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OP you need to do some other things before you throw your new fan in. Before the recall this was the fan that was catching on fire. It does not catch fire anymore but it still burns up which causes big loads on the electrics. The failure starts by taking out the slow speeds then at the end there is often a large surge which burns the rest. There is a fuse behind the fuse panel in the glove box that has a 50/50 chance of being burned up. Then the wires from that fuse have been heated so badly that the insulation gets hard and can fail.
You might want to take the old fan apart and see how bad it burned up. To give reference this was mine and the fuse was blown and eventually I found a short piece of wire that needed to be replaced but not much. Others have reported much worse where the whole wire needed to be replaced. The fan will still look spotless on the outside.
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This is a thread I started a long time ago and although it is not helpful there are a couple of great linked threads in it. The one thing that you can grasp from it is all the technical writing can't be trusted. From the fan that should have been in the car to the temp of engagement of the mechanical fan. Also the wiring in the WDS. The wiring diagram from this thread should work for all cars delivered to CAN(all CAN car have the cold weather package, might be why they don't follow the standard info), if yours is a US car it maybe different.
The aux fan should be running 100% of the time(except turns off at a specific road speed) when the AC is on and if yours has not that might have helped cause the heat to climb.
It is climate dependent, in my neck of the woods the mechanical fan may never turn on, likely do to colder ambient air temps.
Without access to INPA is there a way to test the aux fan direct outside of the care. Example checking a spare used?
 

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Not sure I got your full question. The simple way is to turn the AC on with the car warm. Depending on the model year and package it will turn on immediately or there might be a short delay, but it is short. My guess is the later cars use more of the other two sensors that watch the AC system.
Don't think we ever got to testing the supply control voltage to emulate what controls the speed. You need an oscilloscope for that, I am sure this was on 68's list but don't remember him doing it. My tests with a 9v battery did not yield expected results. So as far as I know we have no idea if a short pulse makes the fan run slower or faster. A near 5v constant signal might make it run slow, or a slow pulse of 5v might do that.
Really all you can do is run the AC and see if it works. We also have no idea what the car uses to decide what speed to run the fan at, there are two pressure sensors on the AC, then there is the lower coolant output temp sensor, and last road speed that come into play. If you are having issues with the AC, or road speed sensors, and don't have the cold weather package or you car is later than the official change to the 3 wire system, your results may vary.
That said turning on the AC with the car parked in the drive warm has been a reliable way to find many a bad fan, before they start the burning up process because it seems pretty clear the lower speeds go first even though full speed may last for a longer time before it fails.
My suspicion is the damage to the fuse and wiring happens when only the full speed is working, but that is just a guess from where the damage has been found on the board posts.
Thanks. That was a great response. The car is a MY01 E39 M5. Max ac did not cause a change. I tested with another Fan know to be good. However because it's used couldn't reall rule the fan out yet. Voltage at the fan was about 11.5v. Ac and car on. Just figured there was a way to bypass whatever it is to rule out the Aux fan itself as the source.
 
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