Hello Folks, very glad to have found this forum and I need your kind help on attacking this high idle issue.

Car: 1991 560 SEL with 103000 miles
Problem: idle at 1400 in P and 900 in D.
Action so far: measured idle control valve and get 4.8 ohms and 3.2 V with car running. That give me about 670 ma of current which is too low. I read that the current should be between 750 to 1000 ma.

Based on my search, there are 5 more items to check: OVP, ICU, Throttle switch, vacuum leak and injectors.

Since I don't have any warning lights on nor any problem starting the car, I don't think it is OVP. My next action is to check out the idle control unit. I already took the unit out and could not find any visiable burn or loose connection. It is such a small unit and I don't think I could bent it or flex it as some posts suggested.

Questions to experts: !) is there any way I could use my DMM to measure something on ICU to find out if it is bad?
2) I also read that the throttle position switch is near the cold start injector which is next to ICV. I read that one could measure if it sticks to the open position. Could you please let me know how to do that?

Few things to try. With engine running, unplug the connector on the ICV. Idle should increase. If it does, then valve is working and current is probably ok. This also proves that the OPR is ok since it supplies power to the ICU which in turn provides power to close the ICV. You might still want to remove the ICV and spray some carb cleaner in the valve end to get rid of the dirt buildup since it's possible that it may not be closing 100%. 4.8 ohms sounds about right. Mine is reading 5.0 ohms.

The Throttle Position Switch (TPS) is very easy to test. With engine running at idle speed, pull off the TPS 3-prong connector. Idle should increase. If it doesn,t, either the switch is defective or the ICU is bad. The TPS connector is just behind and to the right of the Cold Start Injector (CSI).

Cold and/or deteriorated solder joints are sometimes a problem with the ICU. If flexing the board didn't produce anything, it may be worthwhile to just go ahead and resolder all the joints just to be sure. If may take awhile but it beats having to buy a new one at around $300. I don't have a schematic of the ICU and don't know anyone that does. Maybe someone on this list has one and could make it available to the members.

Another thing to test. With engine at idle, turn on the AC to its MAX setting. Your idle speed should increase. If it does then the ICU is sensing this condition properly and at least this portion of the ICU is working. If the engine runs slower, then the ICU is not sensing the AC compressor is turning on. This indicates that the ICU is malfunctioning and needs attention. If the engine speed stays the same, then the ICU thinks that the A/C is ALWAYS ON and this could be the source of your high idle. Again this points to a problem with the ICU or even possibly the A/C Controller Unit (ACU) which generates this signal to the ICU.

Apart from the other items that you mentioned, I would like to add another as a potential problem source: The Cold Start Injector. If the CSI does not shut off completely, then it could be "dribbling" extra fuel into the intake manifold. Proper operation calls for the CSI to ONLY be energized when the engine is intially started in order to provide an extra burst of fuel for starting.

Testing the CSI:
After the car has started, unplug the connector to the CSI and make sure there is no voltage present on the pins. Even without any voltage present, it's possible that the CSI may not be closing fully and could allow extra fuel to enter the intake. The only way to prove this theory is to remove the CSI (held in place with two small hex head screws) and see how the engine runs. If you decide to try this you would need to fabricate a small metal cover plate to bolt in place of the CSI during testing. Place the CSI in a small leakproof container to observe while the engine is running. You might even be able to observe that the valve is leaking in this way.

Good luck and keep us posted on your developments.

T.C. Mann (aka TechMann)

Turn the car on and let it sit for about one minute, idling at 1200 on P. Turn A/C to Max and the idle drops like a rock and stables right on 800.

However, it is not obvious to me that this result indicates "that the ICU is malfunctioning and needs attention." Could you please explain?

The A/C Control Unit (ACU) sends a signal to the ICU whenever the A/C compressor is on. When the car is at idle, the ICU is supposed to increase the idle speed to compensate for the extra load placed on the engine by the compressor. Without this compensation, normal idle speed would drop below the recommended limit.

The ICU only needs to make this adjustment at idle and not at normal driving RPMs. How does the the ICU make this determination? It does this by sensing the position of the TPS. If the TPS malfunctions, then the ICU won't increase RPMs in response to the A/C compressor. This is why I suggested performing the A/C test.

The ICU also uses the TPS to trim the amount of fuel delivered by the injectors by way of the EHA (Electro-Hydraulic Actuator) based on the position of this switch. The ICU is supposed to reduce the amount of fuel injected when it senses the TPS in the "Idle" position. This is why when you pull off the 3-prong connector, the idle speed should increase since the ICU no longer senses an idle condition.

To Test the TPS:

When you pull off the 3-prong connector, idle should increase if the TPS is working correctly. Another way to test is to use a continunity tester on the male (stationary) end between the middle pin and the pin furthest from the CSI. At idle or when the engine is off, the meter should show a short between the pins. FYI, the middle pin on the male side of the connector is not grounded but is simply one of the switch contacts. On the female cable side of the cable, the middle pin IS grounded. This is where the TPS gets its ground connection from and uses it to ground the idle sense input pin of the ICU. Make sure the middle pin ground connection on the female cable side is good since this could also cause a problem.

T.C. Mann (aka TechMann)

In other words, since my idle went down upon MAX A/C is engaged, it points to a bad TPS or ICU.

Since my idle is already high, do you expect it to go even higher once I pull the TPS? I have just got myself a digital multimeter. I guess I could measure the resistance "between the middle pin and the pin furthest from the CSI (cold start injector)" when car is not running. If the TPS is good, I should get 0 ohms.

One more question about TPS. Would it be the cable that is bad? When I was at one of these parts web sites, I saw a "throttle switch" and it looks just like a cable.

I guess high idle is one of the most common problems for W126 cars. I hope this discussion will benefit many otehrs. You are a great help. Thank you very much.

One question here! How do you set the Idle Air Control Valve Properly? And how much should idle raise when you unplug the 3 prong switch that tells the computer that the accelerator is released? Mine raises from about 800 to 1100 when I unplug the 3 prong switch! And I at one point turned that screw on the IACV and I think I have it back where it should be roughly but would like to correct it to the proper position! Of course when doing this I have to re-adjust mixture to make everything ok since I change the amount of air going through!

Edit: When I pull the 3 pin plug it idles at 900-1000! When I pull the PLug that goes to TPS I get no change! When I pull the EHA I get a lower more rough idle and when I pull the IACV I get a slightly hiogher idle. Test on the TPS gives me infinite resistance at closed throttle all the way to just before WOT. Then I get 0 Ohms at WOT! This is for a 1986 190E 2.3 8 valve with the KE-Jetronic II not the 1987+ KE-III! Idle is slightly felt when in P or D and stopped! Runs fine everywhere else in the power band! Also new cap, rotor, plugs(copper Bosch), wires, OVP, freshly cleaned main ground, O2, Water Temp Sensor, Engine Temp Sensor, Vacuum thing in sensor group, new water pump, cleaned injectors and ICV, and cleaned IACV, all new rubber hoses from IACV to rest of intake and Zero Position right on target as well as mixture!

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~Jamie
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2003 Pewter C230K SC C1, C4, C5, C7, heated seats, CD Changer, and 6 Speed. ContiExtremes on the C7's.

1986 190E 2.3 Black, Auto, Mods to come soon.....

I have cleaned the ICV with WD-40 etc and it did not help. And when I pull the power plug off ICV while the car is running, the idle does shoot up to 2000 RPM.

Is there any way I could test the TPS? Is the middle tip the ground so I could just follow the V change to see the TPS is bad or not?

I will do both the AC and CSI tests this weekend and I will report back my findings.