Testing & fixing ignition coils - save some $$$ / €€€ / £££ / ¥¥¥

Can't you simply remove the plug caps and measure from plug wire to plug wire?

That's how I have done it and was taught. However, I was looking for instruction regarding the directions above as I get a weird reading often.
I destroyed a coil I thought was bad, but once I removed the wires realized it was fine. The wires were causing the incorrect reading.
 
I think xjwmx is using a Honda coil on his with the stock TCI, the MP08. They have removable plug wires and are a dime a dozen on eBay.
 
A bit of Jerry-rigging could be nessarry in some cases. I test the moulded in wire type at the end where the spark Plug goes to ground....
if the spark plug cap can be taken of do so it may be a resistive type . between that and ground will be the 2ndary wire resistance here all you need to know is that it is NOT an open curcuit.... if it's open its shot ...toss it !
this is for when you get into any motor that has a coil and you have supissions that the coil is bad cuz you can't get it running !
I have on occasions cut the moulded in wire HIGH tension lead and spliced in a better spark plug lead wire... I just twisted the wires together and spliced it like a normal wire ...but I positioned it out away from any metal so it would not Arc to ground through that un insulated part.
..... I have used Automotive coils on bikes before ... they do work and work well but only on single cylinder machines.... However in theory
sense the ignition fires both spark plugs at the same time a single automotive coil should in theory work fine to fire both plugs on our XS650's just splice together 2 spark plug leads into one connection and plug it into the coil like normally.... now both plugs should fire like before.
so a $5.00 coil at autozone Should in theory make your 650 run again..... I have not tried this but I think it would work !
beats a $50.00 part any day and is easily replaced if that coil fails too.
...... A Lot of people think the high tension leads are something special..... their not they just have good insulation around them is all
the best are thick the high tension leads are usually steel wire which isn't the best for transferring electrical current but it works
their multi stranded and somewhat springy being steel wire.... if I was to try and put a single automotive coil on my bike to run both cylinders I would splice the 2 spark plug leads together with a short 2" length to go into the coil at the junction solder them good and silicone around the wires and then cover with black tape. maybe even build it up another 1/2" in thickness because that joint is going to want to arc to ground ! the better insulated it is the better off you are !
Wiring the automotive coil in should be no problem.... just put the primary side wire
from the points or TCI to the - on the automotive coil and 12vdc to the + side of the auto coil.....and Ground the canister /coil itself then... kick it through while holding the spark plug lead if it curls your hair for you it works ! LOL......
...... a coil is just a transformer it transforms low voltage to high voltage just like any transformer does it's not mysterious at all !
.... My suggestion may well over heat the automotive coil, but I doubt it.... it should work fine for years... I don't know if it'll work or not but it should......
Obviously it's not for everyone.... and it's jerry-rigging but If on the off chance that it does work you got a bike that now runs that didn't before and you did it for a few bucks instead of a hundred or so !
......
Great Write up Pete ! well done ! THANK YOU !!!!
.....
Bob..........
EDIT: edited my wiring...dunno what I was thinking at the time ! LOL
 
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Hi littlebill: I guess that simply measuring through the wires could work - and as long as the resistance was finite (ie. not an open circuit) then the coil should be OK.

I would expect a resistance of somewhere in the 10-20,000 ohm range through the plug wires (but electrical folks - please check and correct).

Pete
 
Okay, thought I saw this here on the XS650 sit but had to do a little Googling to find it.:
http://www.xs11.com/forum/showthread.php?t=137

Granted this is talking XS1100 but the same would work on the 650 with half the work!

Note I did not read it all the way through this time so don't know if it is mentioned but I would NOT suggest using the JBWeld steel epoxy, or any other brand with metal in it, for what should be obvious reasons!
 
I think xjwmx is using a Honda coil on his with the stock TCI, the MP08. They have removable plug wires and are a dime a dozen on eBay.
Yes. That came from Purplezinger, by way of the owner of a junkyard he was visiting. The plug wire terminals are spears rather than threaded spears, so you don't have to worry about twisting the terminals loose. If you get them on ebay, make sure the caps and rubber grommets under them are included...

Something I do is before I put terminal caps on I put a zip tie around it real tight to keep the cap from ever splitting. You can warm the end of the wire a little to make it spear easier, or you can split about 3/8" of it. For a final touch I wrap good electrical tape around the wire, then go back over the terminal cap with it, then back over the wire. That helps hold it together further still and seals out the elements. I was taught to do that with wire nuts when I was doing factory maintenance for a bit.
 
