What do you mean by "passenger side fan"? I have '86 300E . It is equiped with a viscous fan clutch. I am not sure if '90 model is the same, if so check your fan clutch to see if it gets engaged when hot as follows as follows: When the coolant temp gets high, lift the hood while the engine is running, turn off ignition while you watch the main fan. If it stops right away when the engin goes off then it works ok. If it continues to run for few more turns then it might have a problem. Also, if your coolant is old you might want to change it.