Ignition System Testing

[Huggins]

I just thought I would share some simple steps in testing these ignition systems. I know there are like threads and very good info type threads for this type of thing, but none of them really cover the specs since I would assume that they are very wide ranged, so I thought I would cover a specific set of saws...

What I've seen more than anything with these saws when there is no spark is the plug wire has come loose and looses its continuity to the coil. To fix this all you need is some Super Weatherstrip Adhesive to reattach the plug wire. While your at it though, I would go ahead and test the rest of the system...

You will need an Ohm meter for the following...

To test the coil you will need to use your ohm meter to either go from the end of your plug wire with one meter lead while touching the other meter lead to the metal housing somewhere or preferably with your plug wire removed, stick one of your meter leads into the plug wire attachment area and touch your other meter lead to one of the wires that come out of the back of the coil as shown with the coil removed.





If your coil is good, it should test out to right around 8 K or 8000 ohm's.

Now moving on to testing your points. This is either going to be a low resistance reading or an open loop or open - test lead type reading depending on what kind of meter your using and what you may refer to it as. With the points closed and making good contact, you should have 0 - .1 ohm's of resistance as shown.



With the points open you should see an open. I know this picture isn't that great at showing the point, but what you can't see is the fact that the meter is flashing 100 or with this particular meter showing an open... I'm using a pair of pliers to hold the points open for testing.



If all of this shows good, then your going to need a condenser.

If you have one of these ignition setups, hope this all helps...

[MCS]

If you don't mind, I like to add a few points for those who will do this while everything is still in the saw.

This is a schematic of a Homelite points ignition system.



The coil has two sides, the primary which is connected to the points, and the secondary which goes to the plug and notice that the primary and secondary are both connected to the frame or ground. When you measure from the spark plug wire to ground you are measuring the secondary side which should read about 8k ohms.
Checking the primary is a little different because there are four things that have to be considered as they are all tied together. They are: the points, the condenser, the primary coil, and the kill switch. The primary will be somewhere in the 1 to 2 ohm range but not 0 ohms.
Testing the primary side.

• Cut a strip of heavy paper and slip it between the points.
  
• Connect one lead of your ohm meter to the nut where the condenser and coil are connect to the points and the other lead to ground.
  
• Set your meter to the lowest ohm range.
  
• With the kill switch in the "ON" or Run" position you should see the 1 to 2 ohm reading on your meter and then with the kill switch set to "OFF" you should see the reading go to zero. If the kill switch was unplugged when you pulled the rewind, you don't have to worry about turning the switch on and off.
  
• Now, with the kill switch "ON" remove the paper from the points and let them close. You should see a very low reading probably less than .5 ohms. This is the point resistance. New points should be even less that that.
  

If you don't have these reading, you will have to disconnect the coil and condenser from the point block and disconnect the kill switch and then test each component individually.
The condenser is the one component that is tricky to test and that is why it is usually just replaced. Some digital meters can check condenser but we don't know how many u farads it should be.
If you disconnected the kill switch, you should test it with the meter to make sure it works. If you put the saw back together with a bad switch you might not have spark

[MCS]

Actual readings from:
XL-903

• Coil - Primary 1.2 ohms
  
• Coil - Secondary 6.8k ohms
  
• Points - used .5 ohms
  

150

• Coil - Primary 2.1 ohms
  
• Coils - Secondary 7.9k ohms
  

Testing a condenser
Although this won't give you the condenser's actual value, it will tell you if it is "Open" or "Shorted." In doing this test, you cannot touch both meter leads with your fingers or you will see your body resistance.

• Place your meter on its highest Ohm scale
  
• Pinch your red meter lead with your fingers to the condenser wire and touch the condenser case with the black lead but don't let your fingers touch the lead or condenser.
  
• Your meter should show a brief reading and then display infinity. Different meters display infinity differently so what you see with the leads not touching anything is infinity.
  
