Why is 40 amp maxi fuse keeps blowing in w202?

Assuming you have not been altering the wiring in the car, you most likely have a ground fault or blown relay. The 40amp maxi fuse is for the air pump and goes to the air pump relay module. First use a meter to confirm that the wire leaving the fuse block (red/yellow) has continuity to the relay. If the wire is chafed or broken, it could be touching the body metal and causing the fuse to blow. If the wire is okay, the relay is the next thing to check. Applying 12 volt positive to terminal 4 (with the gray wire) should cause the relay to close - you should hear it click. If it clicks the next wires to check are the brown wire leaving the air pump (make sure there are no breaks or chafing anywhere) and the brown wire leaving the air pump switchover valve. If these wires are all good I would consider replacing the relay or take it to the dealer. Good luck.