This is caused by a loss of vacuum to the AC control head from the engine vacuum storage canister, the line comes through the engine firewall on the passenger side under the dash, remove the kick panel, then unplug the vacuum harness (the lines are plastic) and see if you have a good vacuum signal of about 18 inches and holds steady when engine is revved up, if not check for a kinked hose where it comes through the firewall,also check for a cracks cuts or splits in the rubber vacuum line to the vacuum storage canister under the hood, it is shown in the vacuum diagram part of the emissions sticker under the hood.
You can use a vacuum pressure tester on the vacuum lines to the actuator for the air blender door. You may have a weak actuator and the force of the air blows the door to def instead of the vent. You can also do this with the egr vacuum hose. Your egr valve may not be opening when it is supposed to. Could be a stuck valve or not enough vacuum.
Has this been happening for a while or is it recent? Have you though about changing the vacuum canister or the pump? Should be under fender wall
That is the vaccum purge canister
The white hose above the fan motor is just a drain hose or air hose. Not supposed to be connected.
Your vacuum box will be either a big round black plastic ball, or a black box like shown below.
Glad to see you have found it! Those hoses do tend to deteriorate. Glad you got it cleaned out and working.
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That is the vaccum purge canister
I feel sure that I have low vacuum. I am having trouble determining what is causing my vacuum to be low. There is plenty of vacuum on the vacuum actuator under the dash that directs the air from vent to defrost when the engine at idle. It acts as though the vacuum canister is not backing up my system. I need tips to trouble shoot this thing. Please help.
Where can I find a pictorial view with the vacuum components named for my 1996 F150 (5.0 Liter engine). I am not a seasoned mechanic but I am a do it yourself person. Thanks for your repley.
Does my 1996 f150 have a vacuum check valve and a vacuum pump. If so where are they located and what do they look like. I am 99% sure that my vacuum backup is not supporting the system. As I said before I cannot find any leaks except for the white hose above the fan motor.HELP!
I proceeded with your advice and found 18 inches of vacuum going under the dash to the air conditioner. When I unhooked the vacuum coupling to check the vacuum I found a blockage in the white line that carries vacuum back to the outside/recirculate door actuator. This white line is the one that had decomposed on top of the blower fan. It had sucked some of the material down the line and stopped it up. I ran a small wire doen the white line and cled it out. All is now working.
Thanks so much for giving me the exact information to make this repair myself. The location of the vacuum coupler was very helpful(Behind the passanger side kick plate) I hope this will help others with this strange vacuum causing ac to go from vent to defrost.
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