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Old 03-30-2008, 02:03 PM
75gmck25 75gmck25 is offline
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Testing GM HEI coil in cap

I'm replacing the distributor cap and wires on my '75 GMC truck, which is a 350 with a stock large diameter GM HEI distributor. I decided to test the old coil after moving it over to the new cap.

1st test- primary resistance, tach to bat terminals is near zero - pass
2nd test - secondary resistance, rotor button to ground - 15k - pass
3rd test - secondary resistance, rotor button to bat terminal - infinite - fail

Every reference I can find says to replace the coil if both of the secondary tests fail. There is no explanation on what it means if only one of the tests fails. Is this coil really bad?

I also tested an old HEI coil I had, and found it also failed the 3rd test, so I'm wondering if I am misinterpreting the readings.

Any input or suggestions?

Thanks,

Bruce
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