From 1975 until 1986 I drove a Model A to work and put approx. 100,000 miles on it. I used a 12 volt battery and a coil from a 56 Dodge truck. It worked great. Now I have a Model A again and can't get 100 miles out of a coil. I have tried 6 volt with a resistor and 12 volt coils. The last one was a Bosch 12 volt which ran Ok if you could get the car started. I then went back to 6 volt with a resistor which started Ok but lasted about 100 miles. What's with these new coils? Any ideas?
Which way are you wiring it in?, is the A still + earth?
The distributor point setting dwell determines the "ON" time of the coil while the "open" period of the points is the "OFF" time of the coil. In general most 4 cylinder engines call for dwell time settings that are way more than necessary to obtain a strong spark and the extra dwell just dissipates heat in the coil if it is already fully saturated. Make sure the coil you are using is designed for a 4 cylinder engine and "cheat" on the dwell to set the dwell time to a smaller number and see if the motor won't run just as well as with a higher setting. This will cool off the coil a bit and might be just the edge you need to make the thing more reliable. If you have a heat reading gun then read the temperature of the coil outside with normal and reduced dwell and see if that leads you to believe you have done some good. Also make darn sure you have the correct value of resistor for the coil you are using. If possible you might also move the coil away from any heat source like the exhaust manifold.
Point gap .018 to .021, plugs about .032. Have you replaced or checked the condenser? Have you checked that the polarity is correct? The coil polarity checker might be cheaper then to keep replacing the coils. Having the coil mounted on the firewall should not be exposing to high heat as the Model A are. The 6 volt coils sold for Model A's (using 6 volt system) should not need a resister.
Unless there is something unique to a Model A coil, I don't see how having the polarity backwards would make the coil fail. The primary current is going backwards through the coil and thus the spark will jump from ground to center electrode if the coil is wired backwards but that doesn't make the coil overheat in a typical distributor setup but maybe there is something I don't know about the Model A system. Heat is generally the big enemy pointing to bad or wrong resistor if one is used or bad or wrong coil for the application. Are you sure that the coil is failing? Sometimes something else is the issue and changing the coil simply makes the problem go away for awhile. Hard to believe you could have so many different coils all failing. Intermittent connection or short inside the distributor was the issue on a friends Model A who had the same issue.
If the condenser is an original made from coiled metal foil they can be shorting internally when hot and when they cool the foil contracts away from the casing and they function fine again. Gives the deception of a dead coil.
Do the dead coils test fine outside of the car?
Fordbarn discussed this a while back and I went with the epoxy filled coil they recommend. It's run well a couple of years on my '31. They said many coils are oil filled and fail because they're mounted upside down in a Model A.
http://fordbarn.com/forum/showthread.php?t=145972&showall=1 is one such discussion
You said you used a 12 volt battery, a you still using a 12 volt or are you using a 6 volt battery? The coils for the A are 6 volt. Could be cross pollination. Check to see if the coil has an internal, as opposes, an external resistor. I use an internal 12 volt coil on my '49 Chevy truck, upside down, like the factory did, no problems. Buy a coil from one of the vendors that are made to fit to the A. You could be buying one that is made for an HEI outfit and it won't last with points and ask me how I know. They don't have internal resistors.
Joe R. Independence, MO
"The last one was a Bosch 12 volt which ran OK if you could get the car started."
I've read that the blue coloured 12V Bosch coils that are made for use on VW's w/out an external resistor are very good.
I have no personal experience w/those coils.
Primary winding resistance of a 12V coil that doesn't use an external resistor should be 3.0 to 3.5 ohms.
Brendan...FWIW while I wasn't having coil problems, I couldn't keep a condenser in my Model A for more than a couple of trips. Got fed up and just replaced the entire distributor setup and problem solved. Car starts better and runs better. Just a thought for you, maybe it'll cure yours too.
Yes, i am still using a 12 volt battery. I have tried two different kinds of standard condensers, three different kinds of distributors, two different kinds of modern points and condensers and still have starting problems with a 12 volt coil and poor life with 6 volt coils using an external resistor.
Brendan, check the polarity of the coil, It is probably backwards to the battery, Model A , as mentioned, uses a + (positive) ground, a 12V coil will be marked for - (negative) ground.
If you are using a 12 volt battery then you must have a 12 volt charging system. A lot of the items sold for these conversions are poorly made junk from the far east or Mexico. Check the charging voltage at the battery with the engine at mid RPM (cruising speed). The voltage needs to be between 13.2 - 14.7 volts at the battery. If it is too high it will take out other components. Too low it won't keep the battery charged.
Bad connections can cause too - high system voltage. A common problem is when someone installs a battery disconnect switch. A lot of these switches - actually all but a few - are poorly made. Driving along the connection is made and lost at every bump, resulting in too high charging voltage, which damages system components.
The condensers and coils on the market again fall into the category of poorly made junk. One thing that can help is a heat shield under the condenser, another is to find older NOS components for the coil and the condenser.
Here's a photo of the heat shield on my Model A - it runs an original Ford coil with a ballast resistor on 12 volts, and an original Ford condenser.
There is one condenser on the market that I trust. Al Lepore of A&L parts continues in his father's tradition of making great parts. His burnout proof condenser is just that. My distributor does not have the heat shield Royce shows. I also have the Autolite waffle manifold heater throwing off heat.
No website but easy to find at Hershey; just stand at the back of the line leading to his canopy.
You've been through so many coils I begin, as others have, to suspect another problem which is affecting the coil as opposed to the coil itself. Honestly the Bosch coil should have been OK. The only way I see a coil affecting starting is if it's only supplied through a resistor whether cranking or running. This might cause lower voltage while cranking and resulting in hard starting. High charge rate is a possibility but if your batts not cooking too that's out. Coil mounting location & condenser need consideration.