PeachParts Mercedes-Benz Forum - View Single Post

dpetryk · #9 05-06-2002, 11:38 AM

In my experience, it could be the throttle body switch. Im my 91 420sel, when the switch was not making contact when it should, the idle was about 1200-1300 rpm. Sometimes it worked and sometimes not. I checked the throttle body switch contact resistance and noticed that it bounced around quite a bit. The resistance should be less than 2 ohms when closed. It was bouncing around2 to 50 ohms and was "noisy" and not repeatable. It was a simple position adjustment of the switch itself. It was on the edge of switching when the throttle was closed - most of the time. After adjustment everything is OK.

Check the resistance of the switch and be sure it is closing when it should.

However even if the electronics is OK the valve seems to have an authority problem. Let me explain what I mean. It started with my 87. The idle was always hi. About 1200 RPM. I could not get it to come down. When disconnecting the valve connector - the idle went up. When applying 12 volts to the valve - the engine died. I connected a variable voltage source to the valve and was able to regulate the idle speed quite nicely. I was then convinced the problem was in the control module since the valve seemed to work. NOT. The valve had the ability to control the idle, I demonstrated that. So why didnt it work?

The control module was fine. It seemed that it was not able to supply enough current to the valve to bring the idle speed down far enough. Thats what I mean when I say it didnt have enough "authority" to control the idle. It was trying but it did not have enough strength. I bought a new valve and installed it and low and behold everything was fine. So now I was determined to understand what the problem was.

I took the "bad valve" apart - I mean completely apart. I could find nothing. I checked the stroke vs current & voltage signature from the new valve and checked it against the bad one. No observable difference. The bad valve had no observable difference to the new one except it was not shiny. So I accepted the fact that there was still so something I did not understand and I let it go at that until my 91 had the same problem.

This time I reasoned that there was something physical in the valve portion causing the problem. Thats when I got real agressive on cleaning the thing. I worked on that thing for hours until I could get no more carbon out of it. And it worked.

There is another phenomom used to drive these valves. The control module modulates the current applied to the valve with something called "dither". Dither is needed to overcome something called "stickshion" . Stickshion is best described as something like sticky friction. Its where the valve does not want to move shoothly and porportionally with the applied current. Because things get sticky it wants to jump rather than slide smoothly. So the electronics applies a low level rapidly varying current to it to constantly wiggle it and keep it free so it wont stick in one place. You can feel the dither when the engine is running. It feels like the valve is buzzing. This buzzing effect makes the valve move smoothly even though it is full of gunk and other sticky stuff.

My conclusion is that over time these valves get crudded up and wear mechanically and the result is that it takes more current to drive the valve shut. After a while the control electronics cannot supply enough current to bring the idle down and hence the idle creeps up. Cleaning it can cure it if it is not worn out. Dither makes the valve move thousands of times per minute and the resulting wear changes the stroke / current characteristic of the valve to something beyond what the electronics can compensate for. So try cleaning it really well and if that dont work, grab your wallet.