Re: Why does a coil blow?

Posted by Bob on November 22, 2005 at 06:04:35 from (64.21.249.40):

In Reply to: Why does a coil blow? posted by 550Doug on November 22, 2005 at 05:44:47:

With very few exceptions, there is no such thing as a coil with an "internal resistor".

If the coil was only running warm, and not so hot you could fry an egg on it, I doubt the system voltage was too high for it.

Coils designed to be used WITHOUT an EXTERNAL resistor are simply wound with more wire eliminating the need for an EXTERNAL or INTERNAL resistor.

Modern alternators generally have their regulators set a bit higher than generators of some years back, but usually that is in the range of what a round-can coil will tolerate. You probably just got a defective coil. (A while back, I had a new one bad right out of the box.)

Another coil "killer" is a bad spark plug wire, or other poor connection on the secondary side. If the voltage is having a problem getting to a spark plug, it will take another path to ground, possibly inside the coil, shorting the windings,

The funtion of resistance in the primary circuit, whether an external ballast resistor, or extra windings in the coil's primary, is to limit average primary current to a level safe for the coil, and breaker points (or electronic ignition), typically in the 3 to 4 Amp range.

The reason for Pertronix to specify a coil with higher primary resistance for a 4-cylinder has to do with "dwell", (for this discussion,basically the % of time the points are closed).A 4-cylinder engine "fires" 2 cylinders per revolution, whereas a 6-cylinder fires 3, and an 8-cylinder fires 4. That means, for every revoltution of a 4-cylinder engine, the breaker points open (for roughly 30 degrees of distributor rotation) one time less than a 6-cylinder, and 2 times less than an 8-cylinder. This results in higher average primary current, and more heating of the Pertronix unit and coil. So, to be safe, they require a coil with a bit more resistance for the 4-cylinders.

No, you can't measure the coil output voltage with a regular voltmeter.

Follow Ups:

• Re: Why does a coil blow? John T 06:34:12 11/22/05 (0)

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