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