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Ignition System

11/11/2007


I'm trying to do my own research on what exactly is going on with my car.
However there seems to be a number of different ways cylinders can get a
spark. Are our SHO's ignition systems (coil-on-plug or direct ignition )? I
hope i was able to give enough accurate information for someone to even give
me a response.
thanks again
Ty
_____________________________________________________________________________
The V8 SHO uses a COP (coil-on-plug) system.

Here's my general take on the system:

The coil receives 12V+ any time the ignition is switched to "run" or
"start", but it is not grounded until the PCM wants that particular
cylinder to fire. When the ground is closed on the coil, the only
part of the circuit that remains "open" is the spark plug gap. The
high voltage differential generated by the coil causes the spark to
jump across the gap. This is "normal" operation.

When a coil fails, it is for one of two reasons. Either the coil
cannot build enough voltage to cause a consistent spark (i.e. -
damaged windings, poor internal connections somewhere, bad solder
joints, etc.) OR the coil builds voltage that should be sufficient to
fire the plug, but it has one or more faults that allow the voltage to
find ground in places other than the spark plug gap (i.e. the voltage
leaks or arcs out the side of the coil housing). In either case, you
are not getting a good spark at the plug gap every time it is supposed
to occur.

Coil failures are rarely complete and sudden on these cars. Usually
it will start out with the coil firing normally 95% of the time and
only faulting every once in a while, in a seemingly random pattern.
This can be brought on my temperature, humidity, etc. which can change
quite drastically and suddenly when you are just looking at the tiny
area surrounding the fault(s). Eventually that 95% figure starts
falling and the coil misfires more frequently. At some point it gets
bad enough that you can plainly notice a stumble when the engine is
running, or in more extreme cases the arcing of the coil voltage
outside the area of the combustion chamber creates interference (EMI,
RFI, or whatever form it takes) so significant that it causes problems
with sensors and/or the PCM. Sometimes one bad coil can generate so
much "noise" in the car's diagnostic/engine management system that the
engine just shuts down, even though if it weren't for the interference
issue, the car would normally be able to get by just fine on 7
cylinders. In these cases, unplugging the bad coil can actually be
better than driving around with that coil occasionally firing
properly.

It's not uncommon for a coil issue to be barely (if at all) noticeable
at idle, but then it gets so severe at higher RPMs that it causes the
car to stall.

Multiply the potential for issues with any one coil by 8, and you see
why sometimes we get severe headaches with this ignition system. And
worn or otherwise defective plugs will exacerbate most ignition
problems.

Dan Carman
 


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