my voltage guage reads about 11.5-12v, but when i read voltage with my DMM directly on the batt and alternator it reads 14.20v

if i roll down a window or use defrost at the same time i use the blower fan my lights dim.

anyonw knows why ?

 

i would like to add that when cold, my guage reads 14v, but when hot it goes down to about 11.5v

 

Lights normally dim a little on extra load. Cant tell {us} by that.

The 14.2 on battery and alt shows your primary charging circuit is good. That is the conn between the engine block and battery.

Now you need to test the power distribution junctions. Follow the wires.. probe the terminal they connect to while lights/ac on.. etc. First place it's lower is sign of bad connection

 

what do you mean by power junctions, and wich/where wires i probe ?

 

See the underhood fuse/ junction/relay? Start there.
Note you can probe tops of fuses for v to see if there's problem between there and batt positve..