battery light stays on - alternator output guage reads low on 1990 Chevrolet 1500 Pickup

Truck has been setting for a while. Charged up battery, starts fine, runs fine but battery light on dash has come on and stays on. Noticed that the alternator output guage is not registering where it normally does (around 14) when engine running but rather reads down around 8. Alternator bad and truck just running off of battery? Thanks.

Asked by for the 1990 Chevrolet 1500 Pickup
check for proper amperage .your alternater might be generating enough juice to keep ibattery charged and running. yet not enough for proper your gauge drops and is really running of battery..(BAD alternator)