Mercedes: 1996 S420 A/C, amp fuse, type fuses

Question
QUESTION: The fuse in my S420 (#21 20 amp under hood)has blown and blows immediatedly on replacement when trying to use A/C even with 30 amp fuse in place.  Any ideas?  Car has 168,000 miles and is otherwise perfect.

ANSWER: Stuart,
You have a high resistance or short somewhere. Does the fuse blow when you turn on the heat and does the heater blower operate normally? If the heat side work as designed, then I would suspect it to be in the AC compressor clutch, or some other part to the ac climate control.
If the A/C compressor clutch has a disconnect then separated it and see if the fuse still blows when you turn on the A/C.
The compressor is engaged to the belt drive via a magnetic solenoid clutch. When power is applied to the coil the magnet engages a slip ring and that hub connected on the compressor shaft rotated  the pump.
If the clutch checks OK you may have a high resistance at the AC Fuse. That circuit may have other things being fed through that fuse.
The ceramic bullet type fuses are notorious for corrosion between the aluminum strip tip and the copper fuse holder clips.
It has been suggested that one purchase a complete set of fuses, clean all the fuse box terminals with some scotch bite pad and install new fuses in every socket.
When you have high resistance you have heat. Too much heat and the fuse does its job by blowing to stop a potential for fire.

Good luck
Let us know how it turns out.
Rob

---------- FOLLOW-UP ----------

QUESTION: Rob:

Thanks for input; fuse is fine until either heat or air is turned on - then it blows if either heat or air are engaged.  Fuse is the plastic yellow colored 2 prong type with inverted "U" shaped area that blows between prongs.  Could it be the regulator?

Answer
Stuart,
The Knife fuses are not as susceptible to corrosion as he bullet fuses. Check your fuse schematic to see if that fuse controls any other circuits.
Is the fuse blow instant when you trunk on the blower. Make sure your have the defrost control off before trying the heat. then make sure the head control is off when you check the AC.
Did you take the AC compressor out of the circuit by disconnecting the the wires before trying the heat?
If the fuse blow is instant then there is a dead short somewhere.
If the blower cannot come up to speed then it may be in the control switch or the rheostat.
Regards
Rob