• Now reverse the leads and you should see brief reading before the meter displays infinity. If you are using an analog meter, one with a needle, you will see the needle bounce to a low reading and then swing up to infinity. Analog meters actually work the best for this because you can watch the needle swing. Switch the leads back and forth a couple of time and watch this.
  

When you test resistance, you are using the battery in the meter to pass current through a circuit. Our circuit is just the condenser but a condenser will store an electric charge. You are actually charging the up the condenser with the meter to a few volts. When you reverse the leads, the condenser will discharge through the meter showing a momentary low resistance reading until it charges up with the opposite polarity.
If the condenser is shorted or leaking, you will see some constant resistance. If it is open it will read infinity all the time.
Have fun ;D

[Huggins]

Great Info...

[MCS]

The resistance is pretty close to the value I read on the 903 coil I used for this example so from that perspective it should be good, however, resistance checks aren't foolproof. Even with a good reading there can be breakdown under actual operation. If the points and condenser are good, the flywheel key has the orientation of the magnets correct, and the high tension lead is good then there should be spark unless the coil is bad. Although coils do fail, I don't think it is that common of a problem.

[MCS]

My SXL style coil tests 6.96k. What does that mean? Does the amount of resistance directly relate to how strong the coil is? Or is this difference due somehow to the coils being different styles?

Thought I would clarify this question. When you measure the resistance of these coils with an Ohm Meter you are only measuring the DC resistance of all the wire that is wrapped around the core. The wire used to make coils and transformers is coated with and insulating film. If you look real close at a coil you can sometimes see this where the wire exits the coil. It will have a reddish appearance. The wire is small in diameter because there really isn't a high current flow - just some high voltage.
Let's say there is a 100' of wire used to make the secondary coil and this wire has a resisance of 50 ohm per foot. Our total resistance would be 5,000 ohms.
Now, after 40 years and 250 cord of wood cut, this insulation breaks down and a bunch of the windings end up shorting together We have weak or no spark so in checking the DC resistance with our meter we get 2,000 ohms. That tells us there is a short in the windings. The other end of the spectrum would be where the insulation failed but instead of the wires shorting, there is just an open. Now our resistance would be Infinity
Hope this helps explain what is being done when a coil is tested with an ohm meter ;D
And no, the DC resistance doesn't tell us how strong the coil is if compared to other part number coils.

[snoghost13]

Quick question about a C5 coil.... Or maybe any coil for that matter. I have a C5 with no spark. Was checking the wire connection at the coil when I noticed what I think is not right. The housing of the coil is really soft and mushy. I have never seen a coil like that but is it normal or a true sign of a really bad coil?

[MCS]

That doesn't sound good. All the coils I've worked with have been firm so something must have deteriorated the casing.

[gcaiola]

Great info here. Very specific detail about coil resistances--not vague--which is helpful and much appreciated. I only wish your attached photos were still available. Were they saved on some photo website that has since stopped allowimg free access? I created a login on that site but still cant find the photos.

I am trying to resolve a no spark issue on a homelike 150 automatic saw. I can confirm coil is OK thanks to your info. But unsure how to even access the breaker points. I guess I have to remove the flywheel? Regardless, thanks again to those who've posted the past info.

[undee70ss]

Yes, you have to remove flywheel.

[sweepleader]

When you get to the points, pay attention to the gap that is there. The rubbing block wears and the points close over time. Also, if the tungsten is still fairly thick, they can be cleaned and function again, they oxidize over time and stop conducting the current needed to make sparks. Use a points file from an auto parts store, not sandpaper. You can leave behind grit from the paper that is also non conducting and foil yourself. Check to see if the points conduct with an ohm meter, resistance with the points disconnected should be near zero.

[dynodave]

A friend gave me a 150. I had to buy a set of points, not because the tungsten was burned but instead because the G-10 lifting arm on the points was worn away and the points would no longer open even at the limit of adjustment. Quite inexpensive and the saw came back to life instantly.
I am actually highly puzzled as to how the saw actually continued to run until this condition of mechanical death? It would have been very retarded and must have had no guts at all!!!