Aftermarket coils, don’t match OEM specs like you might expect.

[Lou C]

A have a couple of spares for my old OMC, and including the old OEM matches the specs in my shop manual….
OEM spec for secondary: 7500 ohms
Actual reading on OEM coil: 8500
Prestolite aftermarket (this engine has a Prestolite points distributor): 15,000
CDI: 8500 but this one must be defective, quickly breaks down under use
Do these differences actually matter? With old boats usually there’s no more OEM….

 

[Scott06]

A have a couple of spares for my old OMC, and including the old OEM matches the specs in my shop manual….
OEM spec for secondary: 7500 ohms
Actual reading on OEM coil: 8500
Prestolite aftermarket (this engine has a Prestolite points distributor): 15,000
CDI: 8500 but this one must be defective, quickly breaks down under use
Do these differences actually matter? With old boats usually there’s no more OEM….

What’s the primary resistance? that is a much tighter ohm spec but I thought generally 8-12 k ohms on secondary was a decent rule of thumb. Be interested to run them and test with a spark gap tester To see where the rubber meets the road. I have original coils for my 65 gto , came with the car when I bought it in 1990 that run just fine. The yellow coil I bought back in the early 90 s is the one that gave out…

 

[Lou C]

I got curious about all this because I had a CDI coil fail on me, it read fine on the resistance in fact of all the aftermarket coils I have, it was the closest to the OEM spec. It would start but as soon as it got a little warm the engine would die out. It was probably getting worse since last summer when I thought all my problems were fuel related but now I think it was a gradually failing coil.
What I found was that the primary resistances were all pretty close, it was the secondary resistance that varied a lot.
I put the original coil back in and it runs fine!