Car bogs heavily when the headlights are turned on?

[cohny]

I just managed to bring my E34 back from the dead with a new battery and starter, however something still seems off. There is a whirring sound coming from the engine bay that almost sounds like the alternator. I also noticed that when I turn on the headlights at idle the engine drops to like 400 rpm and everything dims almost like it's going to stall. This clearly isn't normal. Is my alternator going bad? Is there anything else that would cause the headlights to put such a burdensome load on the electrical system?

[bmwdirtracer]

The engine main ground has been cleaned and rebolted?

Does the same thing happen when you turn the heater blower on full blast?

What is the battery voltage reading with engine running? What about with the headlights on and revved a bit?

[dworthy]

+1

I like where you are going with this Chris, for it too sounds like the brushes are worn in the Alternator to me as well.

[cohny]

Yes when I was working on getting it started one of the things I did was clean the main ground.

The same thing does happen when I turn the blower on full blast, but only if you go from off to 4 quickly. If you go from 2 to 4 it's much less pronounced.

The battery reads 14.15 volts when the car is running.

I remember the previous owner did tell me he replaced the alternator, but I've owned the car for 3 years now.

[MarcoZandrini]

I’m wondering if the previous owner installed a Bosch rebuilt alternator or one from the local parts store. It definitely sounds like the alternator can’t produce enough current (amp) to meet the load. Check the alternatir’s connections. Also, remove the main belt that drives the alternator and spin the alternator to see if the bearings are shot. Check the brand of the alternator while your there. You’ll need a flashlight and a mirror to read the label at the back of it.