[s-cars] Those mortal coils

[Paul Gailus]

This is a multi-part message in MIME format.
--
[ Picked text/plain from multipart/alternative ]
Bill m pontificated:

"If I can find out what might have caused this coil to blow,
I will surely post the answer, cause there's nothing worse
than a suddenly dead vehicle and 0 hp ATW.
I hadnt read of this happening before, but it would be nice
to know what caused it, how to prevent it and how to easily
ID and perhaps disable the offending coil."

One way of keeping a single bad coil with a short to ground
from shutting down the ignition completely would be to fuse
each coil separately. You could then at least limp along on
the other 4 cylinders to get yourself out of a jam.

The "hot" side of each coil primary is brought out of the coil
pack assembly with a separate wire, so this shouldn't be
difficult to do. During his latest surgery Bill m even noticed
that these wires are conveniently labeled with tiny numbers
corresponding to each coil. So you could even ID which coil
was shorted by checking which fuse was blown.

--Some ramblings just in case someone wants to get Haperfused:
Adding a downstream fuse of the same 15 amp rating and type
as the 15 amp fuse near the ECU is not a good idea, since one
fuse will then be just as likely to blow as the other.
It would also be possible that both fuses could blow, or
one could be left a "walking wounded."

Two alternatives would be to:
1. Eliminate the existing fuse, and add 5 separate fuses & holders
near the ECU.
2. Keep the existing fuse, but use smaller current ratings
on the downstream fuses. The thermal time constant of
the ATO "fast blow" fuses is around 1 second, which is
still a lot longer than the ignition pulse widths. So because
of time averaging, you could probably get away with 5
individual fuses each as small as 15 amps / 5 = 3 amps.
However, smaller fuses have higher resistances, leading to
more voltage drop for the same current level.
As far as I can gather,  typical fast blow fuses require a 3:1
upstream / downstream "selectivilty ratio" on their current
ratings to make sure that a fault doesn't blow the upstream
fuse. So 5 amp fuses might be the best compromise.
A slow blow upstream fuse could also be used to reduce the required selectivity
ratio, but then you'd have to use a less commonly available fuse.

Paul
--