The heater in my '77 Dodge D150 works too good. The temp lever is slide as far over to "cool" as it will go, but it still roasts me out. It acts like it is still over on "warm". I tried moving it over farther with a screwdriver, but it acts like there is something blocking it from moving all the way to "cool". I don't really want to take the whole dash apart to repair the heater switch assembly. Any ideas?

 

Moving the selector quickly back and forth from cold/hot you should hear a thump when going to cold and hot, this is the door shutting on both ends of its travel. If you dont hear that then the door is probably not shutting all the way. Its possible that something dropped into the case and is blocking the door, the cable adjustment or a broken door.

I am not too familiar with the design of the heater on this truck but what I have seen a few times is stuff falling through the defroster vents from people throwing things up on the dash. If you can look at the case real good and see if something around the temp door can be removed to maybe get a look at the door and see if something is in there. Sometimes things like the blower motor or blower motor resister can be removed to take a look or get something in there to remove whatever is in the way.

If all else fails you can install a shut off valve on the heater hose under the hood, not the best fix but many people have done this but maybe not for that reason.