Truck looses power when: the fan comes on, mostly on uphills .

truck runs great, only when the fan turns on it brings me down loosing power, i dont know what it can be , has anyone had this problem?

its a paccar mx13 on a peterbilt..

 

its not saying derate, and i been driving the truck fine , its only when i go uphill that fan turns on and looses power, i can go up a hill at 25mph tops.. when the fan turns off, it catches power again and speeds up . im thinking its a voltage problem , between the batteries and the alrernator maybe?? idk honestly whats going on ill try to post a video

 

i check the master battery fuses, and disconected the speed sensor from the transmission and idk wtf now.. im thinking its a low voltage issue but thought id ask

 

13.7 volts ..

 

its saying generic fault code 886-14