Coupe GT ignition cut out problems

[Chris Hall]

Problem:

1985 Audi Coupe GT.  Engine has been stalling due to some sort of 
ignition cut-out.  Tach would drop straight to 0, as if a wire on the 
coil had been pulled / thermo switch had been disconnected / hall 
sender plug had been disconnected.  First time engine stalled, I 
popped the hood, disconnected the ISV's electrical connection, car 
started on the first try.  Figured "that must be it".  Reconnected it 
a week later (after flawless performance) and a day later, car stalled 
in same way.  Popped hood, disconnected ISV plug, started and ran fine 
for a while.  

Then, while sitting at a light WITH the ISV unplugged, car stalled for 
the third time.  I played with all the plugs / wires you normally 
would, connected and disconnected the ISV a couple of times, got it 
started and drove fine the rest of the day.  Cleaned up the manifold 
ground wires, put new connectors on the black wires connected to the 
coil (because it laters seemed that unplugging them and reconnecting 
them got the car to start right back up...)  Car ran fine for a few 
days until I started getting the tach cutting out to 0 with a now NEW 
engine hesitation and occasional BACKFIRE.  

Decided I'd try a new ignition module.  Got one for $10 off ebay, a 
new "Lucas" brand module.  Installed it today.  Car started right up 
and idled great (which it usually would do with the problem...).  Took 
the car on a test drive.  5 minutes in to driving, came to a stop and 
heard a slight engine hesitation.  Watched the Tach, and all the 
sudden, the tach started wildly fluctuating between 500 to 2000 rpm, 
even though the ENGINE WAS HOLDING AT A NORMAL RPM.  Then the engine 
idle started changing... going up to 1200 or so and holding, then 
dropping to almost dying (but catching itself) then up to another 
random RPM, etc.  NOT in a "dirty ISV" way, either.  Did this while 
the Tach needle was still doing it's own odd little dance.  Then at 
one point, I gave it gas and got NO response from the pedal, it 
started to mis-fire, and then died.  Couldn't get it to restart until 
the 4th or so try, and I drove home (couple blocks).  Parked the car 
and let it idle, and the engine RPM held steady, but the tach needle 
started "Dancing" a bit.

So, I ask... Can someone tell me with 100% certainty (like... you're a 
BOSCH TECH haha) WHAT SENDS THE TACH ITS SIGNAL?  I've always thought 
it was the COIL, but obviously the ICM is connected to the coil and 
the hall sender in the distributor.  ALSO, can someone tell me what 
relationship the ECU and the ICM have with one another?  (I didn't 
wanna "unwrap" the wiring to the ECU to look at the coloring unless I 
had to).  

Again, the "dancing tach needle" problem is *NOT* the "dirty ISV" one, 
because it does so with NO change in engine idle, and this problem did 
NOT appear until I switched in the LUCAS ignition module.  Oddly 
enough, when I bought the module, the cross reference the guy gave me 
was the SAME 3 sets of 3 digits, but it was "missing" the LETTER at 
the end.  I asked the guy if this meant it wouldn't work, and he 
claimed that this Lucas module cross references to my car / module.  

So I'm wondering if the new module is working properly, and has 
uncovered a secondary problem (maybe with the coil?) since the engine 
hasn't stalled with the tach going straight to 0 like it HAD BEEN...  
The few times that it HAS died since installing the LUCAS module, it's 
just kind of sputtered out...

I've been trying to figure this out for a month now (and the new 
problem for an hour or so), and although a few people have described 
the ignition cutting to 0 engine stalls, almost no one has posted how 
they fixed it (I'm assuming people talking about this a year ago 
aren't still dealing with the problem...)  One post on Audifans 
mentioned my exact situation with the ignition cutting out, and he 
said that he swapped the ICM and that worked.  

The hose near the ISV is collapsed BUT "inflates" when you give the 
car gas... and HAS been this way for well over a year.  ALSO, 
sometimes when I pull the "spare" fuse for the ECU, the car doesn't 
run any worse with it pulled... although SOMETIMES the engine runs / 
fires like garbage with it pulled (which I'm assuming should happen 
EVERY Time).

Hope someone here can help diagnose what my old (and new) problem is...

Chris Hall
"making girls cry since 1974"