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Howard it's been a while, hope your well.  The defrost vents on top of the dash that defrost/defog the windscreen.  They will not close when conditioned air is selected.  Being full time open really cuts down on the volume of air in the air conditioning mode and as you well know the Jag wasn't built for and doesn't cool very well.  They don't have our summers in Great Britain. We researched these vents in the Series lll manual and what a quagmire of rods, vacumn hoses, electrical wiring, and working levers.  The doors close by hand rather freely but won't move when either defrost or air conditioning is selected.  All fuses intact and the rods controlling the doors move freely.  The manual makes it out to be a nightmare to get into the dash to check the controller for the sequence of events that make the doors close or open.  Looks like both electrical and vacumn source is needed.  We checked all vacumn lines up to the firewall and they are intact.  We don't want to dismantle the dash if there is something else you can think of to check.   The system operates extremely well in all phases of airconditioning and defrost including the gradual changeover from hot defrost air thru the defrost vents to maximum cooling air when selected thru the conditioning vents, but because the defrost vents don't close there is conditioned air wasted on the windscreen. Please advise if you have any experience with this system including looking into your bag of exceptional Jaguar repair tricks. Thank you in advance for the response.

Hi Dan,

I am not an AC expert but have done some work on the ser III AC systems. I remember the defrost flaps only open on "Defrost" setting of the right hand mode switch. Been a while but I also think it works backwards to what one would think in that the flaps are spring loaded open and closed by vacuum from the right hand mode switch. The top dash pad is easy to remove and the right hand kick panel to expose all of the linkage on the right side of the AC box.

First check that operation to see if my memory is correct. I now own two ser III cars and both only have defrost action when the mode switch on the right is in the "Defrost" position. I do feel a minor amount of air out of the defrost vents in all settings but I contributed that to air leakage past the flap. Keep in mind that the AC is in full operation all the time even when high heat is selected. This is to dehumidify the air in heat mode. Cooling and heating is done by door blending of air.

If my memory is correct and the flap is spring loaded open and closed by vacuum and in any other setting other then "Defrost" the defrost flaps open when they are not suppose to, then you should look for a major vacuum leak in that mode which is causing a lose of vacuum on the defrost flap causing it to open when it is not suppose to.

If you get moisture on the windshield from the defrost vents then you have either a stopped up AC water drain or you have a leaking heater core (if that is the case you should smell antifreeze too)

Let me know,


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Howard M. Fitzcharles III


Jaguar from the XK 120 to XJ-6 ser. 3 1987


Dealership line mechanic on MG, Triumph, Jaguar for 15 years, Instructor in commercial mechanics school 2 yr. Product information manager for piston and valve manufacture, Instructor & hotline answer man for import car parts importer 15 yrs.

Associate member SAE EAA member

Import Car magazine

ASE Master Auto with L-1 certification up to 2000

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