Recently, my 1970 Oldsmobile Ninety-Eight's heat stopped fully working.
When I turn on the heat, it works fine for about 30 seconds to a minute, then
goes to lukewarm air. When I turn off the heat, then turn it back on, the heat
works fine for another 30 seconds to a minute, and the cycle continues. The
defrost vents don't work at all, and the driver side vent blows less than the
others. It seems like the controls don't work outside of the lever for how hard
the air blows and the lever for the temperature. I don't think any of the
buttons for AC, Max AC, Defrost, etc change anything.
The ductwork has blend doors that move to get the temperature to the vent where you want it. Your blend doors are sticking or are frozen because the door actuators are defective. You may have noticed a tapping sound under the dash from a defective actuator. Try a spray lubricant on the actuators, or remove them to see if there's a broken sprocket. Might need replacing. It's not a difficult fix.
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