Odd W126 Idle Control Valve with a speed adjusting screw
 

Over the weekend I was digging through a box of parts I bought at a swap meet, and found an idle control valve with an adjusting screw in the top. I was surprised as I thought these things were not adjustable.

Anyway, I soaked the inside with brake cleaner over night, then with WD-40 for half a day, then more brake cleaner to flush out the WD-40.

After I installed the valve I discovered that turning the screw clockwise reduced the idle speed and turning it counter clockwise increased it.

I experimented a bit and discovered that having the screw turned about 1/2 turn out from all the way in made the car idle at a perfect 650 rpms in gear or out of gear. Something it has only done for short periods before. It will be interesting to see if keeps it up or goes back to its old trick of idling perfect in gear, but jumping up to 1000-1500 out of gear.

Has anyone ever heard of a W126 ICV with a speed adjusting screw before? Is it possible that it's from some other kind of car?

Wow... that's interesting. I just came out of a huge ordeal dealing with the idle control system. I looked high and low for valves and control units. My impression of the whole thing is that the control unit adjusts the frequency of the solenoid in the valve so that a mechanical regulator (like a screw in the flow path) wouldn't be needed. I think 1000 - 1500 is kind of high for idling in park. I'd check your original valve to see if it wasn't sticking.

I had one that read perfect in ohms, but got stuck. Granted, I haven't tried to clean it with brake cleaner and wd-40 and it probably is worth a shot, but anyhoo. I'm guessing your original valve got stuck in the range where it effectively limited air flow down low enough to achieve ~700 idle in park.

-Troy

Check out Dave Petryk's creation at the end of this thread over in the SL forum. http://www.peachparts.com/shopforum/mercedes-benz-sl-discussion-forum/243090-idle-speed-air-control-slide-valve-confidence-check-2.html
Does the OP have a picture of this ICV?