MaxPete.......................a very good write up to be sure!
However your explanation is not clear on one key point, but the calculus formula does show it. The key point is that the voltage induced in the secondary winding is equal to the rate of change of the magnetic flux.
As the formula shows, the voltage is equal to rate of change of the flux, multiplied by the number of turns of wire in the secondary.
You also did not mention dwell time, which is very important. Current flow is switched on in the primary winding, and it takes time for the current flow to build up a strong magnetic flux in the ignition coil. The amount of time, measured in degrees of rotation, is the dwell time. For example, the stock dwell, for points type ignition, on an XS650 is 91 degrees IIRC. Pamco ignitions use 60 degrees dwell.
RPM determines how much actual time there is for the dwell period to fully saturate the ignition coil with a strong magnetic flux.
60 degrees is more efficient than 91 degrees. Some ignition systems use a variable dwell period.

In an ignition system the primary current is being turned on and turned off in sequence. When the current is turned on, it takes time for the magnetic flux to build up, but when the current is turned off, the magnetic flux collapses very rapidly. Its this very rapid collapse that gives a very rapid rate of change and produces the high secondary voltage. This is why the spark occurs in the cylinder when the points or electronic circuit stops the current flow in the primary, and not when the current starts.
 
Quite right RG - but I didn't know how many people would be willing to absorb the deriviatives and other oddball math aspects involved with getting into it that deeply.

Anyhow, as you say - it is the speed (rate of change) of the breakdown in the magnetic field is actually what causes the very large voltage spike in the secondary side circuit - which results in the spark being able to jump the gap.

Also, you are quite correct in saying that it takes a finite (albeit small) length of time for the field to form in the core and that effectively limits the RPM of an SI engine. I am not sure what the limit actually is - but I'd like to know more about how Honda handled those incredibly high revving 5- and 6-cylinder racing engines in the 1960-70s. I think those little buggers could rev up to 23,000 rpm or even more - and they stayed together and even ran well at that RPM.

Cheers,

Pete
 
You guys are WAY overthinking this with all the talk of formulas and flux. The ignition coil is an inductor which is the electrical equivalent of a flywheel. When current is low and you try to increase it, an inductor will increase resistance to the circuit to try and keep current low. When current is high and you try to lower it, an inductor will increase voltage to the circuit to try and keep current high. When you open the points on a breaker point ignition it's equivalent to shoving a broom stick in the spokes of a spinning wheel to stop it. It's the same analogy with [non-CDI] electronic ignitions only you are using a steel pipe instead of a broomstick.

When you break the primary current, the voltage of the ignition coil will rapidly rise until it can get current flowing again. The coil does not care where the current flows. Electricity will always take the path of least resistance so EVERY component of the ignition must have adequate insulation to contain higher voltage that what is required to fire the spark plug. If the spark gap is not the weakest insulator in the chain then the current will flow elsewhere and the engine will not fire.

That brings us to the real trouble with refurbishing old ignition coils. A common failure of old ignition coils is a breakdown of the secondary insulation. You won't see this with a low voltage ohm check. Even a megger can miss it. When you see a coil advertised as a 50,000 volt coil, all that means is that the secondary insulation can handle 50,000 volts without shorting out, not that it will put out 50,000 volts. If the spark plug only needs 2,000 volts to fire then the voltage in the coil will rise to 2,000 volts at which point current starts flowing again so the voltage stops rising (it actually drops significantly since it takes less voltage to maintain the spark than it did to initiate it).

The voltage rating of the coil (secondary insulation) degrades with age. A 10,000 volt coil is more than adequate when it is new but now it's 40 years old and can only handle 5,000 volts. The engine will still fire up, idle well, and go down the road without a hint of trouble because the spark plugs need less than 5,000 volts to fire. However, when you crack the throttle you are allowing more air into the engine which means that the dynamic compression is higher and the spark plugs require more voltage to jump the gap. Now you have a funky hesitation that you will likely blame on the carburetors because, "it had a nice blue spark when i pulled the plug." I've fallen for this more times than I care to admit.

Do yourself a favor; just buy a new ignition coil from the start.
 
I have had Coils go bad Just like MrRiggs says.... but I didn't know Why I figured it was just the insulation breaking down , and it was but because the higher compression demanded higher voltage....... that I DID NOT KNOW !!!!!! thank you you taught me something today !
HAHAHAH !
Bob...........
 
Not sure if this helps in this discussion but something I have never done but probably should look into on a XS400 that is very hard starting and my head says the ignition system could be the problem.

This involves what for lack of a better description I will call a "Load Test". I relate this to some work I have done in the past with diesel gensets. It simply involves hooking the genset to an adjustable resistance bank and running the generator while adjusting the load on it. This tells you if the genset can put out the rated power and lets you determine if it is an electrical problem or the motor not being able to do the job.

If you look in a Yamaha manual you should come across reference to a "Spark Tester" one example would be like this one:
https://www.nationaltoolwarehouse.c...-P41027.aspx?gclid=CM_-1rT649QCFQwtaQodZjUM5g

Unlike some that are basically just modified spark plugs this type is actually adjustable in regards the gap between the two point the spark needs to jump.

