Symptoms of a failed Servo?

[DCM]

I have a 1980 300sd with a failed AC and Heating system.

A week or two ago, I posted a question about identifying the proper servo unit in the junk yard and received very helpful responses.

So far, I have not made it to the junk yard but am going this week or next. Last night, the thought occured to me that maybe after finding one (if I am lucky), it might not solve my problem.

Would someone here describe the functions of a servo and the effects of a failed servo?

Also, what model and year of cars might have the newer aluminum bodied unit?

Thanks