Is my Alternator dead?
 

I have a 2002 BMW X5 3.0 with 150,000 miles on it.
The battery is 2 years old, I was driving one day and the car just died and the red battery light came on, I was able to jump it but it would just died after. The car with the engine running voltage is 11.9V and with out the engine running about 13v if I recall correctly.

I tried pulling codes off of it and got:
A8 Thermostat, map cool jammed
Missfire cylinder 3,4

Does the car throw codes for a bad alternator?

Alternator or regulator is NG.

Don't pull "low voltage" codes, many systems fault out with low voltage.

Is the regulator a separate replaceable part? How do I check that

The regulator is part of the alternator.

Based on the OP X5 at 150K, I would say Alt has provided a long life. Ready for replacement.

Best string on Testing a BMW Battery and Charging systems. E39 but everything is the same except some of the air intake system parts
DIY how to test a BMW E39 battery & alternator (discussion) - Bimmerfest - BMW Forums

The alternator in my 2001 3.0i failed very suddenly like yours did, a few months ago at about 170k miles. After taking it apart carefully and fixing it, here's what I learned:

- The only problem was that the brushes had worn out. Surprising to me that this would cause it to abruptly go from working to not working, but that's what happened.

- Both Bosch and Valeo alternators are used in that era. The "alternator assemblies" of both brands are interchangeable - that includes the alternator, the voltage regulator (which is attached within the cover on the rear of the alternator, and is a separately available part, which includes the brushes), and the pulley (different diameters between the two brands).

- So if you want to replace the whole thing, including pulley, buy either brand.

- But if you don't want to replace the whole alternator, you can either replace just the brushes (soldering them in), or buy a new VR (which includes brushes). To be extra clear, the Bosch VR will NOT fit in the Valeo alternator, and vice versa.

- To do that, step one is to find out exactly what brand and amperage you have. Due to the existence of incorrect information, I would recommend looking at the actual alternator in your car. Don't rely on just looking it up. It can be seen by removing some of the intake items (takes a couple of minutes to do that), then with a flashlight, mirror, camera, etc. you should be able to find a label on the back of the alternator and take a photo for comparison.

- Once you have removed the alternator, replacing the VR is easy.

- My June 2001-build, model year 2001 3.0i E53 had a 120-Amp Valeo, so that would be my first guess for you, but as I said, I would look at the actual part in your car before relying on this or anything you can look up on RealOEM, etc.

I posted a summary of my repair on here somewhere.

When mine failed, I did not get any codes. In your case, it sounds like your charging system has definitely failed. My guess would be that the other codes, if they are new, are loosely related side effects of the charging system failure.

Yes (see my previous post). The VR is available as a separate part. It can be replaced by removing the alternator (*), then removing the rear cover from the alternator. Simple as that.

The VR can also be rebuilt, just replacing the brushes, which is the single most likely thing to have failed.

(*) I think that if you really know what you're doing, you may be able to do it with the alternator in place. Even knowing this, I did not even try. Sometimes it is better to do 3x as much easy stuff than to try to do the near-impossible.

Normally if the engine is not running and you have the key in the run position, the "red light will always come on". If you have a bad alternator, the red light normally come on before the engine dies. That will be to alert you that the battery is not being charged.