I am a new owner of a 500 SEC and I am trying to fix a couple problems with the car and could use some technical assistance.

Problem 1) The car intermittantly idle's at 1700 RPM's for no apparent reason. I have had the idle jump from the normal 600 RPM's to 1700 while sitting in traffic. I have heard the the fast idle relay is faulty on these cars and there are also problems with the OVP. I have checked the fuse in the OVP and it looks fine. Is there a way to trouble shoot this so I don't end up buying unneccesary parts?

Problem 2) The heat does not work all the time. If it isn't working I can turn the car off and on and it will work fine. I checked the mono-valve and the rubber gaskets seem intact. I think the problem is with the vacuum switchover valve because the times the heat works I can hear the switchover work from behind the control panel. How would I make this work all the time?

Thank you in advance for your help and I look forward to reading any responses.

For the idle, you may wish to check the throttle switch, at the base of the throttle. It tells the computer the car should be idling. (Switch is closed when the accelerator is not depressed, open any time the pedal is depressed).

The 1985 has a different idle control than my 1989, so I can't help much after this. You may wish to search for idle problems with the same year 380SEL. I think these are the same system, and I recall noting some posts on this model.

Good luck!

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1989 560SEL (172k)
1989 325IC (122K)
2004 Suzuki Volusia LE (3500 miles)
2005 Yamaha Road Star (20 K miles)

If you're getting warm air all the time but no blower motor sometimes when the engine is running and the blower normally will run with the key on but engine off, it may be the (oddly enough) ignition switch. Try jiggling the key, it may come on. Learned this through the search function here, would sell my car without this forum!!

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Earl McLain
'02 C230 Kompressor
'89 560 SEL "Frau BlueCar" (retired April 2004)

I saw that ignition problem as well. The blower will go on but it will only blow cold air. I really think the problem is with the vacuum behind the dash. Thanks for the help.

Check the monovalve diaphram if not getting heat.

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Mike Tangas
'73 280SEL 4.5 (9/72)- RIP
Only 8,173 units built from 5/71 thru 11/72

'02 CLK320 Cabriolet - wifey's mid-life crisis

2012 VW Jetta Sportwagon TDI...at least its a diesel

Non illegitemae carborundum.

I've never found the vacuum switches to be the problem. They are either on or off, there's nothing to go wrong.

If you have a vacuum problem it will probably be one of the vacuum diaphrams has a leak somewhere. Or, the CCU (Climate Control Unit) has a short in it.

Check vacuum first.

I checked the monovalve and everything looked good. The only reason I think it is the vacuum is because the times the heat does work I can hear something click over behind the dash. It could be the switches though. Is there a way to test it?

For 30 bucks it is worth a shot to try to fix the monovalve. I bought a new OVP relay since it was 1/4 the price of the idle control unit. If that doesn't fix the problem I will end up buying the unit. The idle control with replaced in '97 and I don't think the OVP ever has so I thought it was a good place to start. Thanks