Testing Spark Coils

The major causes of rough running engines is poor spark coils

A rig for testing old coils

FIGURE I – A rig for testing old coils

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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.

After removing all the parts, clean the top of the coil box and the bolt threads with a wire brush. Inspect the points, and if there is enough tungsten left, dress down the pits level and square with an oil stone. Clean and polish all the parts, including nuts, spacers, and adjustment spring, on a wire wheel so that good electrical contact will be made at all points.

Check the upper (adjustable point bridge to make sure there is .005" clearance under the cushion spring along its total length. (Fig. 3)). Clean the dirt out from under the spring with a feeler gauge, tap the cushion spring rivet at "B," or bend the bridge back and forth at strategic points until the spring stands away from the bridge the required .005" and bears against the rivet flange.

Install the trembler point first. Run down the nuts and bend the support at "A" (Fig. 3) until the point end stands 1/4" away from the coil core when the mounting nuts are tight. Install the upper bridge assembly over the bolts and run down the nuts. Before tightening the four mounting nuts, move the trembler (lower point) until the points meet squarely and evenly. Turn down the adjustment nut until there is .030" clearance between the points when the trembler is held down against the coil core. Run on the lock nut and tighten in place.

Return the coil unit to the test box and hook up the wires as in Figure 1. The trembler tension is then increased or decreased until a continuous blue spark jumps the 1/4" gap while drawing 1-1/2 amps current. If the ammeter reads more than 1-1/2 amps, decrease the tension by prying up on the trembler support at "A" with a screw driver. If the ammeter reads less than 1-1/2 amps and the spark is weak, bend the support down at "A" by tapping it with a hammer, thus increasing the tension. The adjustment cut "C" should not be moved when setting up the coil amperage as this would alter the .030" clearance required to keep point arcing at a minimum.

We have described the procedure for testing and repairing coils externally. If you are ambitious and do not mind getting a little involved, you may be interested in experimenting with the mysterious works inside that little wooden box. To aid you in following the text please refer to Figs. 1, 2, 3 in the February issue.

If a coil is found to be defective after cleaning and testing as described above, internal repairs are necessary. The windings should be tested first, as it is a waste of time to fool with a coil with shorted or burned out windings. This is where the double-compartmented tester previously mentioned comes into play.

Pick out a coil that is known to be in perfect operating condition and slip it into one side of the tester. Place the coil in question in the other compartment and hook up the wires as shown in Fig. 2. Place a jumper across the points of the questionable coil to short them out of the circuit along with the capacitor inside. Now the windings on the bad coil will operate off the points and capacitor (condenser, if you will) of the good coil. A steady blue spark of equal intensity should arc across both 1/4" gaps simultaneously. If this does not occur the windings are shorted and you should "deep-six" the coil and try another. If a good spark arcs, then either the capacitor is bad or one or both wires have twisted off the bridge mounting bolts inside (G and D of Fig. 3).

Remove the sliding panel side, of the coil box by withdrawing the two little nails at the bottom (E, Fig. 3) and sliding the panel downward and off. Now you are confronted with a great lump of black tar! Study Figure 3 and proceed to carefully dig out the tar little by little with a screw-driver from the area under the four point mounting bolts. You may strike the top of the glass partition (H, Fig. 3) or little wooden spacer blocks. You can dig these loose and pry them out if they are in the way. If you are lucky, the wires will be soldered to the two bolt heads closest to you. If not, you will have to dig deeper to expose the lower bolt heads. If one or both of the wires are broken loose, solder them back to the proper bolts and retest the coil as in Fig. 1. It is not necessary to replace the panel to make the test.

If the coil is still found to be weak the final repair to make is replacement of the capacitor. Dig out all of the tar from over and around the old capacitor and pry it out with a screwdriver. Cut off the wires connected to the top and bottom of the capacitor, leaving tails as long as possible. Remove the glass partition and any loose wood spacers. Do not disturb the tar surrounding the core windings.

The best capacitor to use is a 0.5mf in the 400-800 volt range. These can be purchased at any radio or electronic store. They come in various sizes and makes so it will be necessary to pick one that will fit best. None, however, are anywhere near as big as the old ones. A Mallory or a Cornell fits well if you remove the glass portion. These are about 3/4" diameter by 2" long.

Solder the two leads on the capacitor to the two wires that were connected to the old capacitor and position the new capacitor as far away from the core coils as possible. Make sure that all the leads are well clear of the core coils. If you do not, they will short through the insulation and the tar which is poured around the core windings.

After everything is in position, pack the voids and spaces with wads of paper, cardboard, or small pieces of wood. It is a good idea to replace as much of the glass partition as room will allow. Now retest and adjust the coil as previously described. With the new capacitor, the coil should work better than it did when it was new, providing the repairs have been properly made.

Finally, if you wish to get really messy about the whole deal, you can melt some tar and pour it in over the whole works. Possibly roofing cement would work just as well, except that it would not set up as hard as the tar.

After reading this article, repairing four coils may sound like a tremendous amount of work! Actually it is not. The first one takes the longest, as you have to "feel your way" along. It took me only one evening to renew four coils completely. This included replacing all the capacitors, pouring new tar, cleaning and adjusting the points and repairing and varnishing the boxes. The previous evening I had built a tester and experimented with another coil. So, for the price of four capacitors (30¢ each) and a few hours of tinkering, you can have yourself a new set of coils that would cost about $18.00 to buy.

Incidentally, a leading Model T expert states that the new plastic K-W coils in the market today are apt to be erratic, as they have seen fit to install capacitors of only .25mf to .3mf. He suggests changing these capacitors to .5mf to attain best performance. New "K-W" Ford coil points may be purchased by mail from: Bob's Antique Auto Parts, 7826 Forest Hills Road, Rockford, IL 61111. They are $2.50 per set.

William B. Stafford's originally wrote "First Aid for Flivvers" for his Model T Ford Club publication some years ago. Contact him at 130 Old Oaken Bucket Road, Scituate, MA 02066.