You don't need to "overheat" the engine to test the fan. It should come on with the A/C compressor period, it runs to move air through the condenser at all times as long as the compressor is working. If its not, take it to a mechanic knowledgable about Volvos. Without the A/C compressor engaged it will still come on when the water in the radiator reaches 180-190 degrees, the engine doesn't need to be overheated to reach this temperature. If you are comfortable working on cars you can just unplug the fan and try to jump it with a fused wire to the battery and it should come on, simple test to see if the fan still works. If it works then look at the relay, then test the resistor with an ohmmeter. Good luck ............ Mike