BMW 640i xDrive Gran Coupe Headlights are dim Inspection at your home or office.

Common reasons for this to happen:

• Corroded Ground Wire: One of the most common causes of dim headlights is a corroded ground wire. Ground wires connect the bulb circuit to your car’s chassis (which serves as the ground itself). If corrosion develops on the wire (or the connection is dirty or damaged), it disrupts the flow of electricity, often enough to limit the output of your bulbs.

• Discoloring Deposits: This is particularly common with halogen type light bulbs. Over time, the inert gas within the halogen bulb creates a discoloring film (brown or gray). This builds up on the inside of the bulb and dims the light output. Replacing the bulb is generally the only option here.

• Bad Alternator: Another very common reason to see dim headlights is a failing alternator. If you notice that the lights brighten and dim as the engine revs up and down, the alternator should be checked immediately. If it fails, dim lights will be the least of your problems. The car will begin pulling power from the battery and because it’s not being recharged, it will eventually drain completely. This will leave you stuck on the side of the road.

• Yellowed Lenses: Often, dim lights have nothing to do with the flow of electricity or aging bulbs. Rather, they have to do with the aging of the headlight lens (the large plastic cover that protects the bulbs inside). Lenses yellow as they age, and this affects the amount of light that can escape and limits your visibility at night.

• Loose Alternator Belt: Dimming lights may not be a sign of a bad alternator. It might be a loose alternator belt instead. If the alternator belt is loose, it doesn’t turn the pulley correctly (it slips and then grabs, slips and then grabs). This can be noticed in dimming and brightening headlights. The belt should be checked for excess play, as well as wear and tear.