how can I find out if a headlight being out is the bulb or the ballist

[40v-V8]

crap

[Boston Driver]

if it is the ballast, the bulb will work in the other headlamp.

[40v-V8]

i'm not very bright

[Boston Driver]

...although the bulb is not a cheap item to replace either...

[TeddyBGame]

BD,
It sounds trivial but it is not. It's very tricky for a 1st timer; if you're not careful, you could damage the bulb....then you'd have no headlights...and that would kinda suck.

40v-V8:
Here is my recommendation (i've dealt with these housings on/off the car):

Starting with the headlight with the burnt bulb/balast:

1) just buy a D2S bulb on ebay ($40)
2) remove the black cap (BC) from the back of the headlight housing (you will have two metal straps that will need to be unclipped)
3) slide the BC away from the headlight as best as you can (you won't be able to remove it completely as the balast wiring goes through it)
4) you will have a thick black wire leading from the balast to the inner housing; at the end of it will be a red plug that connects to the back of the D2S HID bulb (low beam).
5) rotate the red plug 90 degrees counterclockwise (I think) until it slides out. If you are careful, the D2S bulb will stay put inside the headlight housing.
6) take your new bulb from step 1 and insert into red plug (be careful not to touch any glass part of the bulb; the oil from your fingers will ruin the bulb)
7) find a friend or family member to turn on your headlights (you can dawn some sunglasses and look away from the beam if you want)

If the bulb lights up, great! Then you can swap out the bad bulb with the new one. In the case of replacing HID bulbs, it is always a preferred practice to replace both at once (like shocks, tires, turbo's etc). You can find a set of new matched D2S bulbs for under $100.

---------------------------------------------

If it doesn't light up...then you'll need a new ballast.

In that case, you can:
1) buy new headlight housing from the dealer for $650+

or....

2) find a used ballast on ebay for about $150. Make sure to buy one from the C5 platform (a6/s6/allroad/rs6) and NOT from a A8, A4 etc..etc. The ballast from a C5 has a certain length of balast wiring that needs to go through the back cap. It would be best to if you can find one on ebay with the small black cap attached to the ballast wiring.

Something like this...see link:

2a) swapping out the ballast will require removing the entire headlight housing (in most cases). Feel free to search the forum..as there are lots of posts/pics/threads on this topic. The balast swap is easy.

Good luck!
-Ted[list][*]http://cgi.ebay.com/ebaymotors/BMW-E39-X5-Aud

[Boston Driver]

...since there is more room to maneuver the bulb out of the headlamp. It is actually quite impossible on a 2.7T on the driver's side wihtout removing the headlamp, but I have never had to do this in the 2.7T. The passenger side is easier if you remove the airbox in the 2.7T.

However, since 40v-V8 has a 4.2, this is not an issue for him...it is easy!!! Also, consider the list of cars in his sig...he is no novice when it comes to cars.

[Boston Driver]

the Phillips are a perfect white for xenon, Osrams are a bit more yellow.