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I have a 1997 Ford Expedition with a seemingly random Hot/Cold air swapping issue. In the winter, when I turned on the heat, it would randomly alternate between blowing freezing cold air from outside, and blowing burning hot air from the engine. Now that summer is here, a similar problem seems to persist with the A/C. Sometimes, it will blow nice cold air from the A/C, and sometimes it will blow burning hot air that's much hotter than the outside temperature even on hot days.

I can't seem to recognize any pattern to the switching. The problem was just in the front vents, with the rear vents always working fine, until just a couple days ago, when the rear vents also started doing the same thing.

I'm assuming that since I still sometimes get freezing cold air, that I'm good on Freon. Also, since I get burning hot air, and never just warm air, I'm assuming that the A/C isn't just intermittently failing. My best guess is that the car is blowing hot air straight from the engine, but I'm not sure what's responsible for that. Does anyone have any experience with this particular issue, or even just the heating/cooling system in general? Any tips at all would be much appreciated!

  • I would bet this has to do with the vacuum assist for the blend door. It probably has a leak or there is a leak in the vacuum line which actuates it. – Pᴀᴜʟsᴛᴇʀ2 Apr 30 '15 at 21:42
  • Thanks. From my limited understanding, the vacuum assist only determines which vents the air will be blown through, not what type of air will be blown through those vents. But I could be wrong? – Timothy Apr 30 '15 at 23:28
  • Most modern vehicles have a vacuum diaphragm which actuates the blend door. If you flip your hot/cold back and forth and hear a vacuum change happening, you know that's what it is. It's either that or the blend door itself is flapping in the breeze. – Pᴀᴜʟsᴛᴇʀ2 May 1 '15 at 0:05
  • I'm not very familiar with how the whole heating system works, but I do know that my particular vehicle model uses a motor directly attached to the blend door to open and close it. Would that replace the vacuum diaphragm you're talking about, or is that something entirely different? – Timothy May 1 '15 at 0:12
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    Yes. Exactly what I'm talking about. Some manufacturers use electric motors. Some use vacuum diaphragms. Others just use a push rod attached to the heat knob. That's where I'd start. Problem is, you have to take the whole stinkin' dash apart (from the bottom) to get to it in most vehicles. – Pᴀᴜʟsᴛᴇʀ2 May 1 '15 at 1:59
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I would bet this has to do with the vacuum assist for the blend door. It probably has a leak or there is a leak in the vacuum line which actuates it.

From my limited understanding, the vacuum assist only determines which vents the air will be blown through, not what type of air will be blown through those vents. But I could be wrong?

Most modern vehicles have a vacuum diaphragm which actuates the blend door. If you flip your hot/cold back and forth and hear a vacuum change happening, you know that's what it is. It's either that or the blend door itself is flapping in the breeze.

I'm not very familiar with how the whole heating system works, but I do know that my particular vehicle model uses a motor directly attached to the blend door to open and close it. Would that replace the vacuum diaphragm you're talking about, or is that something entirely different?

Yes. Exactly what I'm talking about. Some manufacturers use electric motors. Some use vacuum diaphragms. Others just use a push rod attached to the heat knob. That's where I'd start. Problem is, you have to take the whole stinkin' dash apart (from the bottom) to get to it in most vehicles.

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