I have a compassitor that runs my sound system that has a guage on it and it reads the same as my multi meter when im driving it's charging at 15 and I've seen it drop to high 12s as well it's all over the place
before I replaced the alternator it wasn't charging at all at low ram but would charge at higher wich it's never done before after I replaced the alternator it was a little better but coming to a stop it would drop to mid 12s wich is why I figured it's the stock reg so I tried this and now im having other issues

 

Verify that your connections are good, and maybe make sure the regulator itself is grounded good....

 

There's bare metal under the regulator and i even ran a ground wire just to try to alleviate the problem and i double cheked all my connections. I also have it wired to a Keyed on sourse in the fuse box with a fuse tap i checked that and it's sending power and doesn't seem to bad

 

15 is a bit high..... And it shouldn't remain there once the battery is charged. The regulator really isn't doing it's job properly. Why it isn't, is a mystery to me.

 

I just want to throw this out there for anyone.
charm.li has practically every service manual for every car prior to like 2013 or 15 for free. You can download them or view them online. Trying to help.