My brother's 98 f150 w/ 90,000 had a problem. The heater wont work but over the summer it never had any symptoms that the heater core was going out. When the temp **** is on heat it is like the a/c is on.

Could this be the temp switch in the dash or is this just the heater core gone bad w/o the smell?

Any input is great.

 

Sounds like it could be a bad temp. adjustment rheostat. About $12.00 new if you need one and about 15 minutes to replace.

 

It could be your blend door actuator. I also had no heat and took things apart to find the actuator was shot. Once removed, you can open the door by hand and get all the heat you need.