The car is a 91 420sel. It idles to fast. The idle speed is mostly dependant on temperature. The hotter the engine the faster it idles. I have checked all the circuitry associated with the idle control module and everything is OK. All the signals look good. When the throttle switch is open the idle increases which is what I would expect. There is a square wave voltage present on the idle air control valve and its duty cycle varies a little bit. When the air control valve is disconnected, the idle runs up to 1800. When I apply steady 12 volts to the coil the engine quits suggesting that the air valve is operating, or at least has enough authority to stop the engine. But it seems to me that there is not enough authority either electrically or mechanically to properly govern the idle speed when the system is connected normally. It almost seems like it wont apply enough current to reduce the idle speed. If there is an upper limit on the current that the control module will deliver it seems to me that it should be enough to reduce the idle to acceptable limits. But its not happening. Is it possible that there is some mechanical problem with the valve? I have cleaned it out with solvent and it seems OK. The moving part is free. Do I have a weak spring or something like that? Do these things wear out and get soft or is it a go - no go situation? I have substituted two other control modules that I know work and the results are the same. I have also checked for vacuum leaks and found a small one which did not really change anything. Not sure what else to check. I hate to put a new air control valve one the car not knowing if the one I have is really bad. Any suggestions?

__________________
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

I have heard of the solder joints within the valve body cracking and needing a touchup with an iron. Also, check the throttle valve switch -- located right of the valve/3-pole connector threaded into the manifold. Job 07.3-121 in the service manual. This switch not closing can keep the idle high even if the valve and control unit are OK. See mbcoupes.com archives for more details. Cheers, Andrew Seidel.

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86 560SEC
85 500SEC Lorinser
83 300D Turbo
75 240D

Thanks for the tip. I will check the connections. I already have checked the throttle switch. I will let you know what I find. Thanks again.

__________________
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k

I didnt see a way to disassemble the valve body. It is crimped together. How do you get access to the connections ??

__________________
I got too many cars!! Insurance eats me alive. Dave

78 Corvette Stingray - 3k
82 242 Turbo Volvo - Manual - 270k
86 300e 5 speed manual - 210k
87 420sel - 240k
89 560sl - 78k
91 420sel - 205k
91 560sel - 85k
94 GMC Suburban - 90k
97 Harley Davidson Heritage Softail - 25k
00 GMC Silverado 1 ton 30k