coil overheating

H

hangarb7

Senior Member
A guy in our club has a 12 volt system on his A. He keeps having breakdowns because his coil overheats (so hot you cannot touch it). We have tried coils with a resistor and coils with an internal resistor. Voltage at the coil is 13.4 volts when car is running. Car is not overheating.

Any suggestions?

Thanks
 
Is the coil mounted in the right orientation? Depending on the coil they may not want to be Mounted upside down .
 
I would contact the manufacturer of the coil about this problem. The coil is likely epoxy filled, and it may have a short circuit. To determine if there is a short, connect an ohm meter to the coils case and one at a time, to each terminal. Turn the ignition on. There should be no continuity to the case from either terminal (infinite resistance).
I run a 12V system with a coil having an internal resistor. In operation, it gets warm to the touch.
 

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Keep it mind most new coils are designed to be pointing up not down like a Model A coil. I would try to find a tar based one like the originals.
 
Check the resistance of the primary winding, it needs to be ~3 ohms for a 12 volt system.
 
A stand alone coil will be 1.5-Ohms on the primary for operating on 6-volts. It can be used on 12-volts if a 1.3 to 1.5-Ohm ballast resistor is utilized. As mentioned, a 3-Ohm primary is generally made for stand alone use on 12-volt systems.

The primary coil is the only "resistor" on that side of the inductor and it's there to build the electromagnetic field in the coil core. The secondary coil can be more difficult to check depending on how one end of the coil is connected internally to one of the primary terminals. Different coil brands may also have a different number of windings. Normal resistance between the output tower terminal and one of the primary terminals should be from 8,000 to 10,000 Ohms but may be higher or lower depending on which terminal on the primary side is selected for the test but this won't tell you if there are internal intermittent shorts or other problems in the high tension coil. A coil should be tested in a tester that can allow for preheating but preheating can be done by powering up the primary coil for a while. Just don't overheat it by connecting it up for too long. An oscilloscope is still the best way to test the output of an ignition coil. The quality of the high tension spark is a good indicator of condition. It should put out a nice blue spark under test.

A coil that runs hot likely has an internal problem that is causing excessive resistance. Resistance equals heat in an electrical circuit. In a normal running system, the coil should only be warm to the touch. It shouldn't be real hot. The circuit in an ignition system turns on and off so fast that the primary should never have enough time to get real hot. If you leave the ignition on by accident and the points are closed then the coil can overheat and be damaged. A condenser that's in poor condition can also cause overheating but it will also cause poor performance which is usually noticeable in operation.
 
Is the coil oil filled or epoxy ? Oil coils do not like to be tipped upside down. If it has an oil coil I'd recommend changing to epoxy.

Measure across coil terminals, should be 3 ohms for a 12v. Measure voltage out of coil. You mentioned 13.6v to the coil, should be about 7-8v out.

As mentioned the coil will get warm, but, shouldn't be really hot.
 
Most of us just turn the coil over and run a longer wire into the distriburor as well as the other two wires. Those coils don't like being upside down! Ernie in Arizona
 
Bob Bidonde said:
I would contact the manufacturer of the coil about this problem. The coil is likely epoxy filled, and it may have a short circuit. To determine if there is a short, connect an ohm meter to the coils case and one at a time, to each terminal. Turn the ignition on. There should be no continuity to the case from either terminal (infinite resistance).
I run a 12V system with a coil having an internal resistor. In operation, it gets warm to the touch.
Click to expand...

Wow Bob...... That is a spotless engine compartment! NICE
 
Not to change the subject but can you say what color is your car? My coupe is the same but i dont know the name of the color TNX
 
A little history: When cars switched over to 12 volts the 6 volt coil was still used with a ballast resistor. When starting the battery voltage would go down because of the large current draw from the starter motor. A second wire was run from the starter terminal to to the coil so that the ballast resistor was bypassed. This would give a hotter spark for starting.

Since the Model A starter terminal is just a button on top of the starter, it is difficult to run a wire from this point to the coil to bypass a ballast resistor. For my Model T I had a switch to bypass the ballast resistor. (I had a distributor.) It did not seem to make any difference in starting whether the switch was on or off. I used an authentic high quality Bosh coil which I mounted with the nose up. There are coils on the internet that are branded as Bosh but not authentic and are poor quality.
 
How many amps does the coil draw with the points closed and the key ON?
If it's more than 4 amps, then I'd add a ballast resistor.
Also more dwell will make the coil run warmer, as the points stay closed longer, but this shouldn't be significant.
 
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