Measuring COP impedance
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Measuring COP impedance
Namely, how does one go about measuring the impedance/resistance of a COP? I'm in the process of tracking down a misfire--replacing the plugs and COPs as needed and was wondering if one could test the impedance as a guide to determining if a particular COP is defective. Any help would be appreciated. thanks, m
Hotshot
Joined: Jan 2004
Posts: 19,760
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From: St Louis
Resistance readings will show a gross failure, however, most COP failures are not of that type.
Most COP failures are due to breakdown of internal insulation causing internal arcing or shorting, resulting in misfires.
A stress test is the best diagnostic to determine the quality of a suspect COP. The dealer has the necessary equipment.
Hmmm... I wonder if I still have my old yoke and flyback tester from 25 years ago......That would make an interesting experiment....
Steve
Most COP failures are due to breakdown of internal insulation causing internal arcing or shorting, resulting in misfires.
A stress test is the best diagnostic to determine the quality of a suspect COP. The dealer has the necessary equipment.
Hmmm... I wonder if I still have my old yoke and flyback tester from 25 years ago......That would make an interesting experiment....
Steve
Junior User
Joined: Jul 2003
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From: Southeastern MA
I've replaced 2 so far on mine. The failure wasn't the coil, it was the spring. There was no connection/tension where the spring meets the tip of the plug. Seing as I bought a new plug and coil, I noticed the new coil spring would hold the plug somewhat securely, whereas the old coil was barely making a connection on the plug. So, for the $40 for the coil, and a few for the plug and a bit of antiseize & dielectric grease and an hour of my time, it's all set.
I wonder if the heat of the engine causes the springs to expand etc, and after time they don't contract back to their original size. Electrically, with a Simpson, (ohm meter) the coil checked out fine.
But to answer your question, remove the wire connector and put an oh]mmeter across the 2 terminals. If the meter swings, indicationg continuity, the coil is generally good. Jut don't use too high of a resistance scale. I can read the resistance across my body with the meter I have (on the highest scale)
Hope this helps
I wonder if the heat of the engine causes the springs to expand etc, and after time they don't contract back to their original size. Electrically, with a Simpson, (ohm meter) the coil checked out fine.
But to answer your question, remove the wire connector and put an oh]mmeter across the 2 terminals. If the meter swings, indicationg continuity, the coil is generally good. Jut don't use too high of a resistance scale. I can read the resistance across my body with the meter I have (on the highest scale)
Hope this helps
Lead Driver
Joined: Sep 2006
Posts: 7,907
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The measured value can be anywhere from 4000 to 10,000 ohms with about 5 to 7000 preferred.
To use your meter, set the range scale to 1000, short the test leads togather and zero the meter, then do the measurment. If you don't zero the meter, the test is worthless other than telling you the coil winding is or is not open.
The term impedence is not the same in this case.
You are measureing DC resistance.
Impedence is a measure of AC resistance and is frequency dependent among other things. The coil has impedence but is not a useful parameter for our purposes since a regular meter can't measure impedence.
To use your meter, set the range scale to 1000, short the test leads togather and zero the meter, then do the measurment. If you don't zero the meter, the test is worthless other than telling you the coil winding is or is not open.
The term impedence is not the same in this case.
You are measureing DC resistance.
Impedence is a measure of AC resistance and is frequency dependent among other things. The coil has impedence but is not a useful parameter for our purposes since a regular meter can't measure impedence.
Last edited by Bluegrass 7; Nov 9, 2006 at 11:57 AM.
Tuned
Joined: Feb 2006
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I carry a spare coil and the tools when Im out of town - I think I will start carrying them all the time since this is a fairly common problem and while it runs ok on level ground on 7 cylinders, it doesnt do well in the hills where I live. I'll also look into the spare springs and boots - a mechanic at the dealer told me these weren't available -figgers!
