Three problems related to power steering pump have been reported for the 1999 BMW 540. The most recently reported issues are listed below.
Power steering pump broke causing accessory belt to be thrown from engine at highway speeds, resulting in loss of steering and power.
Vehicle was driven at highway speed (55-60mph) and a loud metallic 'thunk' noise was heard as if something struck or bounced off the underside of the car. The serpentine drive belt was discovered to have come off the drive pulleys, and upon closer inspection the power steering pump was discovered to be loose. The upper mounting ear was broken, and the lower mounting bolt was missing entirely. It appears the break in the upper mounting ear cause the lower bolt to vibrate loose, and in turn the serpentine drive belt was dislodged. The result was sudden loss of power steering that nearly caused an accident when attempting to turn at low speed due to sudden and unexpected increase in steering effort.
Started my car and heard a thumping noise and found out it was my belt being chewed up because the power steering pump brackets broke.