Well Vanagonites, it seems that the task I asked you to help in was a bit
too much, no responses yet...
Maybe no one has a spare working ISCU lying around waiting to be tested,
and probably most are wary of removing it from their vans for testing for
our joint effort. But the problem is I can't make much progress without
feedback from someone with a working ISCU, because mine is defective.

OK, so lets make it simpler. Of those that have the later 1986-1991 ISCU
(NOT the one mounted near the Coil and having two round connectors, but
rather the one hidden behind the right tail lamps and having 10 flat
pins), I need you to do this simple check (this time, without dismantling
anything) and posting your results as replies to this thread:

1. This test requires that your idle is behaving OK, i.e. the throttle
switch is adjusted OK, and ICV & ISCU functional.
2. Turn on the ignition (don't start the motor!), go to back of the van,
remove the motor cover, and listen/feel if your ICV is buzzing.
3. If it is, report here. If not, it's either a problem with your system
or the ISCU is tied to the fuel pump relay and not to the ECU relay. To
test, you'll have to check it with the motor running but it will be more
difficult to hear the ICV buzzing.
4. Move the throttle out of idle (by pushing where the cable is connected)
and check if the buzzing stops.

report your results here.
Thanks,
TM