Hey. u need to understand how teh car airconditioning works.
IN simple terms, the AC unit has a belt going on to your car engine. when you turn the ac on, ur engine actually has to move that belt too. So that is why the car heats up a little bit.
PLus the needle somewhere in teh middle of teh radiotor gauge is actually the optimal temprature for engine to work in.
However, just to be on the safe side, here is what u can do.
1) get ur radiator flushed and use a good coolant. and check it every 3 days.
2) when ur drviing and u stop at an ISHARA, turn the AC off so that you ac doesnot put too much load on ur engine.
THese were ofcourse my 2 cents and im pretty new here. im sure that the experinced members here will be able to help you in a better way....
