Battery light is on after installing a junkyard alternator....why is that?

Baaaad alternator!!!

If you get an alternator at a junk yard it might not be any good, you can take it to an auto parts store where they can test it.

With GM cars in general, I have found it is best to change the battery and alternator both out together. If one is weak it will take out the other in no time. These parts are both dirt cheap at a junkyard. Believe it or not, a junkyard delco alternator or battery is much better than a duro=last silver rebuilt in mexico.

You may also still have wiring problems remaining from the previous failure. GM dummy lights and gauges are notorious for being inaccurate, to the point that they hardly even function. At least rig up a volt meter that you can keep an eye on while driving, for a few days, to verify your alternator is actually outputting the right voltage.

the ground wire from the body to the engine is bad

I had a problem like this. My alternator tested good, but the no charge light came on a lot.

My pulley had a wobble, It looked difficult to replace the bearing so I replaced the alternator
with a second hand one from the junkyard that was in good condition. Then the no charge
light didnt come on anymore.

Check the alternator pulley while the car is running and look for a slight wobble of the pulley.
Also, remove the belt and pull up and down on the pulley to check for movement. Im not certain
this is your problem, but it sounds very much like the problem i had with the same symptom.

And have the battery tested at your local auto parts store.

fit a new reconditioned alternator and not one from a junkyard

the problem is either in the batterie or alternator. junkyard alternators are a 50/50 chance of working. you should go to your local parts store and have them do a batterie/ alternator test. they dont require you to take them out the car.

Optomum Alt Charge is 14.5 so yours is Low And Junkyard Electrical Parts I NEVER Recomend I Would Replace Alt w/ Reg. NEW

new altenator or junkyard one.if new return it, if junkyard just ignore light as its odds on they will not change it .the red light is caused by the difference in the voltage between input and output voltages .Quick test --start engine and run at 1500rpm disconnect battery and turn everything electrical on and if engine stops then change altenator if it stays running then altenator is OK