Level VI Member
Join Date: Jan 2017
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No heater control valve, the core is constant flow like said.
Heater uses an air mix control and the cable comes off the temperature dial, the extra load you feel is the action of the cable, at first it operates one air door then later in the dial movement it also operates another to make two moving, you feel the difference in that. One door controls the amount of heater air, the other thinking puts the air down low for feet or high for defrost, or closes off a/c. Compound action there.
Possible sticking door at heater too, they seal the door edges with stick-on foam and sometimes in the heat the foam degrades to let the gum sealer ooze through the foam in a gummy mess, then the door can be almost impossible to open when it sticks shut due to the 'glue' on it. Fords had hell with that and the cables broke all the time trying to open the door for more heat.