The fluid shouldn't have a factor in kickdown speed (but should be checked anyway!). And to the best of my knowledge, there is no kickdown fuse.
Check to see if the transmission is kicking down at lower speeds... IE, go 15 mph, floor it (into kickdown) and see if it kicks down into first gear.
You might need to adjust something with the transmission.
2006 E350 w/ 155k miles (Daily Driver)
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)