92 500sl Aux Fan problem(s)
 

I asked this question 9 years ago and I still have not figured it out yet :( Here is the question: Aux fans come on when the air comes on they do not come on w/o the air. I thought that is how it was designed....wrong What I understand is that they should come on stage 1 when the coolant temp gets to 105C or 220F? If they still do not come on w/o air what can I check to get them working properly? I know there is relay and I know there is a ballast resistor where are these located and how do you test them and what else could it be? is there an error code for that?

If your cooling system is operating properly, you shouldn't be seeing the Aux fan coming on at all if the A/C is not in use. Your coolant temps should never be getting above 100˚C anyway, the engine driven fan should be keeping temps in check. If you're having overheating problems when the A/C is not in use (no aux fan), the fan clutch should be a prime suspect.