Bendix brake valve question

[CFR__AND_SONS]

They found the problem with mine. It was the ABS relay valve underneath the 5th wheel. The guy replaced it with a used one (he said the new one runs like $500). No more air leaks so far.

This might not have anything to do with it but this had never happened until they swapped out this valve... I had a voltage spike yesterday. It was raising past 15.5 v so I pulled over and turned off the truck. When i got back on the road, it was running good. Mine runs at like 14.8 all the time. But later on in the day it was going past 15v so i was getting the buzzer and volt light on. So I would just turn on the AC full blast to bring it down. Any ideas? About to run a search on the forum on this.