Since several of us have commented on coil problems, I am not sure to whom
you are asking the question. But I'll chip in. I don't have any idea what
causes coils or motor windings to fail, other than the obvious overcurrent
(not likely in a car I don't think) and the likelihood that somewhere in a
thousand feet of wire, maybe, per coil, and of millions of coils, there are
bound to be a few bad spots or assembly mistakes. Anyway, I don't know what
makes a wire break in a coil but I know what they act like with a broken
wire. Working on British cars for my whole youth taught me the symptoms.
They won't start until it gets warm and the expansion of the windings get
close enough together that they touch or the voltage will jump the gap. So
they run fine. But when they quit running they cool off and the windings
retract, and you won't get a spark until the coil is warm.

I can't think of a VW of mine that has had this problem but I did have it
happen on a 1977 Porsche 911. But like I say when a car won't start in the
morning but will when it warms up and it stays running OK no matter what
the ambient temperature is, it's most likely the coil.

Jim

On Thu, Nov 19, 2015 at 9:33 AM Larry Alofs <lalofs@gmail.com> wrote:

> 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>
> 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
> >
> > >
> >
>