Fluctuating (low) voltage: what does it mean?

[gathermewool]

With the A/C on yesterday, the V was dropping below 12, which scared me. I ended up buying a new alternator and installed it 2 hours before picking my sister up from the train station.

FWIW,at idle, fully loaded, the new alternator was only reading in the mid to high 12s. I checked a buddy who has the same exact car with a recently installed reman'd alternator and his was similar.

>1,000 RPM and on the highway is where the MAJOR difference was. Before, it would fluctuate from low to mid 13s. After the install it stayed planted at 13.9V +/- .1V at anything >1,000 RPM. I feel a lot more confident that the Subie won't leave her stranded any time soon now. : )

I got her to 206,500 miles. Hopefully, she'll taker her to 300kmiand beyond!

 

[mechtech2]

If you absolutely need more voltage at idle or very slow speeds, you can get an overdrive pulley for the alternator.
It's simply a slightly smaller pulley.
Rebuild shops will have hundred of odd ones lying around.

 

[SteveSRT8]

That's one of the coolest things to see in a newer vehicle.

On hot sunny days when the AC is roaring our GM products will wait a minute or so at a light and then very slowly ramp the idle up until the voltage comes up to near 14.