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Old 09-04-2003, 11:02 PM
Mike Murrell's Avatar
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Join Date: Feb 2000
Posts: 2,141
Question 91 300SEL - Idle drops when A/C is on

Ign. system checked out last summer - plugs refreshed at that time. Do not believe this problem is related to ignition. Car starts very easily all of the time - hot or cold. No missing out or other poor performance once underway. Approx. 105,000 miles on the clock.

At cold start up, car idles at approx. 700-750 in D with A/C on or off. I see little in the way of idle speed variation with A/C on or off when cold. This is what I've experienced since I purchased the car in '97.

A month or so ago and after 30-40 miles of highway driving in hot weather with A/C on, I pulled into the driveway and found my idle around 500 in D. Sorta rough, but engine not stalling. Turned off A/C and RPMs shot up to 750 approx. Turn A/C back on and RPMs dropped down again to about 500 - again - sorta rough idle, but not stalling.

The problem has occurred maybe 3 times in a month - sporadic, but always when engine is good and warm after a fairly long drive.

Tonight this reoccurred after returning home from a 30 mile highway drive with A/C one. I let the car sit about 2-3 hrs., then restarted and activated A/C. No idle drop.

OVP is a Siemens(Austria) - single 10 amp fuse model - part 201 540 38 45.

MAS is a 22 pin unit - part 011 545 79 32.

I do not have an ABS light on condition that can be associated with OVP problems. Just for the heck of it, I reseated the MAS a few weeks ago. I never lose A/C by the way, just get the idle dip when switching A/C on when hot.

I hate throwing parts at problems, but the intermittency of this makes diagnosis tougher. Searching the archives, I'm inclined to suspect the OVP.

Could a faulty OVP be behind this?

Thanks for your time.
Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC

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Old 09-05-2003, 07:20 AM
it leaks, its german
Join Date: Mar 2002
Location: raleigh nc
Posts: 1,111
OVP's on 116 &7 mills make 'em idle high, not low. On 103's they will drop, however OVP issues are normaly accompanied by ABS lights and such. The idle control unit might not be seeing a/c signal. Check the wiring diagram for your car, locate the input feed for compressor signal and see if the signal is there. If you clean the ISC valve and have a/c on signal to the unit, about the only thing it can be is the unit.

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Last edited by joe p; 09-05-2003 at 08:32 AM.
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Old 09-05-2003, 08:27 AM
Join Date: Mar 2000
Location: Gainesville FL
Posts: 6,844
The idle control system is more intricate than the service info implies. Let me tell you a bit of its characteristics.

You can read that there are an idle switch and a decel switch involved. They let the controller know when to control. But! There also is involvment of the hall speed sensor (on the speedo) and the airflow potentiometer.

I'm not sure how much the hall sensor plays in the idle control decision, it is involved with decel cut-off. The airflow potentiometer does seem to play a significant part in the decision to "Control".

"Control" can be measuered with a tach and the same arrangement used to measure EHA current. The readings will be in the 600-900ma range. Control will mean that a motor running at 650rpm at 600ma will see a 50ma increase when the AC is turned on. It will do a feedback control of engine speed under varying load (trans in gear, PS turning) by varying the current up and down to maintain engine speed.

If "control" is not initiated then the system works in a fixed state, usually around 6-700ma. In this state the idle valve is positioned for a fixed amount of air. This is the state the thing is in while driving down the road. Remember if the idle tried to maintain "control" as the car drove off the throttle (in the valve)would be held shut and a sudden stop would stall the motor.

So, your condition seems to be either one of no "control" or one totally dead (these are not the same condition although they give similar results). If the idle valve is not powered at all on the 103 cars it will assume a "limp home" position. This position is slightly greater airflow than a warm idling motor would see unloaded and in "Control". This will give a slightly greater idle speed unloaded but will of course not compensate for for load. Adding AC and placing it in gear might stall the motor if the steering wheel were then quickly turned.

The initiation of control while needing the idle switch and decel switch relies on the airflow pot for the real decision. It seems that condition will not happen till the pot voltage gets below .7v.

Tons of threads on this.
Steve Brotherton
Continental Imports
Gainesville FL
Bosch Master, ASE Master, L1
33 years MB technician
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