Alternator over charge

I have a 09 389 peterbilt that I put a c15 in about a year ago and everything has been fine until now. Starting a couple of weeks ago every time it rains it starts over charging. It has cooked 2 batteries so far. I’ve changed the alternator twice and all the cables from and to the batteries and the starter.. even put an extra ground to the alternator. I switched all 3 batteries out with another truck I have and still same. I am at my wits end.. can anybody please help?

 

I’ll have to check and see what brand of alternator it is.. I just usually take whatever brand my local peterbilt store sends me. I have put a multimeter on it to confirm it and the gauge is right. It goes up to around 15.7 and after a few minutes in the dry it goes down to 14.5 at the batteries and 14.6 at the alternator.

 

I don’t have the voltage sensor wire but it’s never had that wire. I have always thought it wouldn’t go over 14.5 anyway.
I’ll try putting a wire on it and see what that does. It just goes from batteries back to alternator right?

 

I changed all cables even the jumpers from battery to battery.

 

Well I guess you were right, I ran a wire from the batteries to the bottom post on the alternator and it's charging at 14.1 and I've sprayed it with the garden hose and it doesn't change. Thank you !