VW New Beetle

One of the functions of the alternator regulator is to compare the alternator output with the battery voltage. If there is a problem it puts the light on. Either the battery is overcharging or undercharging or there is an broken earth(ground) cable/strap connection between the bodywork and the engine. Start with the easy and end with the difficult. Take it slow and follow the safety rules in your manual. After every stage, make safe then see if the fault has cleared.

If you have access to one, connect a DC voltmeter or battery tester across the battery. With the engine running there should be a reading of about 13-14.5 volts. It won't be, but should be later. Switch the engine off.

A physical inspection (prod/poke/pull) is always a good place to start. If nothing else you will get to know what the engine bay looks like with everything in place and with the covers off. Whilst looking around check for wiring that comes close to the engine and make sure the insulation hasn't been rubbed/melted away. If all the connections are good and the alternator drive belt is tight, go for the next easiest bit.

Check the connections on the battery and (if not sealed for life) check the battery acid level in each cell, top-up as necessary with distilled/denatured water. If you need to clean the battery connections remove the negative one first. Remove, clean and replace the positive connection then clean and replace the negative connection. A battery has a nominal life of 3 years. Much depends on whether it does a lot of starting/short runs, the climate and range of temperatures it has to endure. In fact, while the battery is disconnected you should take it to your local shop or battery place and get them to test it; normally for free. Also, while the battery is disconnected, check out the cleanliness and security of the cables on the back of the alternator. If the battery is good, the drive belt is tight and the earths are good the only things left are the alternator and loom wiring.

The alternator is a tricky beast and not that easy to diagnose if you don't have the right tackle. Remove it (follow the book) and take it to an auto electric shop/repair facility for testing. If it needs repair they will either sell you the bits, repair it for you or give you an exchange deal on a new or refurbished unit. You could take a chance on one from a breakers but there are no guarantees. On any repair the biggest cost is always labor. Doing it yourself saves money, skins your knuckles and satisfies your soul. You probably won't be able to get the parts at trade cost but you will still get discounts to put a smile on your face. The good news is these faults are rarely loom wiring, they are either connections or big swap out bits.

When warning lights turn on [except during starting when the key is in the "start" position] indicates that there is something wrong in that system.

It cannot tell you what is wrong, only that something is not right, and...

You need to have a qualified automotive technician to troubleshoot the electrical system, identify the defect, and properly correct it.

This answer applies to a 2004 Beetle. It might work for other Beetles, too. It's a surprisingly easy fix, even though it means removing the entire headlight assembly.

1. The first step is to open the hood. If you're attempting this repair, you've probably already opened the hood numerous times to fill windshield fluid, check and add oil, and for other engine-compartment maintenance and repair. It should also go without saying that you should do this with the lights in the off position, and with power off in the car.
2. Under the leading edge of both the passenger and driver's side fender inside the engine compartment, you can see part of the headlight assembly. There is a hook-like lever extending at about 45 degrees toward the hood. This is the most difficult part - identifying the lever. It's very hard to take a picture of it, and it's difficult to distinguish from other parts if you haven't done this maintenance before.
3. Pull the lever toward the fender and hood. Don't force it. Apply gentle pressure, perhaps using the side of a screwdriver or other tool, until you feel a gentle "pop" or a disconnect of the locked plastic parts.
4. Gently push the headlight assembly capsule forward toward the lens, using your hands or a tool. Some people describing this procedure find it the most difficult - you are sliding the assembly on its tracks which are most likely caked with dirt, and against the pressure of a rubber seal and some adhesive (Note: don't attempt to remove the lens or any outside part of the headlamp; this will just break the internal seals and possibly subject your new bulb to wet and dirty conditions).
5. Once the lamp assembly slides out of the fender, you'll see the power wire (near the front) attached with a plastic clip. Carefully unlock the clip and remove the wire connector.
6. Place the headlight assembly on a working surface with some protection to avoid scratching the lens.
7. At the back of the housing is a fairly large black plastic cover with a plastic clip. Remove to reveal the bulb assembly.
8. Unlatch the wire clip holding the bulb in the larger assembly. Disconnect the wire clip on the back of the bulb and install the new bulb. Note the orientation of the bulb base (it is basically a circle with a slice out of one side). As you may know, touching the glass of halogen bulbs usually reduces their life because of the oils and other substances on your fingers. However, unlike many home light fixtures, the ample metal base makes it unlikely that you'll have to touch the bulb itself (if you need to, use a cloth or paper towel).
9. Reconnect the power cable; you may also wish to clean the guide rails and lubricate them with dry lubricant before you return the assembly in its place.
10. Line up the assembly guides with the guide rails (this may take several tries; and there are three, the first two engaging at 9 and 6 o'clock positions). Some guides suggest lifting the lever to engage the assembly, although a firm push may be all that's necessary.
11. You may have already tested the lamp on step nine. If not, put the key in the ignition and turn on the lights. Also, check the rubber gasket where the assembly meets the fender, and adjust it for best fit. If this is the second time you've done this, it's taken you all of 5 minutes; if not, this may have been an hour repair/replacement.