Hi Wbain 5280,
The digital servo update is a nifty although expensive device that is sold by Unwired Tools (
http://www.unwiredtools.com) to replace a troublesome climate control control unit used from around 1976 - 1981 in some of the Mercedes vehicles. It was developed by Chrysler Corporation for the 1971 Chrysler Imperial, and slightly modified and adapted by Mercedes, they called it the ACII climate control system. The control unit combined blower speed control, vacuum pod control, and heater valve control all in one unit that was driven by a small electric motor which was operated by a differential amplifier. Unfortunately, the heater valve tended to bind up and/or leak, causing the little electric motor to keep trying to run, even with the key switch turned off, which would run down the battery. Also the 9 or so vacuum lines going into the unit would start leaking at the connector over time, killing the whole operation. If you don't have one of the cars with the ACII system installed be glad! Happily, the digital servo update takes care of all the problems except for the vacuum leaks, which can still occur, usually due to a leaky vacuum pod after 25 years of service. I hope that explanation clears it up for you!