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Old 09-04-2003, 11:02 PM
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Mike Murrell Mike Murrell is offline
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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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