1985 745t

I've been having this funky issue for awhile and can't figure it out. Basically at idle my dash voltage gauge (yes yes, I know, it's not made for specific accuarcy) will drop down and then fluctuate around 13 volts or so (going by the gauge top and bottom numbers). It just won't stay stable.

However, when I'm driving it will generally stay stable above the middle, say around 14.5-15 volts solid. On longer drives it doesn't even fluctuate, regardless of what I turn on or off in the car.

My question is whether this is the harmonic balancer/pulley or something else. I would think that, if it was the balancer, I would experience the flcutuation in all situations... correct? I did a visual inspection of the pulley, but its hard to see whats up at that angle.

I can say that the crankshaft pulley is the only piece in there that is rusted (fairly throughly, although it seems to be only surface rust on the non-belt touching parts and appears solid).

Any help is greatly appreciated.

Thanks,
rt