Homelite C-5 Ignition Coil Continuity Question

[ZeroJunk]

This is an aftermarket coil that slips over the laminated core that crosses the magnet in the flywheel. It has two wires coming out. One red, one white. I assumed the red went to the points and the white grounded where the condensor mounts.
Problem is that the red wire and white wire coming out of the coil show a dead short between each other.

Is this coil bad or am I missing how this is supposed to work ??

 

[heimannm]

The primary lead (to the points) and the ground are only a few turns of wire apart and so have very low resistance. Unless you can measure something in the range of 0-2 ohms accurately it will appear as a dead short.



I will add that working on a number of XL and Super 2 models recently I have realized that for whatever reason I cannot reliably test the coils for continuity as an accurate measure of whether they are good or not. Several would not register any continuity between ground (or primary) and the secondary lead (spark plug wire) even with new wires installed so I assumed they were bad. After three in a row I thought I'd better test them on saws to make sure and low and behold all of them produced a nice spark. I have never encountered this on any of the McCulloch coils I have tested so I guess in the future I'll just have to test the coils on a saw to know if they are good or bad. I can still check the points (open/close) and condenser (capacitance) to make sure they are good before assembling a saw.

Mark

 

[ZeroJunk]

Thanks for the reply. I don't have spark. The condensor and coil are new. The continuity between the red and white wire on the coil is about .3 ohms on a Fluke 87, basically the same as touching the probes to each other.

The continuity reading at the terminal for the points doesn't change in either switch position until I remove the coil wires and then the resistance is infinite with the coil out of line and the switch off. So, the switch works...

I suppose I could take a nine volt battery and see if DC will pass through the coil.

 

[heimannm]

"The continuity reading at the terminal for the points doesn't change in either switch position until I remove the coil wires and then the resistance is infinite with the coil out of line and the switch off. So, the switch works..."

I do not follow your process/reasoning here. There will always be continuity from the points terminal/primary lead to ground (very low resistance) and from the points terminal/primary lead to the plug wire (high resistance).

Mark

 

[ZeroJunk]

"The continuity reading at the terminal for the points doesn't change in either switch position until I remove the coil wires and then the resistance is infinite with the coil out of line and the switch off. So, the switch works..."

I do not follow your process/reasoning here. There will always be continuity from the points terminal/primary lead to ground (very low resistance) and from the points terminal/primary lead to the plug wire (high resistance).

Mark

It was just a test to see if the switch was not making/breaking.

I tested it again a few minutes ago. Actually I show .8 ohms through the coil and .1 ohms just probe to probe. Not, much reactance but may be all that it is supposed to be.

On the other hand I show like a meg ohm through the closed points. So, I cleaned them with some 1000 grit and the resistance is still over 500K ohms. So, I'm thinking it may have been the points all along.

Closed points should just be a few ohms I would think. So, I ordered a new set of points.

Lot about this I don't understand , obviously.

 

[KASH]

Idealy the points should read zero reistance the new pint sets are of very poor quality.Clean the contacts of your old points with a point file or emery cloth finish with fine emery cloth spray the contacts with brake cleaner let dry and pull a clean business card threw the points several times.Gap to 14to 16 thousands and if you have spark you probably will never have to touch the points again in your life time.
Kash

 

[scottr]

Thanks for the reply. I don't have spark. The condensor and coil are new. The continuity between the red and white wire on the coil is about .3 ohms on a Fluke 87, basically the same as touching the probes to each other.

The continuity reading at the terminal for the points doesn't change in either switch position until I remove the coil wires and then the resistance is infinite with the coil out of line and the switch off. So, the switch works...

I suppose I could take a nine volt battery and see if DC will pass through the coil.

If you charge the primary with your battery and then remove the charge the collapsing magnetic field will create a high voltage in the secondary winding .

 

[ZeroJunk]

Thanks guys. I assume the red wire is secondary to the points and the white wire is ground, only because the original HV is red, but the ground wire is some tiny gauge bare that terminates at the screw that holds the condensor..

I'll put a better cleaning on the points. I'm a little timid because most of the contacts I deal with are on magnetic relays and are silver plated and if you get too rambunctious you will clean the silver off.

When I pulled the fine grit sand paper through it left a white residue on the paper like some kind of oxidation. I'll get them clean.

 

[scottr]

It was just a test to see if the switch was not making/breaking.

I tested it again a few minutes ago. Actually I show .8 ohms through the coil and .1 ohms just probe to probe. Not, much reactance but may be all that it is supposed to be.

On the other hand I show like a meg ohm through the closed points. So, I cleaned them with some 1000 grit and the resistance is still over 500K ohms. So, I'm thinking it may have been the points all along.

Closed points should just be a few ohms I would think. So, I ordered a new set of points.

Lot about this I don't understand , obviously.

Closed points should be less than 0.5 ohms .

 

[ZeroJunk]

The problem with the points was that there was no continuity between the points and the stator plate or whatever you call it. Loosened the screw, moved it around a little and fixed that.

But, still no fire. I tested the cap with an LCR meter and it shows about the same as several others, around 200 nF . The DC resistance between the primary is .9 ohms, about normal. The DC resistance to the secondary is 10.85 K ohms on both legs. Switch is working.

Still no fire.

And on top of all that and the reason I leave this stuff alone in sane moments is that the bottom of the starter cup broke last time I tightened it. In a hurry and not aligned. $50 for one of those on ebay

I wasn't going to charge the old man if I could fix it, but Good Lord.

 

[heimannm]

What do you have for a gap when the points are open? I am not familiar with those models but I would guess 0.015-0.020 should work. Is the flywheel key sheared and the timing way off?

Mark

 

[ZeroJunk]

What do you have for a gap when the points are open? I am not familiar with those models but I would guess 0.015-0.020 should work. Is the flywheel key sheared and the timing way off?

Mark

No, the timing key is fine. Gap set at 15 thousandths per spec. I'm wondering if the magnets in the flywheel are bad. Even if the timing key was off it would fire somewhere. It is one of those three magnet models .I bought a new starter cup and think I will just tell him I can't fix it. Which is true. It was a sentimental thing anyway.

 

[scottr]

Do the points have a good ground ?
Are you using a known good spark plug ?

 

[ZeroJunk]

Well, to all who responded I finally got good fire. Not 100% sure why. I swapped the leads from the coil primary. White to the points and red to ground, although physices wise I'm not sure that it would make any difference. I bought a new starter cup since I broke the original being stupid, but that would not have anything to do with it.
So, I pulled it a couple of times and thought I saw a little spark. Tried a third spark plug and got a good blue arc.
Runs with mix squirted in the carb. Now, I get to rebuild that old Tillotson HL.

I think from the start the points had lost their ground. Then I proceeded to go in circles for a while.

Looks like I'll get it though. Thanks