Burning Out Ignition Coils

[Corley]

The difference between a negative ground and positive ground coil is which terminal (negative or positive) that the high voltage side returns to. IE:, a coil has two windings, the primary side and the secondary side (also known variously be other names such as the high side, or high voltage side). Each winding in the coil has two ends to the wire. The primary winding has one end of the wire connected to the small terminal that says negative or (-), and the other end of the primary winding is connected to the small terminal that says Positive or (+).


The secondary winding, or high side, also has two ends to it's wire, one connected to the high tension terminal (the large center post on the coil). It is the other end of the secondary windings connection that determines if the coil is for negative ground systems or positive ground systems. If the other end of the secondary winding is connected internally in the coil to the small negative terminal, the coil is for a positive ground system. If the other end of the secondary winding is connected to the small negative terminal, the coil is for a positive ground system.


So, you should be able to meter this and determine which it is, right? Well, yes and no. The primary winding resistance is very low, whereas the secondary windings resistance is very high. If you use your typical home ohm meter, a check from the high tension lead to either the positive or negative small terminals, you will see a very high ohm reading. In fact, it is so high, that measuring to either of the small terminal will measure so close to the same, that your meter will probably not be able to distinguish which way it is connected internally. Basically, the primary windings resistance, being so small compared to the secondary's resistance, gets buried in the meter's ability to see the difference.


Here is the problem in using the wrong type of coil: When the correct coil is used, (positive vs negative ground),and the points open, the induced charge in the secondary winding has a path from the spark plug, back through the plug lead, to the distributor, and back to the coils high side terminal. The return path is from the internal high side coil winding, connected to the correct terminal, back through the ignition system to the return. All is well and good. If using the wrong coil for the system type, the return path for the high tension winding is altered. The path becomes longer, as it must also flow through the primary winding to the complete the circuit. This longer path, in addition to having more resistance, also produces a counter EMF (electro magnetic force) which reduces the output of the secondary winding, creating a smaller voltage to the plug. Not by a lot, but by enough to cause issues on some vehicles. Most will probably work and run OK, but it is not optimal.


So, buy the correct coil for your application, and you will be a happier camper, they are not expensive. (Really sorry if all this is not easily understandable by everyone. If you don't get it, you are not stupid, just lacking in some electronics theory that allows one to understand it. In this case, consider yourself lucky because you for sure have some different talent/knowledge base that is actually useful in real life, and not this trivial electronics BS.)

[Corley]

Originally Posted by Bob Bidonde

My understanding is that the terminal connected to the points should have a marking the same as the electrical system's ground.

This is correct, but not the whole story, since coils are built for positive ground application and coils are built for negative ground applications, and to be optimal, you need the correct coil type and voltage.


To put it in simpler terms, if you are converting from positive ground to negative ground, you really should get a negative ground coil, and not just switch the leads, if you want an optimal ignition spark.

[30 Closed Cab PU]

Originally Posted by Corley

This is correct, but not the whole story, since coils are built for positive ground application and coils are built for negative ground applications, and to be optimal, you need the correct coil type and voltage.


To put it in simpler terms, if you are converting from positive ground to negative ground, you really should get a negative ground coil, and not just switch the leads, if you want an optimal ignition spark.

Thanks for explaining. The reason I asked - checked Brattons and Snyder's catalogs. Both carry only one model 6v coil, and do not mention type of ground (positive or negative). I was thinking of picking up a spare off EBay.

[katy]

Not all coils are marked + and -, some are marked Bat and Dist.