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Author Topic: How can you tell if a coil is bad  (Read 1681 times)

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Offline Deacon Thunder

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How can you tell if a coil is bad
« on: October 27, 2011, 04:13:03 pm »
The subject line says it all. I had the stock coils off my 1977 750F checked by the local Honda shop. I was told that the coils were weak...whatever that means. I suppose that compared to some of the newer high output coils they might be weak...but not bad. What say you guys that really know. The techs here are half the age of most of my socks so they may not be in tune with older technology.
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Offline racemx51

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Re: How can you tell if a coil is bad
« Reply #1 on: October 27, 2011, 06:55:10 pm »
Well Im just a youngin, 26 years old but has been in the bike business and racing bikes my whole life so...Anyways, I would check first to see if you have spark at the points... If this bike has points? And then check to see if your getting power going TO the coil. If you are then check if the plugs are sparking, if they are not Id say its either the coil or one of those connections at the coil. You could also try plugs. Ive had no spark before and chased it and chased it and then put some new plugs in and bam.. Spark.
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Offline CoachDoc

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Re: How can you tell if a coil is bad
« Reply #2 on: October 27, 2011, 06:59:49 pm »
I believe there is a Hondaman post in FAQ that tells how to use a multimeter to evaluate coil status.
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Offline Retro Rocket

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Re: How can you tell if a coil is bad
« Reply #3 on: October 28, 2011, 12:51:02 am »
It will have a really sneaky look on its face and your watch and wallet will probably go missing..... :P ;D
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Offline TwoTired

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Re: How can you tell if a coil is bad
« Reply #4 on: October 28, 2011, 02:10:32 am »
The Ohms reading for the primary S/B about 4.5 -5Ω (Stock coils).
The Ohms reading for the secondary S/B in the 15KΩ range.
These measurements are with the Plug caps unscrewed and removed from the coil output leads.  The plug caps themselves S/B about 5KΩ individually.

The output will indeed be weak if the voltage to the coils is weak.  So, battery voltage S/B measured and the voltage on the B/W wires to the coils should be the same.  A difference indicates voltage losses in the switches and connectors between battery and coil, and should be proven to be in low resistance condition.  (If you feed new coils weak voltage, their output will be weak, too.)

Crusty points and/or poor dwell angle can cause weak output.
If resistor caps and resistor plugs are combined, it can reduce spark current and visual intensity.
The condenser if old and damaged can be leaky or weak which would also lead to "weak spark".

However, simply stating "weak spark" is, in itself, a pretty weak statement.

I'll bet the the profit margin the shop makes on replacement coils is worth convincing customers to buy them.  And, replacing coils is much faster than checking in the rest of the ignition and power circuit.
 
I would ask them to explain why they believe the coils are "weak".  (Well, I wouldn't actually, I'd test them myself and then decide based on reliable data.)

Cheers,
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Lloyd... (SOHC4 #11 Original Mail List)
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Offline dave500

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Re: How can you tell if a coil is bad
« Reply #5 on: October 28, 2011, 02:36:02 am »
the honda shop would love to sell you some new coils,,very odd for a pair to go out,is your bike running?or a project?do you have symptoms?
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Offline Deacon Thunder

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Re: How can you tell if a coil is bad
« Reply #6 on: October 28, 2011, 04:23:13 pm »
So here the deal...I took my original coils and a comparative set of coils from a shop that had them laying around. I went back to the Honda dealer and had them retest mine and test the other set to compare. Before the test I spoke with the tech so he knew what information I was after on the coils. He did the tests in about 10 minutes and the results where a bit of a surprise. My original coils are no longer weak but maxed out on their chart. The comparative set of coils where not as powerful but still usable. Looks like I'll stay with the original coils as long as the plug wires are long enough for my modified cafe setup.
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Offline racemx51

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Re: How can you tell if a coil is bad
« Reply #7 on: October 29, 2011, 01:12:11 pm »
It makes sense if you dont think about it  ;D

These things have a mind of there own sometimes. Maybe you had a crappy connection or something?
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