Testing Spark Coils

The major causes of rough running engines is poor spark coils


| March/April 1982


One of the major causes of rough running engines is poor spark coils. If a coil is in good working order and properly adjusted, it should give satisfactory performance for many thousands of miles with little or no attention.

Most of the old spark coils I tested were found to be in fairly good mechanical condition. A good cleaning, sanding and repair of the wooden box, a coat of varnish, and a new set of points (properly adjusted) should do the trick.

A rig for testing old coils is shown in Figure 1. I made a plywood box of the configuration shown. Instead, however, of a single unit, I built a double one with two sets of contacts and spark gaps in order to hold two coils side by side. This is for checking the windings of a questionable coil through the points and capacitor of a good coil. The foregoing will be explained later. Contacts are 10-32 machine screws with spring brass clips inside. (Pieces of spring brass weather strip for doors work well.) The adjustable gap is made from small corner angles and 10-32 screws ground to a point.

To test a coil, hook up the tested to a well-charged 6 or 12 volt battery (Fig. 2). If the coil is operating properly, it should produce a good continuous blue spark across the 1/4" gap while drawing about 1-1/2 amps. If no spark or a weak and intermittent spark results, further testing and repairing are in order.



It is advisable to install a new set of points, however, the old ones may be used with some success as follows:

First, remove the entire point and bridge assembly from the coil unit. If the "C" and "D" (Fig. 3) are rusted, work them off gingerly being careful not to allow the bolts to turn with the nuts. If they do turn, the wires soldered to the heads will twist off.














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