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GM HEI Coil Autopsy


Hi All,

	I used vicious cutting machines to slice the failed GM coil in half.
These babies are built like rocks.  Extremely difficult to cut open.  I was
not able to find the exact failure site however the following was learned.

	I latter found that when I ran the coil.  My secondary ground wire was not
connected at the other end :-)  The high voltage must have been arcing
through the coil to reach ground on the bottom of the secondary.  the oil
insulation prevented it from finding ground anywhere else.  That sure
didn't help anything!  This may also explain the problem with the high
voltage power supply loosing control.

	The internal structure of the GM coil is almost perfect.  Zero voids in
the vacuum impregnated epoxy.  The windings are well insulated and it is of
obvious very high quality.  It looks easily capable of withstanding very
high voltages on the secondary.  Probably to 250kV!  The coil does show
minor signs of delamination probably due to it's many years of in-car
service.  Probably new coils are best for this hard Tesla coil application.

	There are two problems, however.  There is a design "flaw" in the primary.
 The primary is two layers thick with both terminations on one side.  They
spin the first layer and then spin back over the first layer to the
beginning.  This allows the entire primary voltage to be placed across
adjacent wires at the begining (or end) of the primary.  Normally, 12 volts
probably isn't a problem.  But in my case, I have to be sure the voltage
across the primary does not get too high.  I'll probably put an 1800 volt
MOV across the next coils' primary.  If they would have put an insulating
layer between the two layers this coil would be much more reliable
espicially under unusual conditions (like a spark plug wire falls off the
plug) (I hope Delco is listening ;-)).

	This coil was also defective.  The primary winding was super tightly wound
in a section of winding.  The wire was heavily stretched and crushed into
the lower winding.  It is surprising what ever happened didn't break the
wire.  The cross section of the wire is crushed into a V shape by the lower
winding.  This, of course, vastly increased the chance for the primary to
short and is probably the main reason this coil failed.  This is very good
news!  If this defect had not been present, this coil probably wouldn't
have failed when it did (it would have waited for me to turn the variac up
to 140VAC in :-)).

	So the autopsy went very well.  These coils look like they will be able to
stand up well in this small Tesla coil application.  I will have to protect
the primary a bit from the winding flaw but that is very easy to do and not
serious at all.  Hopefully, the other GM coils are not defective and I'll
ground the secondary the next time which will vastly decrease the chance of
the bottom of the secondary from shorting to the primary.  In Tesla use,
there will be internal heating but these coils are designed for very though
environments and it should be no problem.  Used coils will be less reliable
than new ones.  Hopefully, the used coils will be ok once the other things
are taken care of.  Used coils would be cheaper for the beginner's but the
new ones are going to be more robust and easier to find.  There are a
number of after market coils of unknown quality.  Like the sticker says
"Keep your GM Tesla coil all GM" :-)

	Terry