I own a 2000 Chevy S-10 Pickup 2wd Auto Transmission With the 4.3L V6 Vorc Engine has around 110,000 orig miles on it but few months ago the Alt went bad and there a shop here in town what rebuilds them so i had it fully rebuild then befor i put it back on my truck i had autozone run a few Alt Test on it to make forsure it was going to work and it tested good!! so i put it on the 2000 s-10 then had the slow charge the batt over nite and had that tested witch tested good also cleaned the post & the said post wire on the truck! worked grate for a while but NOW i dont know whats going on
the Volt Gage on the dash keeps acking funny when the motor is cold it's at 14 Volts but it it warm's up its around 13 volt's witch is still good but when i turn the radio & heater on the voltage goes down to around 12 volts?? some where in there.. also when the motor frist starts for the frist time in the morning the volts only goes to around 10volts then stops but if i rev it a few time it goes rite back up to 14.. but what would make the volt keep acking up? 2 days ago i had Autozone run a Batt Starter & Alt Test on my truck witch they said & showed me everything is puting out good but the batt is a little low on charge only charge 88% so i took the batt out & had the charge and then i pu it back in the truck and had them test it all again tested good!! so last night it started doing it again witch a frist of mine who work's for a autozone told me to bring it up there and he would test The Starter Alt & Batt tested good so ok.. he asked me up put the alt under a hard load turning every light and Radio and heater on high as it will go and put the truck in Drive with me holding the brakes well he tested the Alt Still Showed up Everything is good!!! but as i showed him the volt gauge on the dash it only around 12volt's with the insaid lights kinda dim not really dim just a little bit more then stock!! so i told the guy what rebuild it whats going on today and he tested it and pasted he even checked the fuseable link witch it's putiing out what it should he said.. Any idea's what could be going on? all the other gauge's work fine! Water Temp,Gas,Oil,Tach,&Speedo work's grate nothing else is acking up at all.. Any Idea's Would Help!!

I have replaced the belt & Pully about 1 year ago! Dayco parts and the belt Tensioner seem's to still be good