On 10/26/2016 5:41 PM, whit3rd wrote:
On Wednesday, October 26, 2016 at 3:19:48 PM UTC-7, Ivan Vegvary wrote:

I thought if I could see all the ignition activity at the same time

(or at least 4 at a time) I could look for anomalies and identify a

poorly firing spark plug or semi-faulty wire.


Yep, the low-voltage terminal of a spark coil does give that kind of info.
You can identify (or so I'm told) variations in fuel mixture from cylinder to
cylinder that way (higher breakdown voltage for rich fuel mixture). It
takes a 500V scale for the o'scope, though ('low voltage' means +12 to
minus 300 or so...)
A transformer clip-on probe can trigger your 'scope from cylinder
1, which simplifies the analysis if your auto has a single coil for all the
cylinders.

You have to be careful NOT to assume normal conditions when detecting
faults, which by definition, are NOT normal conditions. Stuff happens.
It's best if that stuff doesn't happen when you're probing around
with an expensive digital scope.

300V might be a fine number under normal circumstances, but,
if a wire is bad and the plug fires at a much higher coil
secondary voltage, the primary may also show a much bigger spike.
And there's always the temptation to crank up the sensitivity
to see some smaller wiggle on the trace. That may be a bad idea
in this case.

A plug either fires or it doesn't. If it fires, the thing you
are interested in is the current. After it arcs, the voltage
will be determined by the plug and wire/coil resistance/inductance.
High current fat spark good. Of course, that assumes
that the current is all going thru the plug and not arcing
to ground thru a crack in wire insulation.

If you have a clip-on current probe of known transient response,
you can clip it on the coil output and look at the relative currents
for all the cylinders. Properly insulated current probe designed
for ignition service should be safe to use on your scope.
If you try to move the probe to different plug wires, you'll probably
see more variation in the coupling than in the actual signal.

It's a vintage car. If it runs, drive it proudly.
If it runs like it has ignition problems, change the plugs. If that
doesn't fix it, work your way back toward the battery.
Run it in the dark and poke around the wires with a grounded
probe to look for insulation faults. You can learn some
interesting things by pulsing the coil with the engine off
and listening to the coil. But, you can smoke the coil if you're not
careful.

Rule number one...
If it ain't broke, don't try to fix it using your expensive scope.
Rule number two...
If it IS broke, don't try to fix it using your expensive scope,
except as a last resort.