1994 Other GMC Models Battery Light and gages

I have a 1994 GMC 3500HD with a boom mounted. (Old NYNEX truck). There is a an auxillary battery mounted in the engin copartment. Inside the cab is a botton to use the auxillary battery. When I start the engine it runs very rich and loud, and the volt meeter gauge is at the 10v area. If I press the auxillary battery botton it seams to switches over and the volt meeter gauge goes up to 13V+. If I step on the breaks it switches back. If I use the turn signal the gauges start going crazzy, up and down... The alternator is brand new and I just replaced the primary battery which seams to help with the volt meeter staying higher. When I use the the turn signal the gauges still jump back and forth.