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If you look in the FSM there's a detailed section on how to test the air idle control valve...
...I've been meaning to do it on my W201 for a while now but I need to make up a test lead so the current can be measured.
I have a bit of an uneven idle that I've been chasing and I'm of the opinion (but a bit stuck trying to check and confirm) that this is due to an uneven voltage going to the air idle control valve. I did change the alternator to a new one so I'm pretty sure that the source of all electrickery is good...
...for some reason or other summit else is disrupting the signal to this valve. I put it down to either the fuel "hydraulic" sensor (can't remember what it is called) or the oxygen sensor causing the grief.
At the moment I can't really help - all I can say is that this CIS system is infuriatingly simple and yet so difficult to diagnose and fix.
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A pointer for your problem could be
1) The joint temperature sender - this should detect coolant temperatures below 80 degrees C and then tell the computer to bump up the idle.
2) Air / vacuum leaks in the hoses
3) There's some sort of connection between the automatic transmission and the CIS brain I think
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