If you check in the Yamaha manual you will see where the "plug gap" is set at .7mm-.8mm. Now if you go look at the spec to use for the "Tester Gap" you will notice that the minimum gap is 6mm, note that there is no"." before that 6! It doesn't take much to imagine the voltage needed to jump around 9 times the gap will put a much higher "load" on everything in the ignition system. As it has been mentioned before the energy needed to jump a gap is much higher at high pressures of compression. This test would simulate that by increasing the distance the spark is required to jump.

Guess my point would be just because you see a spark jumping the "normal" .7mm-.8mm gap when the plug is laying on the head when you kick the motor over slowly it does not guarantee you are going to have a good running motor while under hard acceleration on a hill.

Back to the genset analogy, just because that 10,000 watt generator lights up one 100 watt bulb does not mean it will be able to light the 100 100 watt bulbs it should be able to light.

If someone wants to/or can trim out all my rambling on here and re-post the real important stuff so it makes more sense feel free I can use all the help I can get!
 
The physics people tell me that it takes about 10Kv / mm to sustain a spark. So, a 6 mm gap would take at least 60,000 volts. It must be a misprint - I don't think that even the largest SI engines use a gap that big.
 
Simple load tester: Unplug the wire from the spark plug, plug in a second spark plug, hold the test plug against the plug in the engine and spin the motor over. The two plugs are in series, if it can't jump the gap inside the engine then you will see no spark in the external spark plug. This is useful for for troubleshooting no-start conditions.
 
if I doubt the ignition system I widen the spark plug gap real wide and test the spark if it still sparks I then return the gap to normal.
its a quick Load test for the coil.....
Kshanses ...no need your explanation was great !
I have a spark tester in the shop somewhere ( god only knows where though!) but it is sat by the factory ( non adjustable P.O.S. at right near 1/4" and I've used it many times and had the thing arc through that gap with no problem....
.....
I think that 10,000volts per millimeter is referring to a Continuous arc not a short arc in time.... Pete ! it takes way more voltage for a sustained arc....
My Arc welder I had ( before the fire) was a AC/ DC stick welder and I measured the voltage on it it was 55vdc now you wouldn't think that is enough voltage but the Amps were 225 amps on it's highest setting....
the stick welder I have now has a voltage of 110vac and has 200 amps..... if you raise either the amperage or the voltage you can get an arc
and sense it's much easier to raise voltage that's what they do and they use a coil instead of a big hanging 75lb transformer like in the welder
to make that arc on the bike !
and sense it's not a continuous arc, it's even easier....
.....
I have a question though.... the capacitor/condenser used in a Points engine .... what roll does it play precisely ?
does it give a Jolt to the points when they break open or is it for the coil to help collapse the magnetic field ?
I don't think i've ever understood the capacitors roll completely in a points system I do know if you take it out the points will burn in short order.... so is it just to keep the points from burning ? to act as a cushion for the points ? because the voltage would be divided between the line to the coil and the capacitor when the points are closed and power is being fed to them.... in turn that lightens the load on the breaking points doesn't it ? Ugh ! as I said Never understood the need for the condenser ! LOL
......
Bob.........
 
I have a question though.... the capacitor/condenser used in a Points engine .... what roll does it play precisely ?

It's funny you mention arc welders when asking this because that is the analogy I use to explain the role of the condenser in points ignitions. An arc welder is a constant current source, like an ignition coil. When arc welding, you touch the electrode to the work piece to start current flowing. When you lift the electrode the arc welder will increase the voltage to maintain that current flow. If you lift the electrode high enough you will exceed the maximum voltage and the current will stop (arc goes out). The exact same thing would happen with a set of points because they open relatively slowly. But if you stick a condenser (capacitor) in there then the current will continue to flow through the coil when the points open, not across the points but into the uncharged capacitor. The primary current ramps down fairly quickly as the capacitor charges and hopefully the points are open far enough at that point that the current can't flow any longer in the primary so the coil voltage will rise enough to initiate current flow through the secondary (spark plug).
 
The physics people tell me that it takes about 10Kv / mm to sustain a spark.
Riggs is saying it depends significantly on the density of the air. There are better things to ponder than all this. My advice is get an MP08 if you need a TCI coil... They were used at least 'til 2000 that I know of, maybe later, so they're not necessarily old.
 
The physics people tell me that it takes about 10Kv / mm to sustain a spark. So, a 6 mm gap would take at least 60,000 volts. It must be a misprint - I don't think that even the largest SI engines use a gap that big.

Not saying that a Yamaha manual could not have a misprint, but I did just now take the time to check an article I had read in a magazine just a couple months ago. In that one the tech adviser for the magazine in response to someone with a different brand of motorcycle was telling the person how to check for a possible "lack of spark" problem. In the article he suggested testing with an 8 mm gap.
 
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