Aux fan on slow or fast speed? If it is on fast speed then it is possible that the thermostat switch on top of the engine block is faulty or shorted. It is a 3-prongs switch which feed the Kilma relay with temperature info. You can try this

1) Remove the 2 prongs connector. The aux fan should stop. It is to turn on cooling when engine reaches 100C or 105C, depends on what part # is installed.
2) The single prong is the kilma input. Remove it and see whether the A/C comes on. It turns off A/C when it reaches 105C. This may be the problem.

Good luck.

__________________
Not MBZ nor A/C trained professional but a die-hard DIY and green engineer. Use the info at your own peril. Picked up 2 Infractions because of disagreements. NOW reversed.

W124 Keyless remote, PM for details. http://www.peachparts.com/shopforum/mercedes-used-parts-sale-wanted/334620-fs-w124-chasis-keyless-remote-%2450-shipped.html

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