i have an 89 f-350 460 c-6 and the more electric i turn on the more my volt meter goes down. replaced altenator and still no change. it also seems to affect truck idle and dies sometimes when going from park to revese or drive. idles to high or to low when warming up

 

If you replaced your alternator, then your next point would be the ground cables on your electrical system. With bad grounds, the more current you are using (turning things on) the added resistance at the cables causes a voltage drop.

Check the ground cable from the battery on the engine block. Then check the engine block to frame cable. All should be clean, free of rust, and tight.

 

You need to put a voltmeter on the battery. If the alternator is working then the voltage should something like 13.5 volts. If the alternator is not charging then the voltage well be about 12 volts and everytime you turn some else on the voltage will drop.

 

I agree with what the other guys said about your voltage problem. I did stereo installation for many years and have owned several Fords. Of all the vehicles I worked on or owned they have always been the worst about electrical issues. Check everything that is easy and cheap to fix first, then move on to the expensive stuff!