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TOPIC: ignition source coil

ignition source coil 11 Mar 2019 10:54 #1

  • oldmanfun
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Friends.. after a long break for a bad knee and double hernia surgery, i'm back to looking at riding my 73 ct3 for this summer season.... i couldn't last year as i had no spark and still don't... i've gone through all the checks for points operation, timing, and the such... last thing i remember was that, off the bike, i had some continuity between the source coil windings and the core.. so i got gutsy and rewound it... a very careful job for certain, but i got 10 layers of 40 winds each using new 24 ga wire; the same size wire and same number of windings... checked it for ohms and it is now at 3.8 which from what i can tell is too high.. questions are- what ohm reading should i be getting and would this amount of resistance cause a no spark condition... appreciate any input on this one... thanks for your time and thoughts.....

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ignition source coil 11 Mar 2019 15:31 #2

  • RT325
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Good question--i've experimented with using the lighting coil as a source coil on a yamy ag100 farm bike & had it running well using the lowest lighting wire output & anything higher caused the points to arc like a welder--or like a faulty condenser. I would have thought you'd get 'some' sign of life--just not sure what sign sorry. I'd think original coil would only have about half of what you've got but not sure how critical it is within reason. Original coil would test to earth as the coil 'is' earthed at the end, but i dare say it shouldn't show continuity to the pole if the coil end--both ends--were disconnected. like un-solder from condenser & unscrew from earth then should show nothing to the pole, Hope that makes sense--having trouble understanding myself sometimes haha. I'll have the factory resistance here somewhere, i'll go look later but i suppose you had them at the start of your rewinding. Would be under two ohms i'd say.

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ignition source coil 11 Mar 2019 18:29 #3

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rt325... thanks for the reply.. it does seem too high to me.. my continuity and ohm readings are off bike ones.. i can take a cordless drill and set it up to turn the crank with my multitester going to the source coil lead (unattached to the rest of the bike) and get a definite open/closed reading from the points, and when i set it up with a dial indicator in the spark plug hole the piston timing goes right along with the electrical... i just am not getting any spark though.. new ignition coil as well as tested with older good one, tests also done without on/off switch (my own-- i got this bike in boxes and tried to restore it a bit.. first ride out i came back with the headlight inside my t shirt and half the bike's 'extras' had fallen off ... it was not an easy ride... so i redid things with no instruments or anything else.. made my own light and ignition on/off switches and made a bracket for a 55 watt halogen light (6 v)... rode it each year for a few and then took my 'break'.... came back to it and it did start up and then it stopped and hasn't started since... no strange noises or signs of a mechanical issue.... i get voltage from the other two coils that should be for lighting and battery charging.. have checked grounds , etc.... only thing i could come back to was that coil and since i couldn't find another one i rewound the old one... don't try it unless you have a lot of patience! will see if i can find any documentation for ohm's on the coil; if you have some around then would appreciate that info when you can find it! me? my mind is like a sieve these days! summer is coming up again and i want to ride !!! thanks again

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ignition source coil 11 Mar 2019 18:54 #4

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I got lost at "i had some continuity between the source coil windings and the core.. "

That sounds normal to me? Don't the windings ground to the core on one end?

Also curious about the statement: " can take a cordless drill and set it up to turn the crank with my multitester going to the source coil lead (unattached to the rest of the bike) and get a definite open/closed reading from the points

What kind of reading do you get on the meter? Are you measuring voltage?

Since I'm the type that tends towards tearing the whole engine apart and rebuilding it when it won't start... instead of noticing I forgot to turn on the ignition switch... :Ugh I really try hard to look at simple things like dirty points or bad condenser first....

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ignition source coil 11 Mar 2019 19:34 #5

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MarkT wrote: I got lost at "i had some continuity between the source coil windings and the core.. "

That sounds normal to me? Don't the windings ground to the core on one end?

Also curious about the statement: " can take a cordless drill and set it up to turn the crank with my multitester going to the source coil lead (unattached to the rest of the bike) and get a definite open/closed reading from the points

What kind of reading do you get on the meter? Are you measuring voltage?

Since I'm the type that tends towards tearing the whole engine apart and rebuilding it when it won't start... instead of noticing I forgot to turn on the ignition switch... :Ugh I really try hard to look at simple things like dirty points or bad condenser first....


