I'm having trouble diagnosing a high idle on my 1990 300SE and reaching the end of what I'm able to do myself.

Replaced all the vacuum lines and connectors, new plugs, wires, rotor, new fuel pump and filter, coolant flush, tried three idle control valves (no difference what so ever between them and tested to be working) changed both idle control valve hoses, new idle switch, new potentiometer, filter, oil, transmission flush and filter. Nothing I've done has the slightest improvement or change over the RPM, in P and N at around 1800RPM and in drive at 1000 RPM.

Ordered an overload protection relay and should be installed tomorrow .Car has 85K miles

Brought it to a local mechanic and he told me it could be the MAS Control Unit. Could this cause high idle and what does it do and can it be tested? Can't find anything about this module online?

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

Based on the schematic from the FSM, it doesn't look like it:

http://i.imgur.com/g9RFq1A.png

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(4/11/2020: Hi Everyone! I am still taking orders and replying to emails/PMs/etc, I appreciate your patience in these crazy times. Stay safe and healthy!)


82 300SD 145k
89 420SEL 210k
89 560SEL 118k
90 300SE 262k RIP 5/25/2010
90 560SEL 154k
91 300D 2.5 Turbo. 241k
93 190E 3.0 235k
93 300E 195k

Thanks, very helpfull. It was loose in the socket and I secured it again, didn't change anything.

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).

Jorn - you've likely seen this thread:

89 300E, W124, M103 High idle... SOLVED

There's a bit of sobering truth by Frank Reiner in post #7

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Mike Murrell
1991 300-SEL - Model 126
M103 - SOHC
"Fräulein"

I did, I've spent nights reading up on it. I think I'm getting close, today I got the idle down to almost normal by adjusting the Air Flow Meter-Potentiometer, only for it to go up again when I restarted the engine...

See my other post: Air Flow Meter-Potentiometer OEM vs Ebay

After reading that, I decided it was maybe time for professional help...

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1979 Black on Black, 300CD (sold), 1990 Black 300SE, Silver 1989 Volvo 780, 1988 300CE (vanished by the hands of a girlfriend), 1992 300CE (Rescue).