Many newcomers working with magnetos are stumped when it comes to coil testing. An automotive oscilloscope has a coil and a condenser tester on it that may be used toas a coil tester for a magneto. Coil testers are available on the market, but they are either scarce or very expensive. There is an alternative for that "just once in awhile" coil tester. Please note that this information is intended for the novice and not an experienced magneto repairman.
A very effective coil tester may be constructed with a conventional contact point type automotive distributor. Any distributor from any car would serve the purpose, as long as it had a serviceable contact set and condenser. You would have to check to see that the contact points were opening about 0.018 inch. The other items needed would be:
1) Three pieces of test leads made up of approximately 18 inches of 14 gauge wire with battery clips on both ends. Alligator clips would be suitable.
2) One foot of 7mm spark plug cable (use metal core, not TVRS cable), with alligator clips on one end and spark plug wire terminal on the other end.
3) A test spark plug, available at most automotive parts stores with a battery type clip attached. This provides a test gap of 1/4", that is 0.250 inch (high voltage spark tester).
4) A six volt dry cell battery available at any parts, hardware, or department store. The dry cell is much more convenient to use than a wet-type battery.
With the test leads made up and the distributor clamped in a vise to make it convenient, make these connections:
A) Connect one test lead from the battery to the distributor (as a ground). Rim of the distributor is okay.
B) Connect a test lead from the distributor terminal to the ground metal of the coil to be tested. Sometimes there is a ground wire coming from the coil, as well as the insulated primary lead wire. In this instance, connect the ground lead wire to the metal core of the coil and clamp them both together.
C) Connect the spark plug wire to the test plug, which is clamped to the rim of the distributor. The other end of the spark plug wire is attached or held to the high-tension terminal of the coil.
D) Connect the third test lead from the other battery terminal to the primary (insulated) lead wire of the coil to be tested.
E) Rotate the distributor shaft to open and close the points. There should be a blue spark jump the gap on the test plug every time the points are opened and closed.
There is an alternate method to using an automotive distributor. The only parts of the distributor that are essential are the contact points and condenser. If you obtained a set of contact points that were an integral unit, that is both the movable insulated arm and the stationary point unit were attached on one base plate, and a condenser, you could make up a tester. This tester would require the contact points and the condenser to be clamped or bolted to a face plate- a piece of metal, perhaps 1/8" thick and 3" wide x 7" long. The connections would be similar to those outlined previously except the distributor terminal would now be the terminal that was on the movable contact arm.
The base plate would be the same as the rim of the distributor. In place of the rotor and shaft to open and close the points you could either use your finger or a screwdriver to open and close the points. If you wished a refinement from the finger, to open the points you could devise a push rod, inside a sleeve to bump the fibre block of the movable contact to open and close the points. The device would, of course, be welded to the face plate. Incidentally, this is the type of device that many of the 'old timers' used to change over an igniter-type engine with a low-tension magneto to a spark plug and high tension coil and battery ignition.
In order to test a conventional automotive-type coil with either the device or the distributor, you merely make the connections to the coil as they would be in automotive use. One terminal on the coil connected to the distributor insulated terminal, and the battery terminal of the coil connected to the battery terminal. The high-tension tower connected with a plug wire to the test spark plug.
This method of testing coils is not infallible, but is intended as a convenient check on coil operation. Usually it is reliable. There are instances where a weak coil may pass this test and fail under load. Also, a coil may pass this test when cold and fail when it becomes hot. It is suggested that a known good coil be compared with the one being tested. As a further check, remove the high voltage test plug and hold the spark cable a half-inch away from the distributor ground. A good coil should jump a half-inch gap.
To test a vibrating point type, such as a Model T Ford coil, no tester is needed. All that is needed is a 6-volt battery and the test leads. Connect one battery terminal to the upper lead of the box-type coil. Connect a lead wire from the lower lead of same side of the box coil; this is the high tension lead out. Be prepared to hold the other end of this wire a short distance (1/4-1/2") from a metal portion of the top of the vibrating coil box. Next, connect a lead wire from the bottom of the coil box terminal to the battery terminal. There should be a healthy spark from the plug lead wire to the metal on top of the coil box. The spark may be improved by adjusting a nut on the end of the vibrating coil arm. The contact points may be cleaned by using fine sandpaper or, very effectively, an emery board type of fingernail file.
It is well to point out at this time that there are precautions to be observed when making terminal connections with a 12 volt or 6 volt battery. I have made no reference to polarity of the battery when making coil test connections. The reason is that the coil does not care which direction the primary current flows. However, alternators, generators, battery chargers, jumper cables to start a car, and most electrical appliances are very particular whether a positive or negative terminal is used to make connections. Failure to observe the proper polairy in this case may result in personal injury or damage to an electrical unit.
Contact Bud Motry at 20201 Arthur Road, Big Rapids, MI 49307.