In DEF only, there is a direct electrical connection between the blower and the control unit. The blower speed on other settings is controlled by electrical resistance in the entire system which in turn is driven by the difference in the desired temperature versus the actual temperature. There are also vacuum switches that control current to the blower depending on which setting is selected.
The actual voltage to the blower is controlled by pre-resistors located next to the blower unit.
This is a very complicated system to troubleshoot. For five bucks, you can get a detailed guide to operation and roubleshooting from Performance Analysis Co. Email email@example.com
or call at 865-482-9175.
Personally, I suspect you have a problem with the vacuum side of the system or the servo.
I am working through the same sorts of problems on my 107. Good luck!
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