has to be an electrical issue if you already checked air pump manually & it turns on. I would still change relay anyway & try that first

 

Where is that pump located?

 

on c320 it's located on the front top of engine you can see it after you pull off motor cover

 

Thanks for your response.
I have a S430 and now I know where is that air pump located.
After I pull of the motor cover, I found this air pump disconected. I connected again but realize that it stayed on constantly. I decide to leave it disconected since I know this should run for aprox. 20 sec.
Any ideas?
Thanks.

 

I would try air pump relay I believe located in fuse box under hood on driver's side next to master cylinder