My XC 70 now has a condition where the radiator fan runs at high speed as soon as the engine starts even when it is freezing outside. Where to begin? Stops when I shut off the engine. No CEL, no codes stored.

The logic escapes me: When the A/C pressure gets too low, the engine fan runs on high. It doesn't matter if the A/C is running or not. I suspect after 14 years, the pressure is probably low and the A/C will not work when it is hot. Of course, this time of year it is hard to test.

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Keeping it running is better than buying new

If it turns out to be the AC pressure, is there a way to override this feature without repairing the AC. ( Stopped blowing cold last summer )

No need to 'repair' the A/C, just get a can of R134a at your local auto parts store and add it to your system. Because it is too cold for the AC, turn the dials to defrost - that will engage the AC enough to inhale the can.

If you unplug the pressostat switch, cover the ends with tape to keep dirt and water out. It is a 2 wire switch on the low pressure line.
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Keeping it running is better than buying new

I added a can,the system was so low the compressor would not engage. About 3/4 of the way through the can, the fan shut off, but the compressor never engaged even set to defrost.
Ambient temp is 33F

I hope the can was at 'room' temperature. The gas expands with heat. I would wait for warmer weather before adding an more, so long as the engine fan doesn't start up again.
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Keeping it running is better than buying new

If it's the A/C pressure, or a faulty pressure switch, causing the problem I think you can just unplug the wires from the switch. That way the circuit can never close.

I wish those darn wiring diagrams were still available online.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)

Either one of two sensors, or both, will trigger the fan to ON. One is the pressure switch on the A/C system and the other is the temp sensor in the radiator. You could try disconnecting one at a time and see if the fan will shut off.

If neither sensor seems to be the problem then maybe the fan motor controller is hung up in run mode. In that case you could try, with the ignition OFF, jumping +12vdc directly to the controller and see if the fan runs - it shouldn't. If it does run then I would try a different controller.

Another thought has to do with the A/C pressure switch. If you are running the defroster then the A/C compressor is engaging and applying pressure to the pressure switch. I don't know if that would trigger the fan on or not. Maybe another respondent could shed some light on that possibility.
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Current rides: 2005 Volvo S80 2.5T, 2003 Volvo V70 2.4NA, 1973 Volvo 1800ES (getting ever closer to road worthiness)