Dash Heater & A/C Control Diagram
I discovered a vacuum leak on the heater core water control valve. Searching this forum helped locate the problem. It appears to be a common problem. In my search for how the system works I discovered a Winnebago Service Tip that has a diagram of the vacuum controls. I have attached it to this message so it might help others.
What I found strange is the heater control valve operating modes. The only two positions of the control switch where water does not flow through the heater core is OFF and MAX A/C. This same vacuum port also activates the fresh air damper which makes sense. The heat/AC plenum door is closed in vent, A/C and MAX A/C modes which "should" keep the warm/hot air from flowing into the cab. I have thought about switching the heater core control valve (green line) to the heat/AC door vacuum signal (yellow line). This way the water would not flow through the heater core in Vent, AC or MAX AC. The next time I drive the coach I'll check how warm the dash air is in the Vent position. If it is much warmer than the ambient air I'll know the heat/AC door is leaking. In this case I will definitely change the heater control valve to the yellow vacuum source. That will be much easier than fixing the damper seals.
2004 Itasca Meridian 39W CAT 350
2001 Dadge Dakota TOAD