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Does anybody know why I'd get a spark arching from both the positive and negative terminals of the coil to the coil wire when I disconnect a spark plug wire from the spark plug? Primarily the arch comes from the negative side but I did see it arch from the positive side a few times. It doesn't matter which spark plug wire I disconnect ... the results are the same. And when all spark plug wires are connected I do not get any arching.
Yes, Matt. What you see is what is expected from an open secondary. As Hio said, the arc will go to the shortest path to ground.
When the points break, the collapsing field lines in the coil secondary send the voltage to something like infinity. The spark plug acts as a clamp. That is a .035 gap. Without that clamp you will get an arc somewhere. As far as the coil is concerned at these voltages, a zero volt - primary hookup on the coil is same-same as the + 12 volt hookup. Hence an arc.
It is never good to disconnect a secondary wire and let it arc. In old systems it can perforate the condenser; in modern stuff it can send a transient into aftermarket radios, CD players, ignition modules... Stuff with transistors.
If you want to disconnect the secondary for something like a compression test, take a short length of wire and ground the center terminal on the coil. If you want to test an individual plug wire, hook it to an old plug outside the engine and ground the loop of wire on the plug to a valve cover or something and look for arc.
Never allow an open secondary.
Clean that tower up and get a new coil wire with new boots.
All of that dirt combined with a bad boot and wire will cause the arcing that you describe.
Originally Posted by HangtownMatt
Does anybody know why I'd get a spark arching from both the positive and negative terminals of the coil to the coil wire when I disconnect a spark plug wire from the spark plug? Primarily the arch comes from the negative side but I did see it arch from the positive side a few times. It doesn't matter which spark plug wire I disconnect ... the results are the same. And when all spark plug wires are connected I do not get any arching.
Clean that tower up and get a new coil wire with new boots.
All of that dirt combined with a bad boot and wire will cause the arcing that you describe.
I was planning on replacing the spark plug and coil wires, but didn't CougarJohn say this was normal?
And for the record I replaced the points/condenser with a Petronix a long long time ago if that makes a difference.
I appreciate everyone who's taken the time to reply.
