How it works - vacuum system on a 1982 380SL?

[300sdToronto]

Folks

As my handle suggests I have more of a background in diesels than gas engines, and I am in sales by trade so all of my knowledge of engines - such as it is - has been picked up by reading bulletin boards like this.

I did train as an electrical engineer and so have some basic logical thinking skills. I've successfully diagnosed and fixed problems with the climate control system and the cruise control on two different cards.

I have been diagnosing the dreaded high idle when warm problem and believe my idle control valve is working properly. Folks have been pointing me towards the vac system as a possible issue but I do not understand how it works or how the vac system influences idle speed.

It's hard to test a system you do not understand. If there is a post that explains the operation of the system I would much appreciate a pointer as searching vacuum produces just too many hits!