the metal parts of the car will result in a slight tingle of voltage.
You can watch the sparks at the choke rod and have someone hold the vibrator spring on each coil individually open to see if it stops. It is most commonly the two coils on the far right and left of the coilbox.
In any case remove the coilbox and rreplace the wood back and bottom and replace it with the commonly available non wood replacement parts that do not shirt becasue the are not wood. When coil sparks cross over the wood it carbonizes a track which is electricall conductive. The track may nort be visilble because the original wood is laminated and the tracks can occur between the wood laminations.
These problem are difficult to troubleshoot because they do not act at all logical. If this is your trouble i will tell that when you locate the trouble and fix it you will look back on the experience as one not soon forgotten.
Good luck
Ron Patterson

Carbon tracks once established are pretty much impossible to remove. Snyders sells a plastic replacement kit for coil boxes. Save your old contacts...they're typically a better material. I've found that doubling up the contacts (one original, one new) will help prevent them bending with use.