Idle Stab. system puzzle

[DeWitt Harrison]

The ol' 5kcstq has really got me scratching my head this time.

Initial symptom was low idle rpm. The last time the engine
was operated, a month or so ago, idle stab. valve current
and Oxy duty cycle were dead on.

The first thing I did was to drive the car to see if it would
clear up on it's own. Heh, heh. I should know better.
I proceeded to hook up current meter for idle stab. valve
and duty cycle meter for oxygen sensor control sys. in case
mixture needed adjusting. (The two go hand in hand as
many if not most of you know.)

With the engine at temperature, I saw that the idle stab. valve
current was 100-150 mA too high, 430 mA being optimal.
This seemed odd but I tried tweaking things anyway. Shortly,
the current suddenly climbed over 1000 mA and then switched
to zero mA. ??? This has happened 3 times now, once with a
spare controller. One question is whether the idle stab. controller
has a current limit shutdown feature. After some amount of
ignition-off time, the idle stab. valve current comes back for a
time. Another question: would opening the idle stab. valve
more, that is driving it with a greater duty cycle (more current),
cause the idle rpm to rise or fall?

Things found good so far include zero ecu error codes, most
signals to the idle stab. controller, oxygen sensor wiring and signal
to the ecu, idle/full throttle switch and wiring to ecu and idle stab.
controller. At the moment, I do seem to get a good oxygen duty
cycle signal from the ecu although, initially, it seemed not to be
present at the test connector in the engine compartment. I did an
ecu output test and the oxygen frequency valve buzzed nicely.
Btw, is there a test to force a fixed oxy. control system duty cycle?

Things not checked include the rpm (I think that is what it is) signal
from the ignition coil primary and a signal of some sort from the
ecu to the idle stab. controller. Can you just connect an rpm reading
meter to terminal 1 of the coil module (green wire)?

Thanks for any thoughts.

DeWitt Harrison
'88 5kcstq