AC/Heater question
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Senior User
Joined: Jan 2011
Posts: 145
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From: Dothan, Alabama
AC/Heater question
I have a 78 f150 custom and it has factory air. The heater and air work BUT I can not get it to switch from defrost mode. After it has been on a while it will move to the floor and vents (sometimes) when switched but I have to play with the selector back and forth to get it to work. I thought maybe the diaphragm was stuck? Any ideas? I think I found the diaphragm located on passenger side way up into the console? I am assuming this is it. Thanks
MSEE
Joined: Apr 2004
Posts: 10,386
Likes: 35
From: Austin, TX
The heater/AC system in these trucks is vacuum operated. I think defrost is what the system defaults to if vacuum is lost, but I'm not sure. If moving the selector switch back and forth is what fixes it, it's possible the vacuum switch is not routing vacuum to the different vacuum motors properly.
There is also a check valve, as well as a vacuum reservoir under the hood to maintain vacuum to the heater controls during periods of low vacuum, such as wide open throttle.
There is also a check valve, as well as a vacuum reservoir under the hood to maintain vacuum to the heater controls during periods of low vacuum, such as wide open throttle.
