My 1991 Dodge Ram 350 Vans check engine light came on. Like a dummy I changed alternater. Now light is off, alternater puts out 14.4 volts for about 5 minutes, then dose not charge at all, and light is back on.

 

There are about 2 dozen things which will illuminate the CEL. You must have had a reason to suspect the alternator.

There is a process where you retreive the computer's codes by counting flashes of the CEL after quickly turning the key on, off, on, off and back on and wait for the CEL to go out

Flash-flash flash pause.........Flash flash flash flash flash

would be code 35

You can expect to see code12, and code 55.

Post any other codes you see.

Eliminate the possibilities of bad grounding.

Clean the battery terminals.
Clean the battery to engine ground
Clean the battery to frame ground.
Locate the frame to engine ground, and clean it. Add one if you cannot find one.

There are fusible links behind the brake booster. They are lengths of wire designed to protect the wiring. When they blow, the insulation will look all burnt and fugly. It should stretch out. When aged, they seem to also corrode from within and not show any exterior signs of blowing.

This might be an explanation why it will charge for 5 minutes. After that time the link heats up so much that it loses contact.

Also, you should not stress a new alternator by asking it to charge a nearly dead battery. Fully charge the battery with a wall charger first.

Good luck, post the codes, let us know what you find