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I haven't worked on many Eagles, They don't survive our climate well. In general you have three valves controling the central heat in your bus. Two isolation valves so the engine can be pulled without draining the heating system. These two valves are in the engine compartment, some people will shut these off to insure max cooling from the AC in the summer. There is a electricly controled valve somewhere in the system. It looks like a flying saucer with an electrical coil on top and one inch pipe entering and exiting the sides. It will be in the one inch plumbing usually they mount these up high, out of the wheather and behind a trim panel or in the heat/AC compartment. They tend to stick shut. To test get the engine hot and heat turned right up and hit it with the handle of a hammer. If the heat comes on it needs to be rebuilt. The electrical side should be wired power = no heat, no power = heat. There may be a similar system on the dash heat and AC as well. If it were me I would have to start following the heat plumbing through the roofs ove the luggage compartments to find it. It also could be something as simple as an air lock also or bad booster pump. I don't know if eagle used a booster pump. Good Luck