2006 MCS conv. Last summer my alternator bit the dust. I replaced it with a rebuilt one. The replacement looked brand new and identical to the one I removed. Since my car is in winter storage, I like to putz around with little projects in the winter (external resistor fix for one). When starting up the car I noticed a groaning sound (was afraid it was the supercharger or timing chain). Finally I realized it was the alternator bearing, took it apart and returned it to the shop. They replaced the bearing and said I was good to go.
After the install, I decided to check some youtube videos and test the alternator's charging abilities. At idle everything seemed good (around 14.3 volts) but under load it would drop to 12.5v (when the high speed cooling fan kicked in). Back to the shop I go. One of the diodes was shot, and they replaced it. They bench tested it in front of me and said all was fine (under full load however it seem to show about 13v but the tech said it was providing good amperage ??)
From the videos I watch, they mention to turn on as many accessories as possible. I decided to test the alternator again last night (my car is on jack stands, so I am not able to drive it). Everything started off good at idle. Then turned on headlights, radio, rear defroster, A/C with full fan all good voltage at around 13.8. After about 10 minutes of this the voltage starts dropping slowly to 12.9 V. When revving up the engine, the voltage would go back up. Suffice it to say, after about 10-15 minutes of full load testing, the alternator got very hot to the touch. Does all this sound normal? My battery seems to test ok (with a harbour freight load tester) but it is about 7 years old.

 

Is that 12.9 at idle? Does it go up at higher rpm? Some drop at high load is normal at idle. It’s getting hot because it’s working hard.

 

It does go up to 14.5 when I give it some gas.

 

I feel it all checks out, according to your explanation. They will get hot under heavy load! The running voltage should be around 14.2 volts, as the BMW electronics require that. under full load at idle, it will drop lower.

 

Just keep the load down under idle if you can. Don’t turn everything on unless you need to. Sounds like it’s fine.

 

I would also recommend checking all your major electrical connections for corrosion, make sure they are all clean and tight. Corrosion increases resistance in the circuit(s), which can cause the alternator to work harder.