Dash heater / vent / defroster
I have a 94 winnebago with a chevy engine. The GM dash control for the heater won't work. The fan changes speeds, the cool/heat slide lever works, but the push buttons don't change the airflow from de...
Forum Discussion
fuzzbutt,
(like the name) Anyway, yes, this "Phenomenon" is more common than people think. I don't know if you know that much about how the common engine works and how vacuum plays a part but, without going into a novel here, it's simple. If you've ever used or seen a vacuum gauge in action, while driving a car, you'll notice that the more throttle you use, the less vacuum on the gauge there is.
Full throttle usually displays almost "0" vacuum. While an engine is idling, there's upwards of 25" or more of vacuum. So, since many automotive A/C and heat systems utilize what's called "Vacuum motors" on the controls of the heater doors and vent doors etc. (because they're cheap, easy to manufacture and, don't require any source of power) they are effected by a loss of vacuum. And, there's a few reasons they loose vacuum. One, as stated, full throttle, two, a broke or, fallen off vacuum hose, or, problem that happens on occasion, the "vacuum canister" develops a leak.
I forgot to mention, the reason for a vacuum canister. As stated, when at full throttle, there's little to no vacuum in the system. So, when that happens, your statement that said someone told you that the default setting of the control doors in the A/C and heat system is do send the heat/A/C to the defrost, is correct. The reason, in an emergency situation or, in your case, just an incident, the industry standard is for a driver to be able to "see" where they're going due to the defrost working and no other ducts.
Because in the loss of vacuum, those doors can control flaps are spring loaded to aim every thing at the defrost side. Well, when there's little to no vacuum, and you still need heat or A/C at the ducts or feet, vacuum is "stored" in those vacuum storage canisters and utilized, through a series of valves, in that situation. And that stored vacuum is what's used to help control the direction of the air flow during low engine vacuum conditions.
Ok, now that you have a somewhat understanding of the system, then you'll be able to maybe do some investigation as to the loss of vacuum. It's a good move to start with the source of the vacuum for that system. And that's the vacuum hoses etc. right on the engine. If you don't find a faulty, broken or fallen off vacuum hose, than aim your next part of the investigation at the canisters.
I had one, one time that was completely round, about the size of a soft ball. It had a seam that was "glued" from the factory all the way around the middle. Well, by luck, I removed that canister, applied some slight air pressure to it, under water in a bucket and found the leak. I applied some silicone, let it dry and that cured my "no vacuum" at throttle conditions. So, I hope this helps some.
Scott
P.S. Please post what you find out so others may learn.
(like the name) Anyway, yes, this "Phenomenon" is more common than people think. I don't know if you know that much about how the common engine works and how vacuum plays a part but, without going into a novel here, it's simple. If you've ever used or seen a vacuum gauge in action, while driving a car, you'll notice that the more throttle you use, the less vacuum on the gauge there is.
Full throttle usually displays almost "0" vacuum. While an engine is idling, there's upwards of 25" or more of vacuum. So, since many automotive A/C and heat systems utilize what's called "Vacuum motors" on the controls of the heater doors and vent doors etc. (because they're cheap, easy to manufacture and, don't require any source of power) they are effected by a loss of vacuum. And, there's a few reasons they loose vacuum. One, as stated, full throttle, two, a broke or, fallen off vacuum hose, or, problem that happens on occasion, the "vacuum canister" develops a leak.
I forgot to mention, the reason for a vacuum canister. As stated, when at full throttle, there's little to no vacuum in the system. So, when that happens, your statement that said someone told you that the default setting of the control doors in the A/C and heat system is do send the heat/A/C to the defrost, is correct. The reason, in an emergency situation or, in your case, just an incident, the industry standard is for a driver to be able to "see" where they're going due to the defrost working and no other ducts.
Because in the loss of vacuum, those doors can control flaps are spring loaded to aim every thing at the defrost side. Well, when there's little to no vacuum, and you still need heat or A/C at the ducts or feet, vacuum is "stored" in those vacuum storage canisters and utilized, through a series of valves, in that situation. And that stored vacuum is what's used to help control the direction of the air flow during low engine vacuum conditions.
Ok, now that you have a somewhat understanding of the system, then you'll be able to maybe do some investigation as to the loss of vacuum. It's a good move to start with the source of the vacuum for that system. And that's the vacuum hoses etc. right on the engine. If you don't find a faulty, broken or fallen off vacuum hose, than aim your next part of the investigation at the canisters.
I had one, one time that was completely round, about the size of a soft ball. It had a seam that was "glued" from the factory all the way around the middle. Well, by luck, I removed that canister, applied some slight air pressure to it, under water in a bucket and found the leak. I applied some silicone, let it dry and that cured my "no vacuum" at throttle conditions. So, I hope this helps some.
Scott
P.S. Please post what you find out so others may learn.
