'90 300SL fan on all the time
 

New to me car and even on a cold start the cooling fan is on even if the a/c is off. On a side note, the a/c always turns on when I start the car even if I have turned it off the last time it was driven. Is the fan controlled by the engine computer or directly from a temperature sensor. Will dig out my WIS soon.

Your's is an early M104, I believe they are still very similar to the M103 block. Check the coolant temperature sensor wires at the end of the block psg side. With mine the plug is the culprit, wiggle them while the fan is running at low temperature. That's the problem with mine, I'm leaving it for now to keep the AC colder during traffic this summer.

hey there, sounds like you might have an issue with the coolant temp sensor. i had something similar on my old 300e. might wanna check the wires on the passenger side of the block. sometimes just wiggling them can help.