1990 200 TQ Overheat problem

[KWSTURTZ]

My Audi Quarto 200 overheats and the fan does not come on to cool until 
2 late(3/4 of temp gauge)  and does not stay on long enough to cool it 
bellow boiling i drive on freeway i can keep it cool until i get into a 
traffic jam , Come to find out if i turn on heater THEN (no mater if 
overheat or not )the  engine / radiator cooling fan comes on and it runs 
like a champ 1/4 -1/3  on the temp gage at all times.  So what i think i 
need to know is how to make the fan run properly without having to turn 
on the heater.  Work done on  car in last 120 days is  new head, new 
water pump,   new  radiator and  new cooling fan. All reply's greatly 
appreciated.

Majic