Yeah, w/ this kind of weather lately, you'll need both fans.

1st - The mechanically driven fan that is mounted directly the engine. The fan clutch is filled w/ a wax like substance that gives in w/ resistance...hence why it sounds like an airplace taking off when you leave a stoplight, in the morning, etc. When they fail, its usually due to the wax leaking out or seizing. When it leaks out, the best test is something simple like taking a newspaper and seeing if that stops it (w/o breaking off blades of course) and/or if you can spin it freely when the motor isn't running. It should have some resistance.

When it seizes, it always sounds like an airplane...low rpm's, high rpm's. This is actually the worse of the two situations because people sometimes confuse it w/ the aux fan and continue driving the vehicle. I've seen at least two 740's where the owner continued driving the vehicle and the centrifugal force on the fan actually broke the plastic blades off, literally sending them through the hood. In the process it would break the radiator hoses, fan shroud and radiator itself...not pretty.

2nd - The aux fan a 3 stage electric fan that runs either from running the A/C and/or increased outside temps. I can't remember the code now, but the engine codes is basically saying that the 3 stage resistor isn't working.

Either way, w/ the temps here in Boise being around 110* lately, my buddies 750iL has been sounding like an airport w/ his fan going. You're aux fan is there to assist the mechanical fan w/ cooling just like the battery(besides starting) is there to assist the alternator when you're running extra electrical loads.

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