The fan is extremely flaky on my SL. The culprit in my case is the pushbutton unit. If you push around on enough buttons for long enough, it'll kick on finally, then later crap out again. I've been too lazy to pull the unit and clean the contact and resolder the circuit board.

The Aux fan is the electric cooling fan in front of the radiator. If you still have the old aluminum fuses, hit the trash with them and replace with brass or copper. The aluminum ones degrade and contribute to lots of strange problems due to poor connections.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)