heating issues 91'APV

I have done a total cooling system flush, tried 3 different thermostats... My heat gauge will only stay around the 1st. make and am getting poor heat,fairly cool especially when on defrost,when it is on vent it is somewhat tolerable but still to cool. I decided to start the van from a cooled down state and within 1-2 min the radiator fan comes on so.... I decided to un hook the power to the radiator fan and now I get just the right heat in all ranges,the gauge stays about in the middle and things seem to be just as they should. Now I know that the fan is there for a reason but what is it that kicks it on waaay tooo soon? I believe that is the root of my whole problem. What part controlls the temp at which the fan turns on??? I am fairly sure if I can replace that part all will be well for real.
Thanks for any help with this, I have been batteling with this for 3 winters now and I think it's time to do the right thing:banghead:. BTW my Lumina APV only has 235K on it and other than this heat thing I have no complaints.
The coolant level is always good, no loss of coolant. I just think the radiator fan kicks on waaay tooo soon!

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Thanks for such detailed info, I will look it over and let you know the results.