Coil Tester

Coil Tester
By Tom Wesenberg
Saturday, 24 October 2009

Note: The following Coil Tester description and design originated with Tom Wesenberg, it is reprinted here with permission of the author as a service to fellow Model A hobbyists. On behalf of MARC I would like to thank Tom for his generosity.
Frank Pollack
2009 President, MARC

Here is a coil and condenser tester I made for about $9. The 6 volt relay is 35 ohms and the capacitor connected to the relay windings is 330 microfarads. This combination makes the relay vibrate at a rate to give the coil the same frequency as a Model A running at 1,000 RPM. The higher the capacitance, the slower the relay will vibrate. If you use a 6 volt relay with a different ohm reading, you’ll have to experiment with different capacitance to find the frequency you like for testing the coil.

I use an old award plaque found in the trash for the Black Walnut boards used in the tester.

Notice that when a pencil is placed in the spark path a yellow flash is given off on the spark plug side of the pencil lead when the coil is wired with the correct polarity.

I am showing a slant pole coil as used on 1928 and 1929 Model A’s. In November 1929 the Service Bulletins shows the wiring changed so the driver’s side of the coil now is the negative power side terminal and the passenger side terminal will lead to the points, and + ground. To test the later coils I would simply reverse the 2 coil terminal clips.

This tester is small and easy to carry to swap meets if you want to test coils before buying them.

Parts List:
1. Wood — free
2. Screws — $.88
3. .015″ x 1/4″ x 12″ brass strap — $.84
4. Relay — $2.99
5. Capacitor (330 microfarads)— $.015
6. Push button — $1.29
7. 6 alligator clips — $1.50
8. Wire — $1.00

Editor’s Note: For portability to test coils at a swap meet and etc. a 6 volt lamp battery should work fine as the power source.  10/10/2018  Power Relay  No. K10P-11D15-6,  Potter & Brumfield,  Coil Resistance (Ω): 40, $12

Color Coded Wiring Diagram