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How to test your coil

Old 06-27-2011
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Default How to test your coil

This is a easy test, you only need a DVOM and a small amount of time. If your engine is stalling, stumbling, cutting out, starting hard, misfiring, or has a lack of power. It's possible the coil is going bad or has failed.

First locate your coil.



Then set your DVOM to 20k ohms. Touch one probe to the coil post and the other to the tach terminal. This is your secondary. This is a good coil. If you have a heat gun, try heating up the coil. The resistance will go up a bit, perhaps about .5 ohms or so. If it goes real high, then the coil is bad when hot.


This is a bad coil.


Now set your DVOM to a lower scale and test your primaries. This is a good coil


A bad coil will show open.

Now test battery voltage to the coil. Set your DVOM to DC volts and turn your key to KOEO. Connect one lead to ground, and the other to the positive coil terminal. You should have at least 90% of battery voltage. If less then 90% locate the problem and repair.


Now check the resistance between the tach connector plug and ground. The resistance should be greater then 10,000 ohms. If less then that repair the short.

Last edited by danthurs; 07-08-2011 at 05:45 PM.
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Old 06-28-2011
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I added a few more tests to this write up. If I missed anything, just let me know.
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Old 06-29-2011
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There is voltage across the coil terminals in your last picture causing the meter to read negative resistance.
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Old 06-29-2011
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I just had the leads connected backwards.
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Old 06-29-2011
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Originally Posted by danthurs View Post
I just had the leads connected backwards.
A resistor doesn't have a polarity, so your meter is measuring a voltage, instead of resistance. Put your meter on a voltage scale to see what the voltage is.
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Old 06-29-2011
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I will do that.
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Old 06-29-2011
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You are correct. With key off, I have no voltage on either primary wire, with KOEO I have battery voltage on both. I need to double check that one test.

Ack, I misread that test. With plug unplugged, test resistance between the plug connector and ground. I failed to unplug it. Thanks for the catch.

Last edited by danthurs; 06-29-2011 at 02:07 PM.
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Old 07-01-2011
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Originally Posted by danthurs View Post
You are correct. With key off, I have no voltage on either primary wire, with KOEO I have battery voltage on both. I need to double check that one test.

Ack, I misread that test. With plug unplugged, test resistance between the plug connector and ground. I failed to unplug it. Thanks for the catch.
Thank you Dan! Have a great July 4th!
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