For a moment let's forget about spark timing, spark consistency, spark energy, point geometry, point condition, point gap, vibrator tension, upper point cushion spring gap, upper point cushion spring tension, capacitor health, or current draw.
For now we only want to know if a Model T Ford ignition coil is rebuildable.
(it doesn't even need points installed to perform this test)
This is not a 100% foolproof test, but it's a good place to start! This simple test will help determine if the coil is rebuildable or not, i.e. "good" or "bad".
See below ...
Find your multimeter and set it to measure resistance (ohms).
Digital or analog, either will work!
If your device has more than two ports, plug the BLACK lead into the Common/"Com" port, and plug the RED lead into the port with the Omega symbol (looks like an upside down horseshoe).
There are many different brands and styles of these meters. They may have slightly different displays (2 digits, 3 digits, 4 digits) and settings, but they will all function the same.
Touch the leads from the multimeter to the primary and secondary contacts on the coil, and hold them there ....
like this ----->
Measuring resistance is not directional, so it doesn't matter which lead touches which contact.
Make sure your contacts are clean so the leads can get a good reading.
Read your device.
A Ford coil should return between about 2800-3700.
An aftermarket KW coil should return between about 2100-2900.
Above; here, I have my digital multimeter set on 200k.
In this example, 3.5 = 3500. That coil is in range!
Left, (on the analog meter) simply read where the needle falls within the ohms scale. The reading is not as accurate as a digital unit, but it doesn't need to be. That coil is in range!
If the multimeter returns nothing (no value), then the secondary is shorted/open and the coil is effectively dead. If the multimeter returns a reading outside of the ranges in Step #3, there is likely a fault in the secondary. In either case, we don't want to rebuild it.
Replace it with a good one.
*Like most other tests in the world, this is not without anomalies. About 1 in every 200 coils I see will pass this continuity test and still not function. (I have a theory about why this occurs, but that's a subject for another time.) So, if you perform this simple test prior to sending me your coils, it will likely save you time and money. Thank you!