I disconnected it and measured resistance over it with throttle at idle and "off idle"... At idle: contact, "off idle" no contact...

You need to prise off the plastic covers at either end of the front face of the potentiometer. They protect four screws which you can loosen to allow adjustment of the unit without removing it. I've just been messing with mine in a bid to sort out a rough idle and swapped it with a known good one. Turned out it wasn't the unit that was faulty but the two prongs inside were not making contact with the inner face of the pot. So, although the idle problem sees to have gone away (again) I suspect my Idle Control valve is faulty as I have been unable to make it move inside, even when applying current to the contacts. Anyone have any experience of testing these things?

As for replacing the pot, it is apparently available as a separate unit as a Bosch part, but not here in Indonesia where you would need to either buy secondhand or replace the entire air flow sensor unit. I've read in other forums that you can get the replacement part for $47 from Halsey Imports in the US, who apparently are only reachable by phone. I've got the info at home so if you need the part number let me know.

Good luck

here you go

3437224035

Ok, thanks. I will try to remove it and see if a can get some better function in the one that I've got...

Thats about the same price that i got from my MB dealer here in Umeå, Sweden, today when I went to check the price for an new one... Quite expensive i would say

Probably solved now...

If seemed that the problem was the contact of the connectors at the microswitch, not the switch itself that was the problem. Now the car idles fine! :-)

Thanks everybody! :-)


Edit: English and spelling errors... ;-)

The microswitch or the TVS?

I don't understand what you're saying. Previously you stated that you checked both the microswitch and TVS, and the test results you posted indicated they were functioning properly.

Do you mean that the problem was at one of the wire connectors to the microswitch?

Feedback is important on this issue because idle problems are common, and the "things to check" responses are all over the map.

Duke

With all the technical imput here, I know this answer sounds silly, but I had a problem like that once on a client's 91 300E. It always idled at about 1100 in park. I too checked everything and finally solved the problem with the advice the Germans hammered into me when I was serving my apprenticeship. They told me that sometimes one drop of oil can take the place of 3 mechanics. I solved my problem by taking ALL the linkage apart and lubing it. That means removing the levers from the shaft. The car idle was returned to normal. It's worth a shot.

Peter

__________________
Auto Zentral Ltd.

Yes you understood me correctly. And pardon my english. But it was the CONTACT of the wire connections at the microswitch that I eventually found to be bad. I cleaned the wire connections and with a little force I made the contacts narrower so that they would not loose contact again. The switch itself was ok, and while testing it the first time I never noticed that it was bad contact at the connctors. I guess you understand now(?) :-)

Thanks for the clarification. I cannot in good conscience critize your English, because I don't speak a work of your language.

Duke

Oops. I guessed I overlooked the closed with fix part, before I blithered on about the lubrication.


Peter

__________________
Auto Zentral Ltd.

No harm done! ;-)

If the fast idle returns in the next few days, then replace the switch altogether.

Mine idled normally after cleaning the switch, but the problem returned after a day or two. A new switch permanently corrected the problem.

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2008 E350 4matic / Black/Anthracite

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Gone but not Forgotten:
2001 E430 4matic, 206,xxx miles, Black/Charcoal
1995 E320, 252,xxx miles, Black/Grey
1989 260E, 223,00 miles, Black/Black

Of cource I will keep that in mind since the location of the problem was found!