High Idle 1985 380SL - Mercedes-Benz Forum

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post #1 of 3 (permalink) Old 03-10-2012, 09:10 AM Thread Starter
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Date registered: Aug 2008
Vehicle: 1985 380SL, 2009 E320 Bluetec
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High Idle 1985 380SL

I have posted before on the high idle for my 1985 380SL. I really haven't solved the problem but have some more info.

It runs at high idle cold or hot. I thought it might be the idle control unit but I don't think so. I got one that I know is working and it is still high idle. When I accelerate the vacuum gauge goes to nearly 0 and the car nearly stalls until I let up on the accelerator. I can nurse it up to speed but only with the slightest of pedal pressure.

What is odd is that if I unplug the idle control unit while it is running it keeps running at high idle (even with the unit unplugged). I would have thought it would have died but it did not.

I also pulled the OVP relay while running at high idle and it still kept running too. I would have thought if you pulled that relay it would have stopped also.

Based on that info does any one have any thoughts or things to check to narrow it down?

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post #2 of 3 (permalink) Old 03-10-2012, 09:57 AM
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Vehicle: 1985 500SL + 2009 GLK (previous 250, 280SL, 230TE, 430 ML)
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the Idle Speed Controller adjusts the on-off ratio at the auxilary air valve based on the difference between actual and target RPM, typically 600-750 RPM depending on engine temperature and gear position.
The ISC output transistor is a weak point and often fails.
Power to the ISC is via the Overvoltage protection relay, also a weak point, but a failure here will light the ABS warning light unless the bulb is blown.
The ISC is also powered via a 8A fuse.
The auxilary air valve sets the RPM to approximately 1800 RPM when the ISC fails.
The auxilary air valve is easy to check, apply 12V and listen to a solid click, then measure the voltage when active, should be less than 10V and more than 2V if the ISC works OK.
Good luck !
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post #3 of 3 (permalink) Old 04-07-2012, 09:07 AM Thread Starter
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Appears like I found the fix - ICU Card

I found it to be the Idle Control Unit. I found that if I took the IC card (Integrated Circuit) out of the box and twisted it the idle went back to normal idle. I then took a magnifying glass and a low power very fine point soldering iron and soldered every joint figuring that it might be a cold connection. I can't tell which one in particular did it but one must have because when I put it back on it worked fine. You have to have a steady hand and good eyes (or a magnifying glass) but it is doable. You can't put extra solder on it or you are likely to bridge a circuit and then the IC card is shot. You could also burn out a transister if you use too high a power soldering iron. It was a shot in the dark and it worked. You can probably find a used Idle Control Unit on ebay.

It could be other things too so you may want to be sure its your Idle Control Unit (ICU)before you easter egg hunt. You can try the "twist it" trick and if it works you'll know its the ICU or you can borrow an ICU from a working car and plug it in and if it works you'll know its the ICU too.
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