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Re: ISV problems

From: Meron

>>Your ISV is
>>probably stuck open
>I believe the ISV is designed to CLOSE if there is a problem, if it is
>stuck open (as might be the problem) engine will surge. The ISV is normally
>closed and is modulated by the idle programmer (varying voltage).
>Avi Meron

That's right.  The idle programmer usually doesn't provide a continuously 
varying voltage, it switches between 12v and 0v very quickly (several kHz) - 
the ratio of the time spent at 12v to time spent at 0v controls the amount 
by which the ISV opens against a spring (Note, the ISV doesn't move at 4kHz 
 - its inertia is too high).  If the connector is unplugged, then the spring 
should close the valve, dropping the idle speed well below normal (as 
controlled by the physical idle screw).

1984 Audi 80 quattro
1989 Audi 100 Avant