[Biturbos4] B5 V6 "unilateral" misfiring

[Robert Myers]

Chris,

Let me preface my comments with "I have exactly zero experience with 
that V6 engine and its ignition system" so feel free to completely 
discount what I have to say.

The stock ignition of the earlier 5 cylinder s-cars operates in this manner:

1.      A full time +12v is applied to one side of the primary coil 
of each ignition coil at all times.
2.      In response to a signal from the ECU, the POS breaks the 
connection from the opposite side of the primary coil and shuts off 
the connection between the opposite side of the primary coil and ground.
3.      This collapses the magnetic field of the primary coil and, in 
turn, inductively generates a very high voltage across the secondary 
coil.  This high voltage is what fires the spark plug.

Potential problems include 1. failed POS, 2. failed (or failing) 
coil, 3.  low voltage to the primary.  4. maybe something else.

It sounds like you have fairly well eliminated the POS and coils as 
the source of the problem.  This leaves a low voltage to the primary 
as a possibility.  This could happen as a result of corrosion at the 
connector for the coil (this is, on the AAN engine, at a white 
connector on the firewall) or some other high resistance spot in that 
circuit.  This could be worth checking.

Bob



At 12:15 PM 4/14/2008, Chris Newbold wrote:
>On Mon, Apr 14, 2008 at 12:03 PM, Long, Scott G A1C USAF AFMC 88
>SFS/S4W <Scott.Long2 at wpafb.af.mil> wrote:
>
> > Is there an ingnition control module that tells the cylinders when to
> >  fire?  Do the coil packs on bank 2 share any common ground wiring?  What
> >  about common 12+ wiring?
>
>I believe the components involved are the 6 per-cylinder coil packs,
>the 2 per-bank output stage amplifiers and the engine ECU. So other
>than the amplifier doohicky, there does not appear to be any other
>component which is bank 2 specific.
>
>I'll have to pull out the Bentley wiring diagrams and trace everything
>down to see if there is any bank-specific shared wiring. This seems
>likely, though. I was vaguely hoping that someone might have some BTDT
>experience with this and would say "Look _there!_"....
>
>Thanks.
>-Chris
>
>2001.5 S4 Avant 6spd
>2004 allroad 2.7T
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