What is the heating system doing or not doing?
The servo operates with electricity, not vacuum. If you have a vacuum problem, you will have symptoms such as the fan cutting off when you accelerate. The servo opens and closes various vacuum ports as it moves in response to the demands of the controls and temperature.
The absolute best thing you can do is call George Murphy at Performance Analysis Co. (885.482.4175) and ask him to send you a copy of his guide to troubleshooting the system. It will cost you five bucks.
I would offer to FAX it to you, but it is so good, that he deserves his modest fee.
Falls Church VA
'66 200, '66 230SL, '96 SL500. Sold: '81 380SL, '86 300E, '72 250C, '95 C220, 3 '84 280SL's '90 420SEL, '72 280SE, '73 280C, '78 280SE, '70 280SL, '77 450SL, '85 380SL, '87 560SL, '85 380SL, '72 350SL, '96 S500 Coupe