Alternator bad or something else...?

I have a 1972 Series III. The front headlights lights are dimming and the horn is not working. Neither has been a problem in the past. I put a volt meter on the battery (engine off) after charging it overnight - read 12.4 volts. Turned on the engine and throttled her up a bit. Put volt meter on the battery while running and it read 12.2 (should be 13 to 14 with motor running). Alternator is only 3,000 miles old (see picture). Belt looks great and taut. Spinning like a top with the engine on and not making any funny noises.

What is the best way to "test" the efficacy of the alternator? Advice requested. Other suggestions welcome too. Thanks in advance.

I will note that the horn and headlights seem to be on the same circuit. I would deduce that is the problem if it were not for the fact that the battery is reading 12.2 volts with the engine running

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Sounds like the alternator has died or there is a wiring issue. I'd remove the plug at the back of the alt. and be sure all looks well there. Also be sure the engine is properly grounded to the chassis.

Good advice Erik, as well as engine check that battery is properly grounded, a simple pull of the alt. take to Auto zone they can check it with a diagnostic tool,,, 2P,,,

Finally tracked down the problem. Alternator was fine when tested. It was the dashboard charge light. I removed the bulb from behind dashboard. Ran continuity test on bulb - ok. Checked to make sure socket had power - ok. Put a touch of Vaseline on the bulb threads and screwed it back in. Appears the bulb had previously been perhaps 1/8th of a turn shy of a good connection with the socket....Started it up (charge light illuminated upon turning the key). Voltage at battery was 12.6 engine off, now 14.5 engine running. These trucks can drive you nuts. At least some quality alone time in the garage and a few cold beers. #Winning

All's well that ends well. A few cold ones always helps me, while I wrench on the old LAD!

Am I to understand that all the power from the alternator runs thru this bulb on the way to the battery? And with the bulb having a bad connection, power is deverted? Sure seems like a lot of current or Amps going thru the bulb. My 64 has one of those red bulbs, but never would have thought that's how it works

I am no electrical genius. Actually a novice. My reading and following of Rover forums tells me if the bulb or socket it sits in is shot it creates an open circuit such that the alternator does not charge the battery.

Moral of the story, if battery not charging AND red charging light doesn’t illuminate when ignition turned on (but truck not yet started), check the charge bulb ($1.50) before assuming the alternator is cooked ($125). That could be the culprit.

Others can give more detailed and elegant answers.

The alternator warning light completes the circuit than energises the field coils in the alternator , so if its blown or interrupted the field isn't energised so no output. Some vehicle have a resister in parallel with the warning light which maintains the circuit if the bulb blows. The old dynamo type generator self energises due to residual magnetism and produces voltage as soon as it rotates.