[s-cars] Hesitation -- please help

[Bill Mahoney]

Thanks Paul for this great explanation and seemingly simple coil testing
procedure.
I'm going to try this on my five old coils.  If I can find the bad one, it
would be great to have four spares.
And please forgive my stupid question/s, but just to clarify in my
lamebrain:
I would connect these 27Volts to the coil wires.
Then -
Connect one end of the meter to the coil output and the other to the
laminated steel core and get a reading around 5 milliamps OR zero, then
reverse the connections and expect to get the inverse reading?
Also, how is this 5 milliamp number arrived at I wonder?
And BTW,... anyone with TWO S cars is a big time candidate for the Sfest
pilgrimage.  Heck at least one of them should be running;)  
Cheer's
Bill ~meggered out~ M



-----Original Message-----
From: Paul Gailus [mailto:gailus at mindspring.com] 
Sent: Saturday, August 27, 2005 11:10 AM
To: Bill Mahoney; crameradam at gmail.com
Cc: s-car-list at audifans.com
Subject: Re: [s-cars] Hesitation -- please help

Sorry, for taking so long to answer the red phone Bill. :-)
I just got back late yesterday from Ft. Lauderdale on the
first flight I could get out after Katrina went through.

Adam, one thing to clarify is whether the hesitation is more like
a flat spot ( which is more likely to be fuel related), or is it
more like a fluttering, intermittent thing? (likely to be ignition)

There could still be a high voltage breakdown issue with the
plug boots that you can't really see with just a visual inspection.

As far as testing the coils, unfortunately a Megger is not really
useful because one side of  the secondary winding is internally grounded.
The spark testing you already did is probably the best single test
short of a full coil pack swap. But if you can't arrange the swap,
there's still one more thing you can check on the coils.
There was some info on SJM's site about using a multimeter
with a conductance measurement mode. The root issue here is
that there's a diode (actually a string of individual diodes)
in series with the coil secondary to prevent a spark when the
POS first turns on. To get any significant current to flow through
these diodes, you have to apply some more voltage than most
multimeters typically apply in the resistance or conductance
mode. If you snap three 9V batteries in series to get 27 volts,
and then put it in series with a meter set to measure current,
then you can clearly see if all the diodes and coil winding are
fully intact. You should see somewhere around 5 milliamps
flowing with one polarity applied between the coil output and the
laminated steel core, but essentially no current in the other
polarity.

On my S6, all of the coils measured fine this way.
However, on my 93 S4 nearly all of the coils looked like
they had internal opens in the secondary circuit. They must have
been arcing over internally to complete the circuit under operation.
I replaced all 5 of the coils just based on these measurements,
not because of any apparent problem in running the engine.
However, after replacement the engine clearly runs smoother
over the full RPM range.

I would recommend doing the test above just to get more
info about the state of your coils. If you see any coil(s) that
don't test out properly, they still may not be the actual root
cause of your hesitation. But it would still be better to replace
them anyway.

Paul