HI--I still have a problem with my 87 560sl with 130k miles. It began to idle high about a week ago, and i hope to get some advice on this great site.
I have read with great interest all the posts on high idle, and have checked on each procedure---TPS(throttle position switch), ICV(idle control valve), ICU(idle control unit or speed control), etc. When I checked voltage on ICV, it was 0. I cleaned and connected the ICV with 12V and valve worked. I figured the ICU wasnt sending any voltage to the ICV to reach the correct idle, so the ICU must be bad. Another interesting thing was the car only did this initially after it was hot---idle was correct upon start up until today.
So I'm about to bite the bullet and go buy a new ICU, when something clicks, and she idles like a sewing machine. Voltage across the ICV was 5V. I took off the 3-prong TPS connector and it idled up 300rpm, like it should. I disconnected the ICV completely and it revved way up, and went back to normal when re-attached. So i'm guessing the ICU and ICV and the TPS are working properly.
I drove the car some and then the AC stops. I get back home and put 12 V to the AC clutch and it comes back on, so i know the clutch works. After a few minutes the car goes into high idle again. what the>>>>??? Voltage reading at ICV was zero again, and disconnection everything had no effect.
Am i missing a sensor somewhere thats telling this computer that its overheating or something, and to shut off the AC and idle up to cool?? The AUX fan never comes on (i jumped it and it works) and the car is running at normal temp. I have pulled the connector on the coolant temp. sender and nothing happens. I have checked the OVP relay and the 10 amp fuse is ok. I checked all the solder joints on the ICU and they all checkout.
While the car was running i pulled the ICU out and it didnt make a difference, so i figured the unit wasnt getting any power. All the fuses are powered when the switch is on. What am i missing here?? Could the ignition switch be bad somewhere and not supply power to the ICU like it did the AC compressor?? I have read about the mercedes switches, and on this car we had to "jiggle" the switch to get the AC compressor on, but not recently and it's not working now. I have replaced the AC control unit before, which was a waste because the low pressure switch was bad and was doing it's job. I dont want to replace the ICU and do the same thing. Any thoughts would be appreciated. thanks and I'll keep reading.

As usual, i started the car this morning and it idled normally---AC still did not work. I popped the hood and after it idles for about 2 minutes the Aux fan came on before the temp. gauge even began to register. I pulled the 2 prong connector from the 105C-115C switch on top of the thermostat housing and the aux fan stopped and the AC compressor came on.
I replaced it and pulled the single prong connector on the same switch and the Aux fan continued but the AC compressor came on.
Could this be the problem with the high idle also?? I always suspected that something was telling the car that it was too hot, and could have caused a high idle situation---but the AC always worked until today, but i was always at high speed on hte interstate. I think i will change out the Aux fan switch---i guess the other prong is going to the computer to tell it that something is hot.....would this be a correct assumption??? i hope this is hte fix--its must better than the ICU solution!!! I'll keep you posted. Thanks

well, the new 105-115C switch solved the AC problem. the car ran great and heated to full operating temp. parked it, with idle perfect, and turned it off, then re-started and it went back to high idle again. back to square one i guess. i might change the coolant temp sensor on the rear of the engine- it always seems to go to high idle after it gets hot.

You seem to be doing pretty good so far. The manual on CD gives very good information on the ICU pin assignments and test proceedure.

Trying to relay that information here can be too confusing. Narrating all the ifs, ands, and buts for a technical test is too hard for me.

If you do not have the Cd send me your email address and I'll send you the appropriate pages.

randy, i have the cd, but how do i get inside the glove box to see where these pins are??? does that assembly come out in some way to test it, or do benz mechanics have a special wiring harness that plugs in some way and has long wires on it?

anyway, i think i will have to mess with that this weekend. was going to buy another coolant sensor (left rear of engine) and this mechanic told me it had to be the ICV. i check it again, its clean and is lubricated and i test it with 12V and it's working, etc. I put it back on the cold car---runs perfectly, check voltage across the ICV and it's 4.5-5V, turn on the AC and let the engine get hot for about 15 minutes in the NC sun. running great. Turn the ignition off, crank it right back, and back to 1700rpm. Voltage across the ICV is back to zero. It will not change until it cools off again. When it does, i'll let it idle correctly and stop and start it while it's cool to segregate the heat aspect. If it only does it when it gets hot, i'll just have to check all the sensors going into the ICU. God, they wanted $260 for the ICV--I thought that was lower than the ICU. Man, this is cutting into my fishing time, and i hate to break down and take it in to someone who will start the musical parts game, but maybe thats all i can do......thanks...later

My 380SE did the same thing you are experencing. I reflowed all of the solder joints on my overvoltage relay. That fixed it for a while. Now My Idle control unit gives me trouble when it gets hot. It must have a bad solder joint also. Board expands when it gets hot then no more contact. Then high idle. Seems to be a common problem with this erea of cars. I will try to reflow that unit soon will let you know how it went. $220 for a rebuilt one on fastlane. I can spend some time soldering for that amount. You can take the idle control valve off and clean with carb. cleaner then spray wd40 in it. Helped mine to life. Good luck.

well, after reading all the great posts on this site and figuring it had to be electrical instead of heat related, i think i found my high idle problem. I read about all the trouble with the OVP relay, and started noticing that my antilock light never came on. i decided to replace the OPV, and the car runs great.
i saw in the wiring diagrams that the relay power the ICU and the ABS and check engine light, etc. and the problem was extremely intermittent so i thought i might as well start there, since i was convinced my idle control valve and throttle position switch were ok, after some testing. hope this does the trick, and everything is running great, but my ABS light still will not come on. it may be the bulb, but i have to run that down now. thanks for the help and suggestions. it can get aggravating sometimes, but i love this car, and will probably be glad i have it 20 years from now.