1990 Dodge Truck charging problems

Electrical problem
1990 Dodge Power Ram W150 5.9l 360c.i.

Four Wheel Drive Automatic 100000 miles

The truck is a power ram w150. I have a chraging problem where it wont charge over 11.75v. I have had the alt checked and passed by two part stores. I purchased a new alt to be safe. I also bought a reman comp. and it worked for five mins and the check engine light re-appeared. Then the voltage went from 13.5v trickled down to 11.5 again. I have checked on the back of the alt and reads 11.8v on the large blk cbl. and 11.6v on the green conn. I also checked the blk cbl @ the fussiable link and was 11.8v and between the battery and the f-link is 11.8v also the battery is still @ 11.8v. if i put the jumpers to it from another car it reads 12.8 or 13.4? maybe I got a bad reman comp from au-zone. The code from the comp diag mach said it has a open field circuit or short, which was explained to me to be either the alt. or the comp. but the dash flashes code 12 and 37? can anyone help its chrismas season and i am almost broke.