Heater/defroster control
In my old 95 Aerbus on a Chevy chassis I had problems with the heater when I climbed the hills. As I pushed the accelerator to get more power going up the hill, the heater would "default" or change to the defroster. As long as the hill was short, the lack of hot air on my feet was not a big deal. But for long climbs, it got pretty chilly in the winter months. The same thing happens in the summer with the dash air conditioner.
When we bought our new 2000 Aerbus on a Ford V 10 chassis, I assumed that the problem would be gone. But to my disappointment it was exactly the same. It was not a big deal, since most of our driving was in reasonable temperatures, and the temporary diversion of warm or cool air did not cause a problem. One of the motorhome magazines did have a letter from a fellow with the same problem, who said his mechanic simple added a second vacuum canister for the heater/defroster, leaving the original one for the engine and it’s components.
During a recent cross country trip, the temperature was close to freezing the whole way, and the hills and mountains resulted in long periods of default to defroster, during which our feet and bodies became colder and colder. "Enough," said the co-pilot, "we have to get this fixed."
My mechanic explained that the vacuum is used to run a number of engine and coach functions, one of which is the heater/defroster. The vacuum is created in the carburetor/intake manifold, and that vacuum is "stored" in a vacuum canister for those times when engine created vacuum may drop. When the engine is under load, the vacuum disappears, drains the canister, and the heater defaults to defroster, I am guessing this is because it is felt by the designers that it is safer to be able to see than to have warm feet. A larger vacuum canister is better, but the key is a one way valve so that the vacuum cannot leave the canister when the engine vacuum drops. This will prevent the heater/defroster unit from defaulting to defrost. So he replaced the vacuum canister with a larger one, and added a one way valve so the canister could not be emptied when the engine was under load. The $150 cost was covered by Ford under "warrantee". That was nice.
I wonder if the fellow in the magazine did not have a one way valve included in his second canister, assigned to his heater, since it sounds to me like the valve is the most important item. It does appear to me that most mechanics are well aware of the issue and of the fix.
Well now we are warm and toasty in the coolest weather and on the longest hills. And the same applies to the air conditioner. And it really is an easy fix.
The issue is briefly discussed in FMCA Magazine, Technical Inquiries, December 2002, page 26-28.
Doug