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When truck is in idle, both heater and AC route to floor or panel just fine. When engine is under load (i.e. driving) both switch to defrost. It's my understanding that defrost is the default "no vacuum" condition. My question is, how do I troubleshoot this vacuum issue? I assume that since it works fine when in idle, that all of the hoses under the dash are fine, and that there is good vacuum to them. When I do a visual inspection in the engine compartment all hoses look fine as well. Where else should I check, and what could be the issue?