I wondered if OP had the coil removed or disconnected from earth plus unhooked from condenser & it still showed continuity to the pole, then decided to rewind it. I'll look up resistance---Hang about--2.1ohms resistance on a CT3. that from original yamaha workshop stuff. MarkT--if OP spun it up say with it unhooked from condenser, what voltage would you expect? from this source coil, being a 6volt AC ignition system. Changing tact for a minute & going up top to plug coil i remember on the yamaha coil tester that if it drew less than half an amp then we were to test it on 12 volts. The tester had an adjustable gap & with 6 volt AC coils they'd barely jump a gap with a piddly weak thin spark so i would think it was faulty but then on 12 volts it tested great--& was great--Hardly ever truly had a faulty one & was usualy just dirty points or something. Can only truly remember one genuinely faulty source coil in a mag & that was on a Honda 50 in about 1970 haha.

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ignition source coil 11 Mar 2019 20:24 #6

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Mark T... thanks for your reply as well... i know that at my age i sometimes do not make as much sense to others than i do to myself ! so thanks for the patience... yes, the source coil is grounded to the plate at one end of the winding while the other joins up with the points, the loom wiring, and the condenser at the condenser location.. i found continuity with my tester with the coil off the bike and ungrounded between the lead out and the 'core' and know that shouldn't have been happening.. when i attached my tester to a single lead from only the points and spun the engine using the cordless drill i was again measuring continuity, this time between the lead and the grounded side of the points; this told me the points themselves were opening and closing... my description of using the 'source coil lead' was well, misleading ! sorry... i had already replaced both the points and condenser with new as my first tries at diagnosis; with the testing of the points i feel they are good; with the condenser i do have a function for that on my tester but i had thought that a spark could be obtained even if it had 'gone bad', that it could cause a poor spark but still a spark.. ?? ... ..... and i know what you mean about just tearing it down anyway.. i used to rebuild my vw engine every 30k no matter, just because i felt like doing it one more time! and that was when it was running just fine! i will take the condenser out of the system and do a test on it next... as soon as these rains let up !! thanks for the input!

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ignition source coil 11 Mar 2019 20:39 #7

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RT325... i love forums.. so much good input and friendly conversation! wonderful way to spread the knowledge around... i contribute to others as well and helped run one for several years on construction help for home owners..... okay, back up Reed.... i think i've given a better more precise explanation of what i've done so far in my reply to MarkT now, so some questions have been answered now.... of mention, i had already also replaced the old ignition coil with a new one and have tested with each of them in place with all other parts wired up as well... i kind of figured that with not having ridden this little old beast for about three years i might as well start out with all the standard ignition parts replaced; also did a complete cleaning of the tank and carb as well.. let's see.. also went through the brakes and replaced the old 'shocks' (that after i had taken the coil loose i found that 'shock' had nothing to do with it's ability to keep from happening) with new longer gas/coil units.... and a new battery.... i will be taking the source coil back out and taking out some winds until i find a spot around 2.1 ohms then; perhaps the 3.8 might just be too much resistance to overcome... and will test the condenser as well..... all the more thanks to both of you... reed
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ignition source coil 12 Mar 2019 14:11 #8

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just back in from testing the source coil output wired through the points and condenser... at the speed my milwaukee 12v will turn it on low , i get an increasing ac voltage up to a steady 5.30 ... changed over to another drill at high speed and it went up to over 14 v before i got nervous and stopped... so coil seems to be good; still, how many volts does it put out when i'm just kick starting it? will have to get a neighbor's help to check that..

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ignition source coil 12 Mar 2019 14:22 #9

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Not read all the stuff above but the coil on my DT175MX came wired backwards, it would spark but not run properly....useless information I know but was bored and I know you lot will read anything lol....all I know higher resistance with thinner wire, the electricty flows on the outside of the wire I was told, the larger the wire the greater the surface area and hence lower resistance :)

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ignition source coil 12 Mar 2019 18:13 #10

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update.. looked at points when open with flywheel on and they were clean, even, and gaped correctly.. removed flywheel and desoldered my connections from source coil lead, points lead, lead to loom at top of condenser.. checked continuity to ground from points lead and it was correct closed and open.. checked same at coil and since coil is grounded then got a tone.. checked same at lead to loom and no continuity to ground, so good there... tested the condenser by setting digital multitester at 4k ohms and with black lead to condenser case, touched the red lead to the condenser top.. signal of 15???? value, was only there for a moment and then went to offline.. repeated tests showed same results; i read that this shows a good condenser... so with those mag components tested as okay, i'll put things back together again and try it again with each of the two ignition coils i have....
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