1991 Other Opel Models cooling temp

Dear All, My car, Vectra GT 1991 is having the following problem, while driving inside the city and the air cooling system is running, the temp increased over 90, so i turn of the air cooling and the temp returns below 90. The car has one fan infront of the engine and the air cooling system is not original with the car (the car is without air cooling system form the manufacturer)

Regards