Bad Coil?
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
I don't think I ever tried to test just a coil for spark, I have several used ones I just swap if I suspect a bad coil.
If I had to guess on a way to do it I would say put 12 volts the pos term , put a coil wire in the center and position the other end 1/2" from ground, then attach a wire to the neg term and with the other end of the wire quickly scratch ground and watch for the arch at the coil wire. Thats just a guess.
If I had to guess on a way to do it I would say put 12 volts the pos term , put a coil wire in the center and position the other end 1/2" from ground, then attach a wire to the neg term and with the other end of the wire quickly scratch ground and watch for the arch at the coil wire. Thats just a guess.
Tech Contributor
Joined: Jun 2004
Posts: 20,804
Likes: 959
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
I don't think I ever tried to test just a coil for spark, I have several used ones I just swap if I suspect a bad coil.
If I had to guess on a way to do it I would say put 12 volts the pos term , put a coil wire in the center and position the other end 1/2" from ground, then attach a wire to the neg term and with the other end of the wire quickly scratch ground and watch for the arch at the coil wire. Thats just a guess.
If I had to guess on a way to do it I would say put 12 volts the pos term , put a coil wire in the center and position the other end 1/2" from ground, then attach a wire to the neg term and with the other end of the wire quickly scratch ground and watch for the arch at the coil wire. Thats just a guess.
Ya beat me to it, Roger.
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Thread Starter
Instructor
Joined: Jan 2007
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From: Key Largo Florida
Pro
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From: Hayward Wi
Burning Brakes
Joined: May 2009
Posts: 1,232
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From: Tucson AZ
St. Jude Donor '12
Solve this easily through Process of elimination: you already know you have power to the coil. Use one of the plug wires in place of the coil wire and try to start the engine. Of course the cylinder you took the wire from will miss, but:
if the coil is good, the engine will start. - replace coil wire.
if the coil is bad, the engine will not start. - replace coil
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
Solve this easily through Process of elimination: you already know you have power to the coil. Use one of the plug wires in place of the coil wire and try to start the engine. Of course the cylinder you took the wire from will miss, but:
if the coil is good, the engine will start. - replace coil wire.
if the coil is bad, the engine will not start. - replace coil
Burning Brakes
Joined: May 2009
Posts: 1,232
Likes: 6
From: Tucson AZ
St. Jude Donor '12
You make a good point DW. My thought was if he's already had the meter out to check power to the coil, he also inspected under the cap.
The point of my post is demonstrate logical troubleshooting to reach a diagnosis, versus 'guessing' until he gets to the right part.
A lot of time and energy can be spent on hunt-n-peck, where a logical process will quickly lead to a solution.
The point of my post is demonstrate logical troubleshooting to reach a diagnosis, versus 'guessing' until he gets to the right part.
A lot of time and energy can be spent on hunt-n-peck, where a logical process will quickly lead to a solution.
Race Director
Joined: May 2006
Posts: 16,528
Likes: 53
From: Dayton, Ohio
100% I like working through a problem systematically and finding the exact cause rather than changing 4 or 5 things and wondering what actually fixed it. ( I'm a lot **** that way 
. )
Tech Contributor
Joined: Jun 2004
Posts: 20,804
Likes: 959
From: I tend to be leery of any guy who doesn't own a chainsaw or a handgun.
If the OP has 12 volts at the coil+, then a short between the coil and points is unlikely (unless there is also a fault with an open primary winding). Assuming the points are open, there should be 12 volts at both primary terminals.
Race Director
Joined: Mar 2006
Posts: 14,112
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From: Florida
Here is the fastest way I found to test. Takes less than a minute.
Take dist cap off and put a known good spark plug on coil wire and lay spark plug on engine ground anywhere.
Turn ignition to on.
Start with the points closed, take a small screwdriver and touching only the moveable part of the points, open the points.
You should see the points arc a little when opening (you know you have power) and also the spark plug will spark (coil is good).
The only variable would be the coil wire.
The coil might not perform well at rpm when hot though.
Take dist cap off and put a known good spark plug on coil wire and lay spark plug on engine ground anywhere.
Turn ignition to on.
Start with the points closed, take a small screwdriver and touching only the moveable part of the points, open the points.
You should see the points arc a little when opening (you know you have power) and also the spark plug will spark (coil is good).
The only variable would be the coil wire.
The coil might not perform well at rpm when hot though.
