Coil trouble on a VAC

Don Rudolph

Well-known Member
I have a neighbor who is having trouble with a late model VAC. It keeps burning up coils. Every couple of weeks he has to replace the coil with a new one. Someone did change it to negative ground before he bought it. Any ideas? Thanks, Don
 
Coil polarity will not fry a coil, just weaken the secondary spark by it causing to run thru the ground side of the circuit and arc from the side, or ground side of the spark plug electrode, rather than the center. Easily seen on a scope. He does NOT have the ballast resistor wired into the circuit. 6 volt system do not require a resistor. 12 volts do. I assume this has been converted to 12. Most 12 volt coils, particularly for 4 cyl engines will have the resistor internal, in the primary or low voltage side of the coil. Larger engines require an external resistor. Now I'm talking of breaker point systems. Any solid state system conversion does not need a resistor, although some keep them to keep current lower throw the solid state components, thus less heat. 12 volts running across points will give you excess arcing, short condenser life, pitting and obviously, very short point life. Points simply switch the primary circuit in the coil on and off, stopping the magnetic field induced by the coil of wire, and producing the voltage necessary at the plug. The ballast part of the name has to do with the heat generated. The longer the current flows thru the resistor (as in low engine speeds) the more heat, greater resistance, greater voltage drop, less spark voltage. But the higher voltage is not needed at idle or light, slow speed running. As engine speed increases, the time the points are closed is less, the resistor cools, raising the voltage for the higher cylinder pressures encountered with greater speed. External resistors are needed with larger engines so they can be bypassed in the starter circuit when cranking. Large engines, more voltage drop at the starter, combine that with voltage drop thru the resistor and you have lower voltages in the coil, harder starting. By not having the resistor wired properly, full voltage is traveling thru the primary or low voltage windings in the coil, frying the oil packed in there for keeping the bugger cool, thus destroying the coil.
 
OPV,

He is still running 6 volt, but negative ground. Sorry I didn't say that in the 1st post. Any idea what is going on with this tractor? Is it possibly not the coils going bad? He is ready to sell it out of frustration. Don
 
Hi Don - This is a solvable deal. Very basic inductive system that all internal combustion gas engines used. The coil is wrong. 2 bit bookkeepers have spent the last 20 years combining part numbers, to the suffering of us mechanical types. The primary and secondary coil windings have to be "matched" and the coil insulated, usually with oil to help cool it. If the coil gets hot, warm is normal, to hot to touch is dangerous. Back in the early 60s I had a high performance coil (more secondary windings to give a hotter spark at the expense of reliability) on my Hudson Hornet competition engine blow on a dynometer run. Boiling oil is something you don't need all over you...... If you have back issues of Old Abe's News, Dave Erb wrote some suburb tech articles and his basic electricity series for ignition, generator/starter understanding is awesome. He, too, felt that you have to UNDERSTAND the system you're working with or you'll just be a parts changer, with the same thing happening over and over. Have your friend hit the library for an early automotive text. Pre 1980s, after which coverage of basics was limited because of the electronics being used nowdays. No Motors or that type of book. Drop an email, I've a ton of resources I can loan you if your friend is interested. I may even dig up those ol pics of my '41 Commander couple..............
 
If it is a battery ignition system then the coils are very good and almost bullet proof. I my years of messing with engines I have seen only a hand full of coil go bad and most where caused by having a 12 volt battery on a 6 volt coil and no ballast resister.

So it is not likely the coils are going bad unless he gets them from TSC and they are made in China. But if he leaves the ignition on for a long time with out it running that can fry a coil but also fry the points. More then likely he has a points or condenser problem. Or he has short in the system and the coil has power all the time so it burns them out
 

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