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Old 08-13-2019, 12:28 AM
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Give me coil tech 101

Ive been reading up on coils, the testing etc to test set an ohm meter leads on + and - and that reading tells if the coil is good, so to not jade this i wont qoute what i read online this should be, i get a bit confused that its different on 4/6 cylinders or and 8, and different if internal or external ballast and why must a coil have a ballast?

then to test 2nds by putting one test lead on + or - and the other where the distributor lead goes and again i have seen a large range called okay, but what is correct?

so take me to coil class give me the 101 run down please,


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Old 08-13-2019, 02:41 AM
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Ohm testing a coil is inconclusive. An ohm's test will tell you if it's definitely bad but not that it's 100% good. They can ohm out fine and still have a vibration induced problem or just not recover fast enough.

Unless designed for 12v, a standard old school coil will burn up at 12v so a ballast is used to limit the voltage to 6-10v. It depends on the coil/ignition system being used.
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Old 08-13-2019, 03:33 AM
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yep hipster, i have had coils which ohm testing showed good and it worked soso until getting too hot to touch, this was hooked up with ballast if called for.

actually ive seen several coils bench test good that didnt work on a vehicle.
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Old 08-13-2019, 11:22 AM
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Coils are rated on impedance which is read in ohms like resistance, but they are not the same thing. You could think of impedance as how resistance changes when the coil is making and breaking a magnetic field. This changes simple resistance into something that varies with the condition of the magnetic field at any given moment the published figure being an average square root of sum of the squares of the measured points. This being called Root Mean Square or RMS.

The important thing with electronicly switched distributors is that the module and coil be impedance matched ( similar to a streo amp and its speaker load) as when these get mismatched either the module effeciency suffers if driving into too high an impedance or it gets fried driving into too little impedance of the coil.

So rather than grabbing any coil with advertised power suitable for death row executions you need to research the published impedance range of the module you have then purchase a coil that has an impedance in that same range.

If you've been running a mismatched impedance for a while and are having something from inconsistant issues to outright failure the module is likely to be somewhere in a failure mode. The same would be true if you have a coil failure, especially an intermittent failure, this, also, has likely damaged the module.

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Old 08-13-2019, 06:56 PM
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Thanks bogie, great tech,

while they bench test good i have the mindset that all of my coil stash are bad, they are old, so am i lol.

in the case of the electronic module they wanted 3.0 ohm, primary, which all my coils were then 7k secondary which mine were with the exception of one being just under 7k.

so thinking the cheap module was bad i tried points and same thing, more to make me think coil is bad, so i ordered the beru blue coil the vw folks all recommend, to have a known good coil, that is if this is a fire issue,

in the 80s i had a vette with hei do something very similar and it was the module.
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