It took me the longest time to understand that these old style automotive coils are simply autotransformers, with the negative side of the primary directly connected to both the input side of the secondary and to ground. Primary current charges up the magnetic fields, but you don't get a voltage spike and spark to the plug until primary current shuts off. You can test these with a multimeter as long as you know that a pass with a multimeter doesn't verify the coil is good. Testing with a scope takes only a minute longer, is far more trustworthy, and often gives clues as to where the defect lies even if coil is good.
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So if it’s 20 or more it’s a bad ignition coil?
Should the readings be the same on a 6V coil?
What would the readings be for a 40,000 volt coil ?
It took me the longest time to understand that these old style automotive coils are simply autotransformers, with the negative side of the primary directly connected to both the input side of the secondary and to ground. Primary current charges up the magnetic fields, but you don't get a voltage spike and spark to the plug until primary current shuts off. You can test these with a multimeter as long as you know that a pass with a multimeter doesn't verify the coil is good. Testing with a scope takes only a minute longer, is far more trustworthy, and often gives clues as to where the defect lies even if coil is good.
Great vid thank you
Thank you very much, This is most helpful