I've never examined the Haldex system' date=' but I guess, the 'controller' is an electro magnetic clutch in the rear diff? Hence why you see people unplugging it on 2WD dyno days :-) If it works like other 4WD systems, then you still need some load on the transfer box (i,e, a prop or some wheels, diff etc) or it won't work. Bit like an open diff with one wheel staying still. [/quote'] The haldex is a hydraulicly driven clutch, run off of an electric pump, and controlled by a valve which is operated by a stepper motor. Thats why people unplug it.