at low RPM, all of my lights dim and fans slow down. is this likely a problem with my alternator or a problem with my battery?

Could be either, really. But if the battery is say over 5 years old, it could be on it's way out. Would do some checking and cleaning of alt connections, battery cables, (both ends), and clean all of the ground wires in the engine area.

Gte a free battery/alt output test. I agree about the battery being over 5 years old replace it, but it sounds a bit more like an alt problem.

 

 

nipper

If you have a volt meter you can check the voltage output at the alt to see what it is putting out. Should put out about 13.5 at an idle (min.). Another tool that can be very helpful is a inductive amp gauge to see what the amp output is. The auto parts store should at least be able to order one for you. They are relatively cheep. If the tests show the volts down but the amps up then it could be a battery.

COUld also be an old, slipping belt. Check the belt to see if its mangled/cracked/etc

The alternator is only hanging on with two bolts,

 

Undo two bolts and remove the alternator. Take it to any auto parts store and ask them to test it.

 

It's a 5 minute project.

Glenn,

82 SubaruHummer--new alternator

84 GL--new used alternator

01 Forester--factory alternator

Try buying a $2 can of belt dressing and see if it's your fan belt slipping. If so, tighten the fan belt and wheel on.

Bet it's the battery.

Turns out that the alternator belt was slipping.

 

Thanks guys.