If your auxiliary fan is not coming on under the thermal loads you're describing, there's a failure in one or more of the following: (1) the pressure sensor attached to the receiver/drier, or (2) the series resistor, or (3) the coolant temperature sensor on the valve cover, or (4) the fan motors, or (5) the wiring connecting any of the foregoing. As Compress Ignite says, the low-speed fan is triggered by a signal from the pressure sensor (when the compressor reaches something like 15-20 bars), while the hi-speed fan is actuated by the temp sensor (when it reaches about 105C, regardless of what the A/C is doing at that instant).

As for testing the temp sensor, yes there is a test available using a multimeter. You should get the service CD for complete instructions and specs. But because you state that the aux fans were not operating when you were running A/C going up a hill and raising temps to near 100C, it sounds like the pressure sensor is not turning the fans on to low speed as it should. Remember, low fans should come on whenever A/C is engaged and compressor pressure reaches ~20 bars, even if the motor isn't overheating. The CTS (temp sensor) isn't going to do anything until at least 105C, by which time the low fans should already have been triggered by A/C pressure.

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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