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Should the Idle control/stabalizer valve on an '89 S4 be air-tight when closed? Mine is not. It rotates perfectly, no sticking AT ALL, but when in the closed position it kind of over-rotates past the point where it seals tight, and as a result I can fairly easy blow air through it. Is this intended by design, or a sign of failure? Car idles fine and steady. But I have the intake out to fix a leaking/sweating oil-filler-neck, and thought I would inspect everything in there as well.
Oh, and I need to replace the rubber mounts holding the ICV. Read somewhere that there might be a VW alternative. Anyone got a part number for those?
The idle control valve when in the power off position has a default airway designed to flow enough air to support a stable idle without the ac operational. Thus if the ICV fails there will be an idle when hot. If a fail condition occurs and there is a cold start the throttle has to be fethered until the motor warms up.
Fred and Dave, thanks for the quick replies. In that case, I suppose mine is OK then. I will check with power from a 9V battery and if it works fine, I will reinstall it.
The idle control valve when in the power off position has a default airway designed to flow enough air to support a stable idle without the ac operational. Thus if the ICV fails there will be an idle when hot. If a fail condition occurs and there is a cold start the throttle has to be fethered until the motor warms up.
IIRC, I lifted this from a Mercedes forum but the operation is the same as S4-up 928. The armature rotates around to the running position. It is spring closed requiring only open pulses from the LH.
(This design is simpler than the S3 ICV which requires open and close pulses and uses a throttle body bypass for idle airflow.)
I have no idea about the MB unit but the S4 ISV is pulse width modulated and for sure features a spring to close. When closed as in no signal the thing passes enough air to support a warm idle. Given it is pulse width modulated it uses high speed switching to simulate an analogue voltage output, Given this is a modulating control loop there has to be voltages where the air supply is reduced or cut off completely to modulate a situation where the rpms are running high and this is what we see as the shuttle is manually stroked- at part travel it appears to shut off completely. If that position is 4 volts that would make sense. Then when the thing shuts down or fails, the voltage collapses and the shuttle comes to rest with the default opening I advised about in the earlier post.
I have never actually tested the thing with different voltages but presumably from what you are saying that means if one were to load the terminals with different DC voltages the shuttle would move to different fixed postions as per the photos shown. I have always assumed the thing only opens full whack when a direct voltage is applied that is sufficent to drive it such as a 9 volt battery for test purposes. Must pull my spare ISV and play around a bit
IIRC, I lifted this from a Mercedes forum but the operation is the same as S4-up 928. The armature rotates around to the running position. It is spring closed requiring only open pulses from the LH.
(This design is simpler than the S3 ICV which requires open and close pulses and uses a throttle body bypass for idle airflow.)
Ken, thanks a lot. This is exactly the small gap I am seeing at "closed" (well, with power off). Makes sense that the unit has a fail-safe.
04-27-2020, 07:30 AM
Should the Idle Control Valve close air-tight?
