On a tour at the weekend we have to change the 2. coil - it dont works anymore. Now i have tested it with the strobospark tester - no spark. The measurement is correct (capacitor ok / resistor (Ohm) between the points ok). Then another spark test and it works - happy. After a few minutes ... next test ... no sparks. :-( Any ideas what is wrong with this coil? It is a new (9 months) KW from macs.

Kind regards

Michael

When it does not work is it still drawing correct operate current in the Strobo-Spark or does the coil current go to zero at the same time that the coil quits sparking? If the current is normal when the spark in the window of the Strobo-Spark is gone then the coil most likely is arcing internally which is unfortunately fatal if the coil is potted. If the coil is NOT potted then the problem might be visible but once a coil starts to arc someplace internally it will then usually always want to do that. The coil makers like to take short cuts and convince folks that filling the box totally full of hot tar is not necessary but it is what drives out all moisture and seals every nook and cranny and makes the coil a solid reliable device. If there is air in the box, you can get arcing pretty easy and even a small arc turns into something that gets worse and worse typically. I would toss the coil and get a good one since you will pull your hair out trying to find the arc and then trying to make it stop. A good coil will produce a spark that can easily jump exactly 1/4" and a bad coil will arc internally before that.

Michael
There are four manufacturers of new Model T coils in the USA. Three of them are taking short cuts, have improper factory testing and producing a product that will not last. Believe me, I have seen hundreds of them in my shop.
You can email me directly if you would like information to find properly made new Model T coils.
Ron the Coilman