Weak/no spark
Thread Starter
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Dirt don't hurt
Joined: Jan 2011
Posts: 21,066
Likes: 4,568
From: Haslet Texas
Weak/no spark
The bike is a '94 FXSTC
I get mostly no spark but sometimes a weak one will show up.
The coil ohmed out good, new plug wires, tried a different ignition module (no change), the cam sensor is only 1.5 years old.
I get full voltage to the positive side of the coil.
The coil is original I think.
I hooked up a test light to the negative side of the battery, put the probe on the negative side of the coil and when I removed it I still got no spark at the plug.
Not sure what I'm missing.
Anybody got any ideas????
I get mostly no spark but sometimes a weak one will show up.
The coil ohmed out good, new plug wires, tried a different ignition module (no change), the cam sensor is only 1.5 years old.
I get full voltage to the positive side of the coil.
The coil is original I think.
I hooked up a test light to the negative side of the battery, put the probe on the negative side of the coil and when I removed it I still got no spark at the plug.
Not sure what I'm missing.
Anybody got any ideas????
Road Master
Joined: Aug 2007
Posts: 942
Likes: 323
From: Grew up in Texas, moved to AZ
1) Coil might ohm out good, but is on its way out. 23 years, and it's tired. Swap it with a good one and test.
2) Plugs?
3) cam sensor? Solid state stuff can just go out, but you would not get intermittent spark. I would open up to see if it melted or maybe got wet, etc...
I suspect a bad coil.
Dave
2) Plugs?
3) cam sensor? Solid state stuff can just go out, but you would not get intermittent spark. I would open up to see if it melted or maybe got wet, etc...
I suspect a bad coil.
Dave
Last edited by daven9113; Mar 18, 2017 at 07:33 PM.
Thread Starter
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Dirt don't hurt
Joined: Jan 2011
Posts: 21,066
Likes: 4,568
From: Haslet Texas
Even if it ain't the coil I can add one more spare part to my shelf.
Plugs are NGK iridium with about 1.5k miles on them.
Seasoned HDF Member
Joined: Aug 2007
Posts: 8,202
Likes: 3,089
From: NW AR
Your test light/remove on the coil negative won't tell you anything.
A coil swap would not be a bad idea. You could even test it with any automotive coil laying around. A coil is a coil at that point. And the only other thing left is the cam sensor, or the wires between it all.
A coil swap would not be a bad idea. You could even test it with any automotive coil laying around. A coil is a coil at that point. And the only other thing left is the cam sensor, or the wires between it all.
Ultimate HDF Member
Joined: May 2006
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From: Ohio
Last edited by Beemervet; Mar 19, 2017 at 09:30 AM.
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Seasoned HDF Member
Joined: Apr 2011
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From: W. Texas
Would what he is doing with the test light be kinda like opening a set of points on an older car? I remember getting bit by a coil some back in the day.
Maybe the newer stuff don't work that way I feel old now
WP
Maybe the newer stuff don't work that way I feel old now
WP
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Thread Starter
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Dirt don't hurt
Joined: Jan 2011
Posts: 21,066
Likes: 4,568
From: Haslet Texas
Your test light/remove on the coil negative won't tell you anything.
A coil swap would not be a bad idea. You could even test it with any automotive coil laying around. A coil is a coil at that point. And the only other thing left is the cam sensor, or the wires between it all.
A coil swap would not be a bad idea. You could even test it with any automotive coil laying around. A coil is a coil at that point. And the only other thing left is the cam sensor, or the wires between it all.
Don't they normally last longer than that????
Seasoned HDF Member
Joined: Aug 2007
Posts: 8,202
Likes: 3,089
From: NW AR
Anything can fail at any time, so even if it is a year old, it is still suspect.
If you think about the circuit, WP, when you turn the ignition on, the coil should be charging itself because the coil negative should be grounded by the ignition module (or points.) The coil is fired by opening the circuit (ground), mking the coil collapse and inducing a current in the secondary circuit, the high voltage side of the coil. A test light has a bulb in series between the pointy end and the alligator clip. Not much current will flow through the bulb. The coil is charged by a bunch of current flowing through it. Plus, the coil is already grounded at the negative side. So, adding an additional high resistance circuit between ground and the coil negative, then removing it will do nothing at all to the coil if it is still in the main circuit, and damn little if you pulled the coil negative side first.
If you think about the circuit, WP, when you turn the ignition on, the coil should be charging itself because the coil negative should be grounded by the ignition module (or points.) The coil is fired by opening the circuit (ground), mking the coil collapse and inducing a current in the secondary circuit, the high voltage side of the coil. A test light has a bulb in series between the pointy end and the alligator clip. Not much current will flow through the bulb. The coil is charged by a bunch of current flowing through it. Plus, the coil is already grounded at the negative side. So, adding an additional high resistance circuit between ground and the coil negative, then removing it will do nothing at all to the coil if it is still in the main circuit, and damn little if you pulled the coil negative side first.