1977 MGB low voltage when car is running

[rbenjamin]

1977 MGB low voltage all over, across battery terminals, and at the fuse block. 12.5 volts, when the electric fan comes on it drops to about 11.5 volts, goes back to 12.5 volts when fana turn off. Ground at battery terminal was cleaned and mounted, no improvement. The battery is not dying, I’ve been driving about an hour for the past few days, car starts up and drives ok. I think the alternator is weak. Anything I should be looking for before pulling the alternator out and getting tested? Appreciate any suggestions. Thanks.

[Vic Butler]

Check the voltage across the battery terminals with the engine running at 2,000 rpm. It should be around 13.5/8 volts. Turn on headlights and see what the reading is.
Are the battery terminals clean? Dirty ones can cause a voltage drop. Have you checked the alternator plug?. I use electrical switch cleaner to clean terminals, it works very well.