Question about 2003 Mercury Grand Marquis
The vacuum harness connected to the back of the control head is not fitting tightly any more. Pull narrow trim panel covering control head. remove 4 screws holding control head on.
Pull control head back. Fit 2 ten inch tie straps so they hold the vacuum harness (5 hose) tight to the control head. You can remove 2 torx bolts to lift bottom cover if necessary.
Vacuum harness is $20 F5VY-19C827-A, but I doubt you will need it.
Hold it tight with your hand to test before you do repair.
Gov't requires system to fail safe to defrost if a vacuum leak occurs.
I can post a photo of repair if necessary.
If that does not fix, Gino in California fixes internal control leaks.
Posted on Jul 26, 2010
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Posted on Jan 02, 2017
SOURCE: Vent control
You have a vacuum leak.Defrost is the default postion(no vacuum).
See if you can locate the small black vacuum line that goes inside the vehicle on the passenger side near the heater core. The trace it back,should see it going to a canister then to the intake manifold.Make sure it is not disconnected or broken somewhere.
Posted on Mar 02, 2009
needs re-gassing because one line should be hot (feed) and the return cold , This is a specialist job do not attempt to do it yourself because its extremely dangerous with very high pressures involved
Posted on Jun 21, 2009
not blend door, blend door is responsible for mixing hot air from heater core side with cold air from evaporator side of the system.
you have a problem with your panel doors
it may be an electrical problem with signals from you heater control module or a vacuum problem or you may have damaged components inside your dash.
your vehicle will default to defrost setting automatically if there is a problem in the system
Posted on Apr 07, 2010
A vacuum hose that controls the little door that switches where the air blows from is off, its gonna be a real chore the find which one and take the dash out to get to it, but you might get lucky and be able to look underneath the dash from the passenger side and actually see it, good luck
Posted on Jun 19, 2010
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