1990 ford f150

Is the alternator new? Either way take it out and have it tested. Always have them run the test 2 TIMES!

When you turn the key to ON with the engine off does the battery light on the dash come on? If not it's probably blown and need to be replaced as it's part of the "light circut" and it's required to be in working order for the system to charge

Use test light to see if you are getting power to the alternator.

Just replaced a alternator in my 1990 ford f150 also i had the same problem i thought they gave me a bad alternator but it was a small wire coming off of the voltage regulator that hooks into the batterie when the aalternator went out it burn the wire it was a 5 min fix