Ignition Coil Resistance: Am I measuring wrong or are coils bad?

Hi all, trying to finally get my '84 baby 'ceptor running. I had it firing over last week but only on a funnel's worth of gas. It took forever and alot of cranking before it finally fired. I tried to start it again last night, and I'm getting VERY weak to no spark out of both coils. According to the Clymer manual, the primaries should measure about 2.8Ohms, and the secondaries about 21-28kOhms. When I measured with the Fluke, the primaries were perfect, but the secondaries only measured about 14kOhms for both coils. The coil for cylinders 2 and 4 spiked very quickly to 28kOhms but then stayed back at 14k and remained there. Did I fry the coils or is this what they're actually supposed to measure at??

 

My FSM calls for a secondary resistance of 14Kohms so your coils appear to be right on spec.

 

Resistance is a horrible way to test a coil. Especially if its originally specified in inductance (uses heneries for units) or reactance ( uses ohms for units.

Basically what you have proven is that you have continuity and that it has a minimum amount of resistance as specified. In this case more on the high voltage side is better as long as its not in the megohm readings.
I cant remember if the 84 used points, but there should be a ignition controller (like a cdi box) that must have good voltage to work and your coils must have a good ground.
Remember, we charge the coil and it fires when we take the power away from it. Not the other way.

Hook it all back up. And try it again. You may just have ba connections. Check all connection crimps and possibly add sol solder to the ones that need some re-enforcement.
Make sure the batter frame is clean on both ends and that the frame to engine is clean.

If you want to test those coils use a 9v battery and a 1000 ohm 5w resistor in series. Setup a spark plug to be able to see if it fires. Make sure the coil ground goes to the plug too. Never do this without that resistor or youll cook that coil fast!

 

Zen is correct, when the field collapses in the coil, then the high voltage is produced.

Coils operate on induced voltage, can't really measure with OHM meter. Check triggering wiring, points, CDI whatever bike has.

Check high voltage (spark leads to plugs) hairline cracks in old wires can leak HV to ground very easily.

Can't give you a more definitive answer, it's cocktail hour in CA!

 

a nice tool to have is a Thexton spark tester, I had to modify mine with a die, so it would fit into my spark plug boots, once this is done, you can twist the adjustable gizmo and see if the spark will jump inside the inspection tube. Its not an exact science but its something you can add to your arsenal of diagnostic stuff. No substitute for proper trouble shooting with a DMM...Good luck and keep us posted.

 

Thanks Ridevfr,
That is a cool tool that I just ordered to help me figure out my spark issue on my VF500 F that only has spark on the front 2 cylinders.
I see that the Thexton is built only to test plugs with the fat removable top installed. How specifically did you modify it to work with Honda plug wire caps?
Thanks!

Jay

 

Hey there Dr. Jay - I am not around the garage right now, I took the new tool to a bench grinder and slowy ground down the "fat-part." into a consistant round part that would fit into the die I picked oot. I had an appropriate sized die ready (eye balled and tested die on a spark plug threaded part.) then I just went at the part I ground with the die until I had threads on the end of the tool.

I have been known to grind on some craftsman tools in the past, but never to my finer stuff :wacko: Thats just me though, for others, thats the first thing they do is get the trusty engraver oot (sounds like your in a tattoo studio.) :lol: so they are etched with initials, names, etc etc...Funniest one I ever saw was "ANIMAL" and he also did a 666 ound: only to pawn them off!!! You can't make this stuff up...Good luck...

 

Look online and get a pair of smart tweezers. They are cheap and can test coils. Meant for surface mount testing of pcb's but I use them everywhere. Great for coil testing.

 

Thanks Zen Biker,
I've found a pair of smart tweezers on the web for $39.00 but haven't ordered them yet because I don't have a clue as to how to use them to test coils or troubleshoot electronics since they appeared to be basically a very compact multi meter type of device. Could you give a lead to a link or explain how they could be used to test coils?
Thanks again!

Jay

 

Set them to auto and put one probe to each end of the coil. They take a couple seconds to run through all the tests but they will settle on a measurement and that will tell you if that coil is good or bad. Just make sure you post your findings and include the little symbols as each one means something different. If you see a L that is inductance and its measured in Heneries. Everything else is common except how this tool actually tests. It is in no way a simple dmm or vom so dont use it on live circuits. It does not like 12v systems if it doesnt come with the book go to smarttweezers.com and you can download the pdf.

 

Where did you find a pair for $39? Prices I saw ran $379-$399 !!

I see cheap Chinese copies on fleabay

 

I got mine for about 100$

 

Which model do you have ST-5 or ST-3? Looks like a great tool, if I could only justify the need.

 

I am not sure if they have all the functions or are any good but they are at gokimko.com and are these: Aven 18910 Quick Test Auto-Scanning Tweezers.

Jay

 

I have older set of st-2 but was looking at upgrading. When I get back to my pc ill look those ones up. I suggested the smart tweezer brand because I know they have the correct functions.

 

I have found the coils on these bikes to be pretty damn resilient. I find it odd that both coils would exhibit the same symptoms...

 

Considering their age I am not surprised. Get the proper tools to test them before wasting money on changing parts at random.

I did say to ensure that the ignition module had good power and that the connections may be suspect. When in doubt change it all and that includes the wiring. But that is expensive and unnecessary. I will wait and see, that is the best course and since this isnt his daily rider its not a huge hurry to start changing parts. This is akin to teaching to fish rather than just giving the fish. if this was his daily rider I would have him pm me and walk him through everything over the phone step by step.

 

Update: I was checking the coils secondary circuit incorrectly! The FSM has a picture showing the VOM leads touching one primary lead and one of the two secondary leads with the caption saying " the resistance of the secondary circuit should be 14.4 k ohms." Well the correct way to measure the secondary circuit is to check the resistance between the high output leads the plug wires are inserted into. When I did this, both coils checked fine, so I bought both CDI ignition modules on E Bay for $61.00, installed the A unit which was the one leading to the non-working coil and, viola, a nice hot spark to all 4 cylinders.
Thanks to Zen Biker who in another post told me that a bad CDI box could cause just one coil to not fire. It is still a mystery to me as to why an ignition module would go bad on a bike that was running fine when stored but I'll not worry about it too much. Now, on to removing and cleaning the carbs.