Test Your Ignition Coils With A Multimeter

[Vintage Engine Repairs]

I hope it helps!

 

[SteveSr]

Mostly good video. However, you need to add that the way to determine electronic coil from conventional coil is the conventional coil will ALWAYS connect to a set of points and a capacitor whereas an electronic coil will NEVER have points or capacitor.

Measuring "kill wire" resistance on an electronic coil is pretty much meaningless unless you find an open circuit in which case the motor won't shut off. You will also likely get different resistance if you swap the leads. The only real legitimate test on an electronic coil is to check the secondary (HT) side only.

 

[Vintage Engine Repairs]

Mostly good video. However, you need to add that the way to determine electronic coil from conventional coil is the conventional coil will ALWAYS connect to a set of points and a capacitor whereas an electronic coil will NEVER have points or capacitor.

Measuring "kill wire" resistance on an electronic coil is pretty much meaningless unless you find an open circuit in which case the motor won't shut off. You will also likely get different resistance if you swap the leads. The only real legitimate test on an electronic coil is to check the secondary (HT) side only.

Thanks Steve I appreciate the reply, it’s worth mentioning, many coils out there will be connected to a seperate electronic / CDI chip or trigger and there won’t be points & condenser, so you really are just testing windings rather than electronics - it’s like this on some motorcycles - I found it on my dirt bike in fact - it’s electronic ignition but the manual gives resistance values for the coil (attached below). Also some coils are electronic and contained and you can get accurate primary readings from them. In fact it’s rare in my experience that you can’t get good readings from both primary and secondary. Every coil is different though of course.

 

[SteveSr]

In this forum we are mostly dealing with saws and other O_P_E. It is important for folks to remember that ignition systems/coils cannot be confirmed as good or working just by an ohmmeter test. About the best that you can hope for is to confirm that one is truly dead by an open secondary/HT winding. Conversely, showing that the resistance is within specification doesn't mean that the coil is good. There are just too many failure modes such as shorted turns/insulation breakdown in the secondary or electronics failure in the primary to make resistance testing of much use. In fact, Stihl basically says the same thing.

I looked through several of the Stihl repair manuals and came up with the following common denominator in all of them...

 

[Del_]

In old outboard motors the coils in points/condenser magneto ignitions and CDI ignitions can both be tested for resistance of the primary and secondary windings with an ohm meter. Usually we see about .5Ω on the primary and about 5kΩ on the secondary. If there is any 'punch through' of the dialectic in the windings the coil is usually quickly destroyed and usually measures open or shorted.

 

[SteveSr]

In old outboard motors the coils in points/condenser magneto ignitions and CDI ignitions can both be tested for resistance of the primary and secondary windings with an ohm meter. Usually we see about .5Ω on the primary and about 5kΩ on the secondary.

I guess that the point that I (and Stihl) am trying to make is that even if the resistance readings check out the coil still may not be any good.

If there is any 'punch through' of the dialectic in the windings the coil is usually quickly destroyed and usually measures open or shorted.

I would think that dielectric failures say to ground (armature) would be fairly rare due to the physical spacing of the windings in the coil.

The primary failure mode is usually insulation failure in the secondary (HT) side of the coil. When this happens you get turn to turn shorts which will have an insignificant effect on resistance readings. However the shorted turns act as another transformer winding that is shorted and thus sucks all of the energy out of the coil leaving none for spark.

 

[SteveSr]

BTW, If anyone wants a good description of the various Stihl breaker points, Bosch, and SEM ignition systems have a look at the old printed 028, 038 repair manual. It has about a 20 page section including theory of operation for all 3 types.

 

[HansFranz]

I don't know if you mention it in your video, Tom, or if it's been noted in the thread, but IIRC, I read here on the forum that one should never spin over a chainsaw motor without somewhere for the spark to go to ground because it can mess up the ignition coil or something like that (I forget exactly, but made a note of it somewhere because it was news to me)...

 

[SteveSr]

I don't know if you mention it in your video, Tom, or if it's been noted in the thread, but IIRC, I read here on the forum that one should never spin over a chainsaw motor without somewhere for the spark to go to ground because it can mess up the ignition coil or something like that (I forget exactly, but made a note of it somewhere because it was news to me)...

Running a magneto coil without a plug will cause higher than normal voltage at the HT terminal. Normally this voltage drops fast once the spark breaks over. However, with no break over path this can potentially lead to dielectric insulation breakdown and subsequent arcing inside the coil.

