[s-cars] RE: STILL fighting ignition problem

[Varon H. Fugman]

>>This is the first not lethal PSO failure I recall hearing about.

I had a non-lethal POS failure... for about a day, then it became lethal.
;-{

#5 cylinder dropped out while sitting in rush hour traffic one afternoon.
(Verified that is was #5 by disconnecting injectors one at a time.)

After being parked for an hour, I restarted the car it it ran fine all the
way home (20 minutes.)  This actually worried me more because how was going
to troublshoot a problem that had gone away on its own!  (At this point, I
didn't know if it was a POS or coil I was looking at.)

Get home, let the car heat soak for 5 minutes, restart and bingo, cyl #5 is
missing agian.

Let it cool down for an hour, swap the POS modules.  Start the car, no miss.
Go for a drive.  After about 3 miles it started missing, this time on cyl
#2.  Ah ha, problem is POS, and heat related.

However, the next day it started up with a miss immediatly on cold start.
POS chanel fried for good.

Swapped pins to utilize spare channel, worked great for 2 weeks and then
guess what?  The spare channel failed!  This one just failed for good the
first time.

Replaced bad POS module with a used module with only two working channels.
So far so good.  Been driving on it for 3 months.

The point of this whole story is to confirm Bruce's findings that a POS can
go intermittant... before complete and utter failure!

>>+12 volts is present at the firewall connection for the coils and the PSO
is located on the ground side of the coil.  The PSO does not provide a
signal it provides a ground path for the coil primary as a result of
receiving a signal from the ECU.  I was surprised to discover this but
that's the way it is.

Yep, the POS is really just the "breaker points" of the system.  Quickly
open the circuit to ground in the coil primary so the magnetic field will
collapse rapidly causing the secondary to generate a good hot spark.

That's why I think there ought to be a way to homebrew a POS out of a few
parts... just need to figure out what kind of switching transistor to use!
Need to get my hands on a o'scope one of these days to see what the signal
from the POS looks like.  Also need to figure out much current we need to
switch, but it can't be too much given the diminutive size of the OEM POS
circuitry and lack of a heat sink.

>>The POS from the A4 works in the S6.

Brian, good thing you had a V6 A4 to borrow the POS module from... the 1.8T
A4 has a single POS module with a 5-pin and a 4-pin connector... i.e. 4
channels in one module!

The question I have, is there any difference in specs between the Hitachi
version and the Bosch version?  Can we just omit the "A" at the end of the
part number and freely use the (cheaper?) Hitachi version in a turbocharged
S-car?

Varon
'95 urS6

P.S. And remember, don't throw away *any* POS modules!  Even if they only
have 1 working channel... some day we may need to drive our cars with *five*
POS modules screwed to the firewall... each with one working channel!