Most likely what happens is the insulation breaks down and during the high voltage spike some of that energy shorts out and you get a weak spark. It may not be obvious when checking but keep in mind the other losses between the coil and the spark plug, especially the gap between the distributor rotor and cap pin. A bad coil can even test good with an ohm meter. For diagnostics it is best to short the output to ground. Besides safety the open circuit will cause high voltage stress on the insulation. The output current is really self-limited by the resistance of the output coil. And yes coils can be damaged by being energized continuously. 

 

Dennis

 

 

From: Larry Alofs [mailto:lalofs@gmail.com] 
Sent: Thursday, November 19, 2015 10:31 AM
To: Dennis Haynes <d23haynes57@hotmail.com>
Cc: vanagonlist a <vanagon@gerry.vanagon.com>
Subject: Re: crank, wont start

 

What do you think can cause coil failure, besides simple deterioration from age and heat?

  Is it a bad thing to short the HV coil wire to ground while doing a compression test?

Would continuous current thru the primary side cause overheating and damage?

 

When I find old coils lying around in vans that I have bought for parts, I get the feeling that people have changed them attempting to fix other problems.

 

Larry A.

 

 

On Wed, Nov 18, 2015 at 11:34 PM, Dennis Haynes <d23haynes57@hotmail.com <mailto:d23haynes57@hotmail.com> > wrote:

Until recently I have not seen the coils fail often. I doubt I have done a dozen and I know of 5 in the past two years. I keep one on the shelf now. They are just getting old. I always give thought though to what may have caused it to fail.

Dennis

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