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Testing An Ignition Coil

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Old 03-19-2011, 04:30 AM
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Testing An Ignition Coil

Hey everyone I'm working on my 79 and lost all power driving home today. I replaced the dis cap and router thinking that was it but still wouldn't spark. So I'm thinking my ignition coil needs to be replaced but I was wondering if there a way to test it before I buy a new one. I have a volt meter but what do I need to have it set on or is thier another way?
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Old 03-19-2011, 07:35 AM
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You can take resistance readings but the only way to tell if a coil or module is good is to replace it and see that the problem goes away. Coils and modules, by design, may work when cold and change when they heat up.
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Old 03-19-2011, 08:26 AM
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could be the ignition module too. Not expensive either one and if they both are old I would just replace them both.
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Old 03-19-2011, 10:12 AM
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Yeah I think I will go ahead and replace them probably be best. Thanks Guys.
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Old 03-19-2011, 10:20 AM
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You can do a quick check to look for open or shorted windings by using your multimeter to test resistance. You can check the primary winding by measuring between the BATT terminal and the TACH TEST terminal; you should see about 1.0 ohm. In reality you will see a few decimal points higher than that (closer to 1.5 ohm) because the lead resistance of many commercial multimeters is significant in this case. You can check the secondary resistance by measuring between the BATT terminal and center terminal; you should see 8 to 11 Kohms. If your meter does not auto-range, you will need to adjust it accordingly.

However, coils actually rarely fail. What fails is the mechanism behind what makes the coil fire. With the coil connected, connect a test light from the TACH TEST terminal of the coil to clean, unpainted metal on the engine. Crank the engine over with the key, and the light should BLINK. Report your results.

If you don't know what has failed, you don't know what to replace. Simple as that. A few diagnostic steps will reveal that information.
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Old 03-19-2011, 01:14 PM
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Quote:
Originally Posted by raytasch View Post
Coils and modules, by design, may work when cold and change when they heat up.
I agree with this and had a situation last Summer where cold start and driving was perfectly fine and even if the outside temps were below 60 degrees I could drive the truck all day, no problem.

Over 60 degrees (or whatever it was) and after an hour or so of driving, whether non-stop or errands, the engine would start acting up.

Turned out the weird "oil leak" I couldn't trace was actually the coil leaking it's internal oil when it would get hot. Which also would cause an erratic mis-fire.

Stupid coil. Stumped me for the longest time.

Josh
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Old 03-19-2011, 07:11 PM
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Alright guys I got it fixed I tested the ignition coil was good so I took off the ignition module and took it to autozone and sure enough it failed so I replaced that and it is up and running again. Thanks for all your input and help guys I really appriciate it.
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