The EHA, via the ECU, is responding to the exhaust gas oxygen content as detected by the O2 sensor. Some O2 sensors, when in their dotage, have been known to deliver signals that are reversed in polarity from the normal. Unplugging the EHA has removed the O2 feedback of fuel/air mixture from the CIS. It is, in effect, operating open-loop.

Also, there is a connection between the compressor control unit, and the idle speed control unit, as you would expect.

Re: thread title
"delima", or, deli ma: the matriarch of the delicatessen

Oh, I understand that fully, but for some reason, only when the A/C is on is the CIS is doing the opposite of what it should be doing. Ive also replaced the O2 sensor a year ago, and flew thru emissions with no problems this year. I really want to reverse the polarity on the EHA control, but I suspect that wont do too much just for the plain fact that it is virtually just a coil inside. And relays for example activate with either polarity.

Today I did a little test. On my way home, I turned on the ac; and power totally disappeared. I pulled over, unplugged the EHA, and took off again.. Power was almost normal, car didn't have issues with accelerating. SO something is going on.. Ive read thru a few old threads about others 420sel having power issues, and there really wasn't much of an outcome. One person replaced the CIS computer, and the power came back. But none of these were related to A/C use only.

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Just my MB's:
1990 420sel - Winter DD - 410,000km
1990 560sel - absolutely showroom mint - 360,000km!