Ohm testing COP's

[Repzard]

Boy the search is awful,but that goes for alot of forums. A forum member (Ryan####) said I should Ohms test my Cop's. Ive been having a little issue with misfiring. Question I have is can this ohms testing be done to track down a issue with a cop? If so how do you test them with the Ohms meter, do I take them out ? And where do I connect the Ohms meter on the Cop? Also what setting do I set the Ohms meter on (example: 12volt car) Thank You for all replies,Im kinda lost on this test how its done. Typical Chevyman when it comes to Fords ..LOL

This ad is not displayed to registered or logged-in members. Register your free account today and become a member on Mustang Forums!

[cliffyk]

The secondary should be between 4k and 10k Ohms, the primary between 0.3 and 0.8 Ohms...

The primary resistance is very low compared to what many inexpensive multimeters are capable of measuring. To do it right, you need to short the leads and see what the meter says, then measure the COP primary and subtract the shunted reading from the COP primary reading...

[devongarver]

Originally Posted by cliffyk

The secondary should be between 4k and 10k Ohms, the primary between 0.3 and 0.8 Ohms...

The primary resistance is very low compared to what many inexpensive multimeters are capable of measuring. To do it right, you need to short the leads and see what the meter says, then measure the COP primary and subtract the shunted reading from the COP primary reading...

errr....got a pic of primary and secondary locations? Which wire is which? Sorry im having a brain fart here.

[cliffyk]

On any transformer, which is all a coil is, the primary winding is the one that power flows in to. On our COPs the two terminals where the harness connects is the primary winding--resistance between those should be 0.3 to 0.8 Ohms.

Between the coil output and either of the primary pins you should get 4k to 10k Ohms. If you get a proper value between the output and one primary pin, and 0 Ohms between the output and the other primary pin, then the primary is open.

[devongarver]

Originally Posted by cliffyk

On any transformer, which is all a coil is, the primary winding is the one that power flows in to. On our COPs the two terminals where the harness connects is the primary winding--resistance between those should be 0.3 to 0.8 Ohms.

Between the coil output and either of the primary pins you should get 4k to 10k Ohms. If you get a proper value between the output and one primary pin, and 0 Ohms between the output and the other primary pin, then the primary is open.

omg...im a dummy. lol. Yes, that makes very much sense now, i wasnt thinking clearly. the secondary is what was screwing me up, i was like"wtf, theres only one wire..." So is this a fairly sure fire method for making sure they are still operating properly though? If the housing is cracked but the coil is still intact, the coil should read good but when moisture is in the engine it could cause issues because of the cracked housing correct? Is there a safe way to make sure that you are getting spark at each plug? I used to hold the plug against the block on my old tractor to make sure it was firing but thats not exactly the same situation as this lol.

Side note, the transformer(coil) steps up the voltage, but do you happen to know by how much? Also, there obviously cannot be much current flow since the wires are so small, so why are normal plug wires so thick? Just to insulate against the high-voltage running through the engine bay?

OP sorry for jacking your thread!!

[Repzard]

Originally Posted by devongarver

omg...im a dummy. lol. Yes, that makes very much sense now, i wasnt thinking clearly. the secondary is what was screwing me up, i was like"wtf, theres only one wire..." So is this a fairly sure fire method for making sure they are still operating properly though? If the housing is cracked but the coil is still intact, the coil should read good but when moisture is in the engine it could cause issues because of the cracked housing correct? Is there a safe way to make sure that you are getting spark at each plug? I used to hold the plug against the block on my old tractor to make sure it was firing but thats not exactly the same situation as this lol.

Side note, the transformer(coil) steps up the voltage, but do you happen to know by how much? Also, there obviously cannot be much current flow since the wires are so small, so why are normal plug wires so thick? Just to insulate against the high-voltage running through the engine bay?

OP sorry for jacking your thread!!

No problem devongarver It a good question you asked and still on the subject in my eyes

[cliffyk]

Originally Posted by devongarver

omg...im a dummy. lol. Yes, that makes very much sense now, i wasnt thinking clearly. the secondary is what was screwing me up, i was like"wtf, theres only one wire..." So is this a fairly sure fire method for making sure they are still operating properly though? If the housing is cracked but the coil is still intact, the coil should read good but when moisture is in the engine it could cause issues because of the cracked housing correct? Is there a safe way to make sure that you are getting spark at each plug? I used to hold the plug against the block on my old tractor to make sure it was firing but thats not exactly the same situation as this lol.

Side note, the transformer(coil) steps up the voltage, but do you happen to know by how much? Also, there obviously cannot be much current flow since the wires are so small, so why are normal plug wires so thick? Just to insulate against the high-voltage running through the engine bay?

OP sorry for jacking your thread!!

You raise a good point because a plain ol' low voltage Ohmmeter cannot really test the secondary winding--the applied voltage is way too low to bring out errant paths that might be a walk-in-the-park for 20kV. To do it right you'd need a "megger", a high voltage Ohmmeter.

However, using a low voltage Ohmmeter will let you find decidedly bad COPS/coils.

The step-up ratio is in the order of 1000:1 to 1500:1--12.0V to 12kV to 18kV for a normal spark. The primary current is some 25A+ peak, however the duty cycle is low so you don't need wires that could handle 25A continuously.

On the secondary side the current is 1000 to 1500 times less than that, 18mA to 25mA; however because the voltage is so high a nice fat insulator is needed to make sure the plug sparks and not something else between the coil and the plug.

The conductor in a plug wire is only as big as it is because of physical strength considerations and that high frequency signals (which a plug firing event is) travel in the skin of the conductor*--the bigger the conductor is, or the more strands, the more skin there is...

-----------------------------------
* - There is a classic and simple formula for calculating the "skin-effect" depth, but it escapes my 62 year-old brain at this moment and I'm too lazy to look it up.

[devongarver]

Originally Posted by cliffyk

You raise a good point because a plain ol' low voltage Ohmmeter cannot really test the secondary winding--the applied voltage is way too low to bring out errant paths that might be a walk-in-the-park for 20kV. To do it right you'd need a "megger", a high voltage Ohmmeter.

However, using a low voltage Ohmmeter will let you find decidedly bad COPS/coils.

The step-up ratio is in the order of 1000:1 to 1500:1--12.0V to 12kV to 18kV for a normal spark. The primary current is some 25A+ peak, however the duty cycle is low so you don't need wires that could handle 25A continuously.

On the secondary side the current is 1000 to 1500 times less than that, 18mA to 25mA; however because the voltage is so high a nice fat insulator is needed to make sure the plug sparks and not something else between the coil and the plug.

The conductor in a plug wire is only as big as it is because of physical strength considerations and that high frequency signals (which a plug firing event is) travel in the skin of the conductor*--the bigger the conductor is, or the more strands, the more skin there is...

-----------------------------------
* - There is a classic and simple formula for calculating the "skin-effect" depth, but it escapes my 62 year-old brain at this moment and I'm too lazy to look it up.

Cliffyk, if you dont have a degree in electrical or electronics engineering, i am feebly humbled, as i knew everything you just said, but had forgotten some of it from school. I used meggers at my old job before i was laid off. We used them to test the break over voltage of different components, even the large bank capacitors we used. As for the formual for skin effect, i dont recall it either, though i know i have it in my notes from work as we used it for calculating conductor size.

Infact, the skin effect properties were so pronounced with our equiptment, we used hollow conductors. Basically like plumbing copper, only it was electrical grade oxygen free etc etc. We flowed non conductive glycol/water solution through it to cool it-allowing literally thousands of amps of current at high voltage levels to be used. Truly amazing. Sorry that was totally off topic. lol.

I seriously think you offered up some great information here, we should make a sticky about COPs as we have many questions/threads about them quite often!

[jagass]

I'm pretty noob about this issue...lol

[cliffyk]

BSME (MIT '71)