I recently received an ISV control unit (1986 and later) for disassembly
(among other things). Thank you Mike!!! This research confirms several
hunches that I had.

The control unit consists of 16 op-amps (4 LM2902 quad chips), a 4 Amp PNP
power transistor for driving the idle stabilizer valve. I would bet
that this transistor is what fails when a control unit goes bad. This
part, a BD438, is readily available. So, I think repair would be possible
in most cases.

My other bet is that if an IS valve shorts, it will blow the transistor.

How many folks out there have a *bad* IS control unit? I'd love one to
verify if it's the transistor that went south.

The unit is much easier to open the a 1.9L IS control unit (which
requires cutting). There are 6 clips molded into the case. Pry these away
from the connector bulkhead, and pull the whole thing out of the case.

Pics of the unit are here:

http://www.kenneke.com/vwfi

I am going to be having some exciting news about all this FI stuff soon.
Stay tuned!

Jon