[jdc1244]

Welcome![:)]

Let’s assume the ACC pushbutton unit is working and the refrigerant level in the system is at normal pressure. You may have a problem with the Klima relay (part number 0035452605) or the 105C/115C switch at the front of the engine (where the upper rad hose connects to the engine). If you pull the switch and the AC comes on, then the relay is ok. If not then the relay will need to be repaired or replaced. Sometimes the electrical contacts in the relay can be cleaned.

You are correct in that the tach works off of this relay (no tach amp, per se) as it senses engine speed for proper compressor operation, such as shutting off the compressor at certain engine rpms, etc.

http://www.benzworld.org/forums/forums/thread-view.asp?tid=295266&posts=2&hl=tach+amp