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Hi Guys,

I'm starting a new topic on coils, and here's why.........I bought 4 new NOS coils and they arrived today. I just got done measuring the primary and secondary resistance of each and here are the results;

Coil #1: Primary = 11.8 ohms, Secondary = 12,110 ohms
Coil #2: Primary = 11.8 ohms, Secondary = 11,850 ohms
Coil #3: Primary = 11.8 ohms, Secondary = 11,240 ohms
Coil #4: Primary = 11.7 ohms, Secondary = 12,030 ohms

The thing to note here is that the primary resistances is NOT the 6 ohms we think they should be and the secondary resistance is not quite the 15K ohms either. My point here is there are probably a number of different primary and secondary resistances depending on who made the coil and when. If I had installed one of these coils in my truck without measuring and later had an unrelated ignition system problem that caused me to measure the resistance of the coil I'd probably think it was bad, when it's not.

So..........if your coil is working fine, or you have a spare or two laying around that you know are good, measure their primary and secondary resistance and make a note of it to put in your glovebox , notebook or just write it on the coil with a Sharpie. This way you won't be tossing out good coils down the road when you go and look up what the supposedly good resistance numbers are for your coil. I think I've got a few used coils in my mountain of parts and if I run across them I'll measure their resistance and post it here. Likewise, if you have a coil or two laying around measure your resistances and post it here. I'm curious as to how many different sets of resistance numbers there are out there.

thanks,
bob

I do that as well. I add the date of the test. My memory is running at full these days.

I have found over the years that 1: Very few coils in all my many high milage and Old vehicles have failed. 2: The failure mode universally would only present itself when the coils was hot. (Open winding) and would read normal after a few degree cooldown. 3: Everyone's meter will read a different reading. If you are within 10% of the spec, you are usually golden.
A man with one watch always knows what time it is. A man with two watches is never quite sure.

Hi Just,

I agree. But.........the problem is "the spec" in that it only applies to certain 24V coils (meaning those made back in the day). Since it appears that different vendors made them with different primary and secondary resistances, if you don't measure it when new, you'll never know if it's good or bad when you have engine problems and look up "the spec" for an older, different coil.



regards,
bob

Bob, your numbers mirror my reproduction Newstar coil readings.

ZG,

Roger that.........thanks!

bob

Bob,
Here's some more data for you.
Measured 2 OE coils:
Coil A, Pri. 6.6 ohms, Sec. 13.8 ohms
Coil B, Pri. 6.8 ohms, Sec. 13.9 ohms

Andy