More insight into resistors

January 1, 2011 | By Richard Prince


As a follow-up to a resistor question in the August issue of Auto Restorer, to understand why a points-type ignition system has a resistor you first need to understand the nature of ignition coils. An ignition coil is a transformer. Transformers are used to transform power; in the case of an ignition coil, it transforms a low primary voltage to high secondary voltage—a voltage high enough to jump across a spark plug gap and ignite a mixture of fuel and air.

A transformer can only transform alternating currents and its primary resistance or, more properly, impedance, is proportional to the frequency of the alternating current applied to it. With a frequency of zero (the engine not running) the impedance would be very low, and should the points be closed, the primary currents will be high. A resistor inserted in the circuit limits the current flowing in the primary circuit and therefore assures a reasonable life for the points and coil.

A point ignition system should always have series resistance added. It can be an external resistor, resistance wire such as GM used, or a resistor that’s internal to the coil itself.

One more tip: never search for a misfiring cylinder by disconnecting plug wires. The high voltage will still seek a path to ground and can do so within the coil destroying it immediately or setting it up for failure at a later date. A better way to troubleshoot misfiring cylinders is by using a test light to selectively ground out cylinders by back-probing the plug wires on the distributor cap.


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