> What I have found is that the idle stabilizer control unit has a burnt
> transistor or two. I'm guessing this could be the culprit, though I
> can't say for certain. What I can say for certain is that the price tag
> for a new ISCU caused me to spit a mouthful of miso soup all over my
> computer screen -- so now I have that problem too.

If you can't find one at a pick-a-part, probably your best bet is to watch
ebay for a used one to come up. They go for around $70 there.

I'm tempted to try to fix this unit or try to find one in a boneyard.
> Does anyone have any kind of schematic for the 251 907 393B? I saw a
> great post on this from Jim Davis, circa 1998; but only limited
> information was available.
>

I started to draw up a schematic with the intent of emulating it digitally,
but I gave up after I got about 1/4 through it. It's a mind-bogglingly tense
design. It uses a series of hand-tuned analog feedback loops that keep the
engine turning a certain number of revolutions per minute, all without
actually being able to count either revolutions or minutes! The real
giveaway that it's too touchy a design to fool with is those two resistors
soldered into standoff clips. Those mean that some guy at the VDO factory
took the new units off the line and performed some series of arcane tests,
probably with a custom made Type2/T3 idle controller tester, to decide what
values of resistor should be soldered to those clips. Every one I've seen
has had a slightly different combination. It's a marvel of german
engineering, but it's a complex 1970's solution to a problem quite easily
handled by a contemporary 25 cent digital microcontroller. I am currently
mapping the behavior of the idle controller with the intent of someday
creating a simple digital replacement. Really, it can't be TOO hard: count
RPM, if too low open idle air valve a little more; if too high close a
little. Simple, right? We'll see, I guess...

--
John Bange
'90 Vanagon - "Geldsauger"
"We'd tell a monkey how to peel a banana, if he said he was peeling one in a
Vanagon."