My Dad still has his 71 280SE...how do you like yours?
Sorry for going off on another tangent...I'm fighting with overheating on my 300E as we speak.
All I know is that I can keep temps under control if I DON'T turn on the A/C. It's HOT in Texas, so that is really not an option, but at least I can rule out some things.
Other things to look at is the radiator cap, coolant mix ratio, or air in the system (not likely unless you just did a flush and refill recently). You just had your radiator re-cored, so you can rule out dirt in the cooling fins. Check the ones for the A/C radiator, just to make sure.
As with you, I haven't found anything in the Haynes manual that specifically states whether or not the aux fans work exclusively for the A/C or not. But like you, I've started pointing the finger at the temp sensor (also it's the only part of the cooling system that I haven't replaced). If you unclip the wire from the sensor, the fans should run at max speed.
I'm still testing the system to see what gets the fans going...
2009 ML350 (84K) - Family vehicle
2001 CLK430 Cabriolet (71K) - Wife's car
2005 BMW 645CI (124K) - My daily driver
2012 Mustang V6 (60K) - Daughter's car