You may have a bad sensor rather than a bad servo - all or nothing temperature problem. You may also have a a bad vacuum element or no vacuum to the system - works only on DEF. DEF is the "default" setting - bypasses all vacuum and other controls to clear the windshield.

If you call George and pay him five bucks, he will send you his troublehooting guide - this will be a great help to you.

__________________
Chuck Taylor
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