ummmm.... through further research I have found out that the fan has three wires. a positive, negative and a thin wire which provides a square wave pulse from 10hz to 100hz which controls the speed of the fan.

I have tested the positive and negative wires and had a healthy 13.8v there. when I plugged the aux fan back in it juddered fowards and backeards a few times which i guess is something to do with the square wave pulse moving the fan slightly.

now I am not sure if the fan is actually working and it is something wrong with the control side?

I guess a trip to the indy is in order so that they can carry out further diagnostics?

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E90 318i 2007 N46 2.0 Manual
E53 X5 2004 M54 3.0 Sport Auto
E83 X3 2004 M54 2.5 Auto