Technical Articles

Rebuilding Model ‘T’ Coils

This information was e-mailed by a reader, without signing his/her name, and the e-mail address gave no clue. Thanks any way. It looks good, but the HCCA has not tested or verified the information, and cannot be responsible for those using these directions

Model T coils which still work well are a rare find. However, a few simple tests with an ohm meter can usually tell you if the coil is good, bad or indifferent.

First, to test the secondary winding: Set volt/ohm scale to “x 100” and zero out the meter. Then touch the leads to the two contact terminals on the front of the coil. The reading should be about 3,200 ohms, plus or minus10%. A zero reading indicates an open circuit.

Next, to test the primary winding set volt/ohm scale to “x 1” and again zero out the meter. Either put a thin piece of plastic between the two contact points to insulate them, or remove them altogether. If you are going to reuse your points the contacts probably need to be cleaned anyway.

Touch one lead to the contact terminal on the bottom of the coil and the other lead to one of the screws that hold the top point on (make sure the rust and corrosion have been removed first. Also, if the points are removed, one of the screw posts will be dead) Your meter reading should be about 1/2 ohm. A no reading will indicate an “open” circuit

Then, test the wire from the points to the terminal by setting the volt/ohm scale to “x1” and zero out the meter. Touch one lead to the top terminal on the front of the coil and the other lead to one of the screws that hold the bottom point on (one screw is supposed to be dead if the points are removed) No reading indicates an “open” circuit.

To test the capacitor, the number one cause for non-working coils, set your volt/ohm scale to “x 10,000”, zero out the meter and touch one lead to the top terminal on the front of the coil and the other lead to the terminal on the bottom of the coil. No reading indicates the circuit is “open”, but if the reading does not go to infinity, the capacitor is bad. Either way coil surgery is required!

If you are going to replace the capacitor, the coil only needs to pass the first three tests. For instructions to replace the capacitor, continue reading.



The best replacement capacitor to use is a .47 mfd 400 -630 volt Mylar Capacitor. These can be found at most electronic supply houses.

Carefully remove the sliding side of the coil box, which is held in place with a small nail. After removing the side, DIG out the wax on the left side with a sharp knife. Do not try to “melt” the wax out as you may ruin the wood case. (Chilling the coil in your refrigerator first will make it more brittle and easier to chip out.)

Beneath the wax on the far left is usually a wood insulator, a large paper capacitor, and a glass insulator in the center (middle of coil) Remove the paper capacitor by cutting the top and bottom wires from it, but leave sufficient wire at the top and bottom to solder your new capacitor to. If necessary, also remove the wood spacers and glass insulator. (Some times removal of the capacitor is sufficient to fit the new capacitor in.)

It is not necessary to refill the spaces left by the old wax you removed.

Reinstall the wood side panel and refasten with the nail. The coil gap should be .30, and if you have new coil points, so much the better. The adjusting screw allows further fine tuning of the points while the coil is buzzing. Be careful you don’t get zapped!