A word of advice


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Posted by Sherman in Idaho [72.47.153.105] on Saturday, November 23, 2013 at 01:40:03 :

In Reply to: Re: coil posted by derek [68.122.10.35] on Saturday, November 23, 2013 at 01:31:44 :

The resistance of the primary of an ignition coil is pretty close to the resistance of the leads and contacts of a typical voltmeter (between .1 and 1 ohm). If the coil is open, the meter will detect it, but if there's a shorted turn or two in either the primary or secondary, you won't find it with an ohm meter. To get any kind of accurate reading of the primary resistance, push the probes good and hard against the two coil terminals until you get the lowest reading you can on the ohm-meter. Then push them both against the same coil terminal and see what reading you get. Subtract that from the first reading, in order to cancel out the lead resistance.

Shorted turns in a coil are insidious because even though they don't change the resistance much, they dissipate a large part of the energy stored in the magnetic field that should have gone into making a spark. Most electromagnetic devices like motors, transformers, and coils, fail in one of two ways. Either they totally open up (easy to detect) or adjacent turns short against each other from the effects of heat and vibration. When this happens, the device will get hotter and hotter and less and less powerful, causing more turns to short, until it eventually burns out. But long before it gets to that point, it won't be working very well.

Coils are cheap and easy to swap out, and it's worth keeping a "known good" one on the shelf for testing purposes since an ohmmeter doesn't really provide a very good test.



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