[xj40] Condensation on windscreen from A/C

My 1988 Vanden Plas forms condensation on the outside of the
windscreen when the air conditioning is running. The condensation
forms mainly on the passenger side. What are the possible causes
and remedies for the condensation?–
'88 Vanden Plas
Queens, NY, United States
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The vent on top of the dash below the windscreen is supposed to be closed
when running the A/C. Either the mechanism that opens and closes the vent
is broken (very common problem) or you have a vacuum line leaking / off.
Repairing / replacing the vent requires removal of the dash. I don’t know
if replacements are available. I had to repair the right side vent on my
car but now the left side is bad. Removing the dash is, as you might
imagine, a bit of a project. Very do-able but you have to be sure and label
every electrical connection and vacuum line that has to be removed to insure
you get them reconnected properly. I used bits of masking tape to label
both sides of the connections (A, B, C, etc.) and made a list that I then
checked off as I reconnected things upon reassembly. That way I made sure
not to miss anything.

Mike Stone
1994 XJ40/XJ6
144,xxx miles
Springfield, Missouri USA-----Original Message-----
From: owner-xj40@jag-lovers.org [mailto:owner-xj40@jag-lovers.org] On Behalf
Of BigE 53
Sent: Sunday, August 23, 2009 12:33 PM
To: xj40@jag-lovers.org
Subject: [xj40] Condensation on windscreen from A/C

My 1988 Vanden Plas forms condensation on the outside of the
windscreen when the air conditioning is running. The condensation
forms mainly on the passenger side. What are the possible causes
and remedies for the condensation?

'88 Vanden Plas
Queens, NY, United States
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Line Books and more !

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In reply to a message from Mike Stone sent Sun 23 Aug 2009:

Surely that vent has to be open to allow air to hit the
screen for demisting? The condensation on the outside is
just because the glass is cold and the outside air is damp -
every a/c equipped car I’ve ever owned does it.–
The original message included these comments:

The vent on top of the dash below the windscreen is supposed to be closed
when running the A/C. Either the mechanism that opens and closes the vent


1990 XJ6 4L Soverign auto on LPG
Wolverhampton west mids, United Kingdom
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In reply to a message from Richard Moss sent Mon 24 Aug 2009:

Richard,

That shouldn’t happen on an XJ40 - it certainly never does
it on mine. On lesser cars, maybe! :wink:

When A/C is selected, those demist vents should be held
firmly closed by vacuum and a similar vacuum should open the
centre fascia vents to blow refrigerated air on to your
upper regions.

When ‘Defrost’ or ‘Demist’ are selected, the air exiting
those vents