Got a 91 D-350 off of a guy who put plenty of aftermarket parts in it. He put in a sound system which seems to be drawing a lot of voltage. I put in a brand new alternator Valuecraft Remanufactured Alternator 14430VC and a brand new battery. Voltage stays at about 11.5v-12v when driving but when I turn the sound system or the AC on it drops to 10v. Been driving it around everywhere with sound and AC off and has no problem starting up. Any idea what the problem is? Do I need a better alternator? Two battery set up? Both?
Thanks for the help.

 

Probably alternator. Do checks in attached TSB and see if that helps.

 

Where is the voltage regulator on that truck?

 

Assuming you are using the volt gauge in the truck? Put a meter on the battery and see what voltage that tells you.