The aux fans are wired in parallel so, if they run at all, they operate together. As macdrone says, it could be a bad high pressure switch on the A/C drier. But it could also be low refrigerant charge, as you asked. The pressure switch on the drier is responsible for low aux fan speed, helping to prevent the temp rising to the point that the high speed kicks on at ~105C in the first place. But that pressure switch monitors pressure buildup in the A/C condenser, and if the refrigerant charge is too low, it will not produce the pressure levels adequate (around 15 bar) to engage the switch to tell the fans to come on low speed. At least, that's how it works in my W124.

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1988 California version 260E (W124)
Anthracite Grey/Palomino
Owned since new and still going strong and smooth
MBCA member

Past Mercedes-Benz:
1986 190E Baby Benz
1967 230 Inherited from mom when she downsized
1959 220S Introduced me to the joys of keepin' 'em goin'

There are only 10 kinds of people in the world--those who understand binary and those who don't