2000 Ford E350 Econoline Club Wagon A/C Heater Control switch replacement

I had the same problem and also thought it to be the control switch, however, further investigation proved differently. A minimum of 17" NEGATIVE PRESSURE (Vacuum) is required through out the system to operate properly. My actions were as follows: 1) Remove access panel on top of dash and check VAC by removing the manifold and connecting an inline vac pump with gauge, I believe the black tube is the main supply. 2) If you have 17" there then the problem lies down line, if not then the problem is forward (as was my case). 3) I had a crack in the accumulator bottle, replaced & cured.