The red idiot light for the battery comes on and shows low, I have replaced the old battery with a new one but hasn't solved the problem how can I tell if my alternator is working or not, and how much are they to replace, I have a 3.8 litre Tbird 1991

Try cleaning your wire contacts on your alternator, Look for the wire and white plug on the top of your alternator.sqeeze the plug and remove the plug and wires from the alternator. Take a small fingernail file or small piece of fine sandpaper and clean the electrcal connections, Put the plug back together ,restart the car and check and see if your light has gone out. If the light is still on then take it to your local Auto Zone and have them check the alternator on the vehicle , they do this for Free. I have a 1990 ford tarus that was doing the same thing, it turned out all i needed was to clean my connections..That took care of my problem..Good luck..

Go to your auto parts store and have them test the charging system, it doesn't pay to guess and throw parts into the car.

Starting in 1980 & I don't mean to be
sarcastic here,there were no more idiot
lights

ALL lights from 1980 or 1983 on are very
important OBD1 or OBD2 On-Board-System
lights that you need -- to know when to check
for trouble codes.
You don't clear them,they don't go away,
& you don't ignore them
You won't even be accepted at an inspection
station if there on

You can call or stop by a parts store for price
or use a web site such as NAPA.com

All systems are somewhat different
Some the ignition sw plays a role in
your problem as it is tied to your alternator
field circuit. Some have relays in there

Working is not a cut & dry way to discribe the alt

You can check voltage across the battery
or off the alt output,but that won't tell you
currant output or any problems with diodes
A/C ripple etc that may mess up your car

You can go to a repair shop to test the entire system
or hope the alt is the only issue & remove it & have
it tested at parts store