ISV Questions, 1988 90q (longish)

[KSMITH1@mailgw.sanders.lockheed.com]

Audi CIS Mavins:

I have some Idle Stabilization Valve (ISV) questions which follow after
the background information below.

Last week I was having some routine service done and complained that my 
idle was running at 1200 rpm (warm, w/o AC) and hunting.  My mechanic du 
jour hooked up a digital multimeter into the ISV circuit and adjusted
the %CO screw (in the metering head) until his meter read a sequence of
current values (he said, I didn't check the hookup) that had a nominal 
2.5 mA mean.  He suggested I clean the ISV and reset the current as 
required.  He said, reasonably enough, that the jittering readout 
indicated a properly functioning oxygen sensor.

I cleaned the ISV on Saturday, and hooked an analog meter into the circuit.
I got a steady (the analog meter has much more inertia than a digital
unit) 40mA!  There was no effect from adjusting the %CO screw the small
amounts that he did.  A second meter yielded the same results.  Also, 
while opening the oil cap kills the engine, opening just the dipstick 
improves the steadyness of the idle.  This suggested to me that more air 
was needed than the ISV was supplying.  I presumed that the ISV was stuck,
and removed it again.

Mechanically, the sliding window was free to rotate.  It has a gap 
opposite the direction of rotation that allows some air to flow even if
it is defunct.  

Electrically, it read about 8.5 ohms, either direction.  Testing with a 
power supply at 40 mA yielded no effect.  Testing at 400 mA also yielded 
no effect.  However, installed back into the car, the ISV test using a 
fuse in the fuel pump relay led to a series of clicks as described in 
the Bentley.  Before I hook up an oscilloscope, or unnecessarily buy an 
ISV, perhaps someone knows the answer to the following questions.

I haven't found anyplace in my Bentley documenting this approach to
adjusting the %CO.  Has anyone heard of this "shortcut?"  Is it used
perhaps in VW CIS tuning?

Just what type of signal does the ISV normally get?  A variable current?
Variable duty cycle 12V pulses?  Variable frequency 12V pulses.  Something 
else?

Any other suggestions?

Thanks for your collective wisdom

***                 ...Kirby    (Kirby A. Smith)                 ***
***                      2 X 1988 90q                            ***
***              ksmith1@mailgw.sanders.lockheed.com             ***
***              [=]   kirby.a.smith@lmco.com                    ***
***  Opinions expressed herein are entirely those of the author. ***