If the fans aren't running, the head pressure climbs to the point it trips the high pressure cutout when you're idling. That's likely why you were getting the oscillating temps when the fans didn't run.

Not sure how you're getting 21V to the fan when you're running a 12V system. Perhaps the values given are PWM steps and not volts.

The only way to really tell what's going on is to probe the voltage going to the fan motor with a voltmeter and see what's actually happening. If you have voltage at the motor, but it isn't spinning, you know the motor is the problem.

If you see the CCU has the fan set wide open but you have no voltage at the fan motor, you know the problem is in the communication line or the fan motor controller.

All the CCU readout is telling you is what it thinks its doing, not what's actually happening. If you have an open connection somewhere or the motor controller is bad, the numbers on the display are meaningless.

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Current stable:
1995 E320 157K (Nancy)
1983 500SL 125K (SLoL)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

Gone and wanting to forget:
1985 524TD 167K (TotalDumpster™) [Definitely NOT a Benz]