Mercotronic Coil Testing

[chris-p]

I have a coil that has ZERO continuity on the secondary, yet it will spark on the Mercotronic?? How is that possible?

I do have to have quite a bit of juice on the dial when I get spark, little more than others. Maybe there is just a fracture, and the energy is jumping this fracture in the coil to make spark, but the meter wont read?

[legendre]

quote Chris_P:

I have a coil that has ZERO continuity on the secondary, yet it will spark on the Mercotronic?? How is that possible?

It’s possible for the same reason that the spark plug center contact reads infinite resistance to ground – yet it still sparks. You have a broken connection somewhere in the secondary winding, and the spark energy is strong enough to jump that gap in addition to the plug gap.

So, the explanation is pretty simple, in that sense. Have you ever had a resistor plug cap (normally 5K) that reads wide-open, but still sparks at the plug? Same exact situation.

quote :

I do have to have quite a bit of juice on the dial when I get spark, little more than others. Maybe there is just a fracture, and the energy is jumping this fracture in the coil to make spark, but the meter wont read?

Precisely. You answered your own question.

FYI – It could and likely will continue to run like this for quite some time, but eventually, things will break down far enough that the spark energy is no longer sufficient – or simply fails entirely.

[Mumbles]

Hey Chris, I just picked up an older Merc 200 and one coil reads 500-600 K and has spark while the other coil reads 5.6 K and has no spark. Try and figure that one out. 😯

[legendre]

quote Mumbles:

I just picked up an older Merc 200 and one coil reads 500-600 K and has spark while the other coil reads 5.6 K and has no spark.

There’s at least one simple answer, though it might not be entirely correct in this (or another) case.

The winding in question may have broken internally at one point, but continued to run for some time thereafter. The continuous internal arc has created a conductive carbon path between the broken end of the winding and the coil terminal – or between breaks in the winding, in any case.

My guess is that if you actually tried to push more than a little DC current through that 500-600K winding, you’d find that it doesn’t actually behave like a 500-600K fixed resistor.. that is, it can’t sustain any amount of DC current at low voltages as the current path (just a track of carbon) is quite fine and tenuous.

Much like a resistor plug cap that reads open yet continues to spark, it will give dodgy service and will most likely eventually fail entirely.

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