I have followed the "Spewing Battery!" thread and the "Battery Venting Cloud of Hot Acid" threads and still have a question. I have a 1978 W123 280E with 45,000(really!). I get battery overcharge characteristics ONLY when I run long and hard with the compressor on. I have a new battery, because I cooked the old one I have replaced the voltage regulator and get ~13 volts with a meter at idle and with compressor on at idle.

My question is this. What is the relationship between the compressor and the alternator? Why should I get overcharge only when the compressor is on?I get good air-co when the the compressor blows. The compressor seems to come on and go off as needed.

Any thoughts or good suggestions for further diagnosis?

Thanks, Alex

Thanks for your suggestion. I put in a new regulator about a month ago to resolve issues of unexpected dimming headlights, poor restarting performance and overcharging battery.

I thought I had it licked, but problems still continue. Occasionally, when starting after driving, I get a whole lot of cranking with no fire. Eventually it will start, though many a tense moment as my wife is again about to ask "why did you get this car?" The unexpected dimming of the headlights has been resolved. In addition to new battery, I have new plugs and wires.

The regulator I took out when replacing with a new one looked pretty good. It's "brushes" still had a good 3/8" to 5/8" left othem, so I put it on the shelf.