David,

I'm not sure how much it will help but if you go to page 81 of the following wiring diagram, it shows how the fan system is wired. I wasn't able to access a 2009 specific diagram, but 2008 and 2010 are identical to each other for cooling fan wiring. So I surmised......

http://www.volvowiringdiagrams.com/volvo/XC70%3AXC90%20Wiring%20Diagrams/TP39152201%202010%20V70%20XC70%20S80%20Wiring%20Diagram.pdf

I don't know how deep you've gotten into it so far, so this may be old hat, but from the diagram we can draw some conclusions about how it all works:

First off, note they DON'T show the module separate from the fan motor.

Main power is supplied from the fuse box (15/31) via fuse 11A/43 and for some reason they are showing fuse 11A/44 which "appears" to be unrelated but you might want to check it anyway.

The red/wht wire carries the power from the fuse box to the fan module. Be careful here as that wire should be hot at all times, even when the ignition is off, allowing the cooling fan to run after the car is shut off.

Components 7/8 and 7/16 are the A/C refrigerant pressure sensor and coolant pressure sensor. They tell the CEM when either the refrigerant pressure is high enough or the coolant temp is high enough to require the fan to click on. If either of these two conditions are met, the CEM sends a signal down the violet wire to the fan module which triggers the fan on.

(The coolant sensor's function is twofold as it also provides info for the gauge, which is probably why Klaus was asking if the gauge works properly. Whacky gauge possibly means a whacky sensor. But your car just has an idiot light.....)

The blk/blu wire provides the ground. I see that, according to the diagram, on the 5 cylinder turbo cars the ground wire goes through some sort of junction point on it's way to earth. I wonder where that junction is and what it's condition is. Hmmm.....

Maybe someone that knows better can say whether it's safe to short the wires at either sensor to see if it triggers the fan, and in the case of the coolant temp, does it trigger the idiot light on. It would also be nice to know what the expected resistance is across the coolant sensor at various operating temperatures. In the case of the A/C pressure sensor you could disconnect the wires and check for continuity through the sensor when the A/C is on. I don't know if simply turning the A/C on creates enough pressure to close the sensor, or if it only closes if the ambient temperature is high enough to get the refrigerant pressure up to a certain point. If the latter is the case, then it might just be that you aren't experiencing a condition, with either A/C pressure or coolant temp, that would call for fan on.

I would be very surprised if it was a celsius vs fahrenheit issue. If that was the case, you would think there would be unique part numbers for each, as these things are supposed to be plug-n-play.

OK, I'm starting to ramble on - just wanted to throw out some thoughts.