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Pete, Mercedes Mechanic
Category: Mercedes
Satisfied Customers: 28766
Experience:  Over 15 years experience as a Mercedes dealer technician
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High idle fix for 1985 Mercedes 380 SL

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High idle fix for 1985 Mercedes 380 SL
Hello,I will be glad to try and assist you with this,Welcome to Just Answer.


Check the over voltage protection relay located under the dash on the side of the body on the passenger side,there should be a fuse box in this area and near this fuse box should be some relays,one of these relays will have a fuse in it,check and see if this fuse is blown as this is common and if it is then it needs replaced and should correct this.

If the fuse is good then the other common component to fail on this vehicle as I have had to replace several is the idle speed control unit.This is a small box/.control unit that fails internally and when it does it causes the idle to stay high like this here.

This should be mounted to the left of the blower motor on this vehicle and should be a small black box.

In some cases it is just loose solder joints inside of it where the capacitors attach internally on the circuit board and if you have a soldering iron you can repair these bad solder joints.

Let me know if you need anything else,click accept if you feel satisfied with your answer as this is how I get credited for my time,Thanks Pete!
Customer: replied 6 years ago.

With the engine running can I take voltage readings on the connector for the idle air stabilizer? Can I check the Ohms for the Idle stabilezer and what should it be?


Thank you!

Unfortunately I don't have the info on the tests for this as the data I am using doesn't go back this far and cannot recall the specific test procedures.

You can check the relay for the blown fuse which is located as part of the relay on top of the relay,if it is o.k. then you can remove the idle speed control unit and disassemble or remove the circuit board from it and see if you find any loose solder joints and repair any found.

If you want I can opt out of this question and put it back on the list to see if any other experts can assist better.

Thanks Pete!
Pete, Mercedes Mechanic
Category: Mercedes
Satisfied Customers: 28766
Experience: Over 15 years experience as a Mercedes dealer technician
Pete and other Mercedes Specialists are ready to help you
Customer: replied 6 years ago.
Is the idle speed control box behind the glove box and has a 12 pin connector?
It should be behind the glove box area and next to the heater blower fan.Below is a picture of what it should look close to.Minus the stickers on it.

Customer: replied 6 years ago.
I checked the idle control modual and all circuit board connections are perfect. The overvoltage relay fuse is ok and I replaced it precautionary. I still have a high idle. I need Ohm and voltage check proceedures for the various components in the idle speed circuit.... How can I check to see if I am getting the correct signal or voltage to the idle speed solenoid, coolent temp., oil temp, etc.
I apologize as my info doesn't give me the correct readings or test values.I will have to opt out and put this question back on the list to see if any other expert can assist better on this.

Thanks Pete!
Hi, perhaps I can help. Can you tell me at what rpm the car is idling at?
Customer: replied 6 years ago.



The idle is approximately at 1800 rpm when in park. This high idle moves the car along at over 30 mph which makes it difficult to drive in the city and requires much braking.


I would really like information on testing voltages in the circuit such as wires to idle speed solenoid, power to idle control box and Ohms of the various components such as:


cooant temp sensor

oil temp sonsor

idle stabilizer solenoid

thermotime switch

and anything else I could test in the circuit.


Thank you!

Ok. I'm going to attach the testing for the complete idle control system but the first thing that you should do is check the idle valve itself and see if it works. I assume that when you disconnect the connector to the valve when the motor is idling at 1800 the engine rpm does not go any higher. If that is the case it indicates that either the valve is bad or that you have complete loss of idle control. The various sensors would have nothing to do with the problem. To test the valve you need to get a couple of jumper leads to apply power and ground to the idle valve while the motor is running. Disconnect the wire connector from the valve first and briefly apply power and ground to the valve. The motor should drop down to a very low idle or stall completely. Click on the test instructions below. Let me know if you have any questions.

Testing Idle Control System