I would check operation of “Servotronic” if your car were equipped with one. Every one 4.6is that I bump into had one installed from the factory. The testing process is relatively simple:
1 Tap voltmeter to the Servotronic valve wiring – easily accessible at left strut tower area. With engine, running you will be measuring 8 – 12V.
2. Using ohmmeter measure resistance of Servotronic valve (The one attached to the steering rack). I am not sure what it must read, but expect it to be no more than 100 Ohms.
3. If you don’t get any voltage reading the most probable cause is defective GM3 module. You may confirm this by supplying 12V directly to the valve. Assuming the valve is functional you’ll get very light steering. This is OK for temporally work around , but not a permanent solution.
4. If you get correct voltage, reading at the valve it might indicate defective valve itself. Valve can be purchased separately and replaced on the car.
5. The Servotronic circuit is monitored by GM3 module, so it may throw code if you scan car with BMW scanner.

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2012 X5 x35i My new toy
2005 X5 4.4 Her's (Retrofitted Servotronic Steering Rack)
2003 X5 3.0 Mine (Retrofitted Servotronic Steering Rack and Heated Steering Wheel)
2001 M5 Son's car
2000 540i 6sp Sports pkg, supercharged. Son's car (Sold)
2001 530i (Sold)
1999 528i (Sold)
1975 2002 (Sold)