Hi folks. I have a 1996 C220 that seems to run perfectly except for a mild idle problem. At operating temp with the climate control sytem "on" the car will have a quick and small drop then return to normal in rpms accompanied by a faint "click" sound in the dash (like a relay would make) and a brief dimming of dash readouts. It does not surge above normal idle. If I turn off the climate control or if I switch on the "EC" mode, the problem seems to go away. I checked the overvoltage relay (000-540-67-45) fuse (fine) and cleaned the relay contacts and replaced it carefully so as not to push in the pin sockets. Should I just try a new relay ? I've read all the posts related to this topic. Is there a way to test the relay on this car? I saw other posts mentioning "pin 3" of the diagnostic socket, but the c220 diagnostic socket looks differently. I do have a fairly good multimeter (Hewlett-Packard), but without special auto features. Any suggestions?
The "Fast Lane" parts section of this website lists the "old" 000-540-52-45 relay, but the Parts Shop lists the new one (000-540-67-45)...I don't get it. Otherwise the car runs great...no hesitation, stalling, etc. I'd really appreciate any advice you can provide. Thanks in advance!

Hi there,
Are you sure it isn't just the A/C compressor clutch cycling the compressor on and off? I don't know abut your model, but on my model this happens in all climate control positions except "EC". It could cause the idle speed to vary, and could cause the clicking sound you hear. Why don't you look at the A/C compressor and see if it's the culprit? You should be able to notice the end pulley start rotating and then stop rotating in accordance with your clicking noise.

Richard Wooldridge

Thanks, RIchard.

I'll check it out. Does the a/c compressor cycle on even when the climate control setting would prevent that e.g. a relatively high temp for the thermostat?

Thanks.

Yup... It cycles in all positions and settings of the climate control switch except in the EC position.
Richard Wooldridge

since you have a good VOM i would check voltage at the battery at idle, with acc's on and then off. i found that my 300e was only making 13.2 volts at idle with no acc's on. with them on it droped to 12.7, not enough to keep the battery charged, and make the car run right, as i discovered sitting on the side of the road. i may need to rebuild my alt, wich from reading previous posts is pretty easy. anyway one of the big wigs can tell you for sure. i replaced my battery with a bigger one and have not had any more problems, but my alt output is still marginal at best.

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