I am going crazy here with my 88 300e(w124). My idle is too low at initial start up. Once warm it has a mind of its own. Sometimes it will idle high (only in park or neutral) and sometimes normal. I tryed adjusting the air/fuel mixture but at pin 3 on diagnostic connector I am getting almost 12v even with O2 unplugged. The idle switch is working fine. The car has been boosted a couple of times. Does this sound like classic overvoltage protection relay? Any help or ideas where else to look would be greatly appreciated. I don't want to go through the car and spend a pile of money to find the problem. I appreciate all the help I have had on this forum and think that this makes a benz more affordable.

It sounds like the OVP which you already mentioned. Did you check the red fuse on top of the OVP? The OVP is less than $70, depending where you buy it. MercedesShop should have them.

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J.H.
'86 300E

If you want to use the #3 pin you will have to view it with a DVOM that has duty-cycle readings or possibly with a tachometer.

The signal is a 12v square wave with constant frequency and a varying duty cycle. A voltmeter will read this as either 12v or the average depending on the meters method. If you have voltage you probably don't have an OVP problem though.

What you really need to be looking at is the current being fed the idle controller. It should be around 6-700ma at idle. It should raise 50ma with the A/C and should go to 8-900ma when the throttle is openned. A bad OVP can cause the current to drop to as low as 3-400ma before the engine dies. This kind of OVP failure is the exception to what I stated about OVP as a result of voltage at the pin #3. In this kind of failure the OVP isn't dead it is just a poor circuit.

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Steve Brotherton
Owner 24 bay BSC
Bosch Master, ASE master L1
26 years MB technician

Have the same problem here. I was told to replace the "Idle Speed Actuator" No change then they told me it was the "Cold Start Valve" Still no change. The idle fluctuates from normal to 2000. Have not been able to find any consistencies on when or why. Sometimes if you restart it goes away other times no difference. The dealer told me the only thing left is the main control unit and the cost is around 3K. I am not sure what to do next. Let me know how you make out.

Thanks.

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Dean

90' 300CE
80' 300D
80' 300SD

Thanks for the response. I am puzzled myself. I don't want to chase around the problem and just buy a bunch of new parts. Have you checked your 02 sensor?

Not yet, do you think that will make a difference?? I am open to any sugestions, havent found a cure yet. It sure gets agravating at times.

Thanks


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Dean

90' 300CE
80' 300D
80' 300SD

Problem solved. It ended up being the throttle position switch. It was easy to test and the connections are right in front of airflow meter. At idle it was not giving continuity. I cleaned the connections and also cleaned the throttle stop screw and voila. Thanks to everyone for all their help.