Prostar alternator, replaced and still showing lower voltage

Check the main cab feed at the batteries. Should be the smaller main red cable that has a cube shaped main fuse at one of the positive posts. Then that main feed cable supplies the mega fuses at the firewall. Connections can get rusted and/or corroded there.

 

Check your grounds.

And see if there is a ground strap on the alternator, it needs to have a good ground to work right.

 

Yes, Prostars have a cab battery feed right from the batteries. I believe it is 2 ought cable size. Should have a cube style fuse right at one of positive battery posts.

 

Brake switches work on grounds in a prostar, so if the signal wire to the body controller is shorted out on anything, it will cause a ground in that wire and make the body controller think you are applying the brakes.

 

Knowing this it really sounds like your grounds are failing

 

Check the grounds from the engine block and/or starter to the frame and all connections on the starter. You have a loose cable somewhere. The starter connections are always suspect number one.