1989 Mitsubishi Truck cooling system

Hi there.I have a 1989 mitsubishi canter 3567cc.4D32P engine. The temperture guage all ways goes to half way and stays there. Never goes up or down. Is this normal?I feel its running to hot. I have put a new radiator in the truck, new water pump. New themostate. New pipes. New radiator cap. The fan goes at a constant speed. Is it a viscose fan?Should the fan speed up as it gets hot?Any ideas what my next step is, cheers brod