BMW 640i Power steering fluid is brown or black Inspection at your home or office.

Common reasons for this to happen:

• Contaminated with Aluminum: As your power steering fluid does its job, it comes into frequent contact with your vehicle’s rack and pinion. The rack and pinion system is made up of high amounts of aluminum which over time can shed into a fine powdered state. This fine powder can contaminate your power steering fluid resulting in a sludgy compound that does not hold the same consistency as your ideal power steering fluid.
  

This shedding of aluminum particles is natural, so you usually don’t need to be concerned with the status of your car’s rack and pinion. However, after having your steering fluid replaced, you should check its color with regularity. If the fluid returns to a dark color quickly, then the rack and pinion should be inspected.

• Water In System: Water causes power steering fluid to oxidize, which diminishes the ability of the fluid and therefore of the system as a whole. As with many components of your vehicle, the power steering fluid reservoir can sometimes trap unwanted water. This water mixes with the fluid, causes oxidization and changes the color of the power steering fluid.

• Fluid Degradation: Power steering fluid is subject to high loads of pressure, as it relies on pressure to do its job effectively. This pressure causes the temperature of the fluid to increase, especially during long drives. As the fluid rises in temperature, it begins to slowly break down as the chemicals separate from one another. This is known as electrochemical degradation and is the cause of discoloration and sludge in the power steering fluid.