Its urgent: Fiat Palio's Fan is runs gud with AC but runing late with non AC

Sir,
my car's fan is working good when AC is on but
when I go non AC then the Fan would not start till the heating needle reaches upto 80 % that comes after 5 km of slow traffic. But if I go on highway then the needle will be down till the speed is good but again fan is not working.
I want that my car's fan will run when needle rises just 25 or 30 % of heating meter.
Thanx in advance!