Most properly designed (i.e. OEM) coils should be designed to handle this situation without failure. However, one that is poorly designed (can anyone say Chinesium) or already damaged and on its way out could be finished off by running it without a plug.

For an electronic coil you also have the issue of where the spark energy goes. Normally it is dissipated by the sparking of the plug. However if the plug is absent that energy may be reflected back into the primary. This may result in a recoverable temporary malfunction or it could lead to electrical overstress that can permanently damage primary side components effectively killing the coil.

 

[HansFranz]

Running a magneto coil without a plug will cause higher than normal voltage at the HT terminal. Normally this voltage drops fast once the spark breaks over. However, with no break over path this can potentially lead to dielectric insulation breakdown and subsequent arcing inside the coil.

Most properly designed (i.e. OEM) coils should be designed to handle this situation without failure. However, one that is poorly designed (can anyone say Chinesium) or already damaged and on its way out could be finished off by running it without a plug.

For an electronic coil you also have the issue of where the spark energy goes. Normally it is dissipated by the sparking of the plug. However if the plug is absent that energy may be reflected back into the primary. This may result in a recoverable temporary malfunction or it could lead to electrical overstress that can permanently damage primary side components effectively killing the coil.

Thanks, SteveSr, for the explanation.

So for saw mechanics, is that something that should never be done, or is it generally OK with modern saws in decent condition? (I'm trying to decide whether to make it a rule of thumb, for example if I flood a saw so bad that I can't start it even under full throttle, and pull the plug and spin over the motor so I can get the excess fuel out of it...)

 

[SteveSr]

Thanks, SteveSr, for the explanation.

So for saw mechanics, is that something that should never be done, or is it generally OK with modern saws in decent condition? (I'm trying to decide whether to make it a rule of thumb, for example if I flood a saw so bad that I can't start it even under full throttle, and pull the plug and spin over the motor so I can get the excess fuel out of it...)

I wouldn't worry about it. The same thing happens when doing a compression test. I have yet to kill one doing this. As I said this condition should be designed to survive.

My aging memory seems to recall that there may have been similar concerns/actual issues with some earlier automotive electronic ignition systems, possibly the GM HE system. Others here may remember this.

 

[Old2stroke]

I don't know if you mention it in your video, Tom, or if it's been noted in the thread, but IIRC, I read here on the forum that one should never spin over a chainsaw motor without somewhere for the spark to go to ground because it can mess up the ignition coil or something like that (I forget exactly, but made a note of it somewhere because it was news to me)...

Here's my explanation from a post about a year ago.
When a spark event occurs, the high tension voltage increases until it arcs across the plug gap but when the arc is established, the voltage will not rise any higher. Any increase in the high voltage gap path will allow the voltage to increase until it arcs the gap, increased gap length may be due to wrong plug gap, spring clip on the plug not in contact with coil lead center wire, break in the lead wire, etc. If the gap is so large that an arc doesn't occur, the voltage can rise to the maximum the coil can deliver (plug lead disconnected) and this voltage might be high enough to break down the delicate insulation on the secondary windings of the coil and eventually lead to carbon traces and coil failure. This is the reason the engine should not be turned over fast with the ignition on and the plug cap off with no path to ground for the high voltage.

Some coils may have overvoltage protection built in but let's not trust that, the best practice is to make sure the ignition sw is off before you pull the engine over with the plug out or disconnected.

Quote Reply

 

[HansFranz]

the best practice is to make sure the ignition sw is off before you pull the engine over with the plug out or disconnected.

That makes sense, and the solution -- to turn off the ignition switch -- didn't occur to me, so thanks.

 

[sean donato]

In the old mercury outboards they just told you to ground the high tension lead. The electricity just needs a path to ground to dissipate. You would burn out a switch board if you didn't. (Stupid youth and ignorance.... been there and done that.) It's simple enough to do. I just use my spark tester during compression tests.

 

[HansFranz]

I'll never forget checking my old Evinrude 40hp twin for spark...my stepfather was holding the plug against the head right next to the plug hole, and I turned the key, and the engine started up (on the other cylinder) and all this fuel mix was blowing out the plug hole in my stepfather's face ... "We got fire!" he yelled... but he didn't drop the plug! LOL I couldn't believe how cool-headed he was.

 

[L34]

If I turn a saw motor(or mower etc) over with no plug I use a fly lead with alligator clips.
1 end clipped to plug spring the other end to the cylinder.
Don't want to risk blowing a coil.