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Old 03-22-2013, 09:14 AM
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HEI Coil Testing

There was a post by 75gmck25 on 3/30/2008 as follows:
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,

I know this is an old post but I can't seem to find an answer to the basic question asked. Does anyone know the answer? Or have a better test for the HEI coil?

Thanks Gregg

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Old 03-22-2013, 05:34 PM
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A search should have covered this.

ohm the outerterminals should read near zero, if not replce.

ohm tach to center rotor, should be near zero, if not replace. Center to ground would be the other side.sold benear zro. ofl would be sabreak in t he cicuit.

Last edited by spinn; 03-22-2013 at 05:41 PM.
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Old 03-22-2013, 07:23 PM
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HEI Coil Testing

Remove and invert the cap.
Measure between the TACH and BATT terminals, using a DVOM, set to OHMS scale, RX1 and calibrated to 000Ω.
It should read less than 1Ω, but more than 000Ω. If not it's bad.
Next measure the secondary side of the coil:
Set your DVOM scale to RX10K or higher.
Put the probes between the BATT terminal of the coil and the carbon pickup inside the distributor cap. It should read between 6000Ω and 30,000Ω. Outside of that range toss the coil and get a new one.
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Old 03-22-2013, 08:30 PM
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I was advised to put a new one in mine when I tuned it up and keep the old somewhere in the car as a backup. because "you never know".
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Old 03-22-2013, 09:32 PM
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The range of the dvom is important.

If you were checking the coil you check the circuit for continuity, and make sure the windings have not broke.

A dvom will mearsure the resistance. A value of Out of Limits or OFL will indicate the break in the coil. You are isolating each part of the coil circuit and verifying that it carries power.

A coil is not really a wear item. By that thinking you would need to carry a module, a fuel pump, starter, etc.. Passenger space would become a premium.

-the outer two terminals are tach and battery
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Old 03-23-2013, 05:20 AM
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Never had a coil fail on me that did not fail open. They internally shorted and lost the ability to energize the circuit.

Yes the terminal get rusty or whatever, but I never had one all of sudden quit because the coil ohmed at 60,000 ohms instead of 6,000.
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