A '79 would not have a monovalve, it has the infamous servo. It may have gone south and it would cause the problems you describe. Another is that you have a vacuum problem and the flaps are not being pulled into place in positions other than DEF. The fan won't come on until the flaps are in place.

All this and more is explained in George Murphy's $5 troubleshooting guide. Give him a call at 865.482.9175.

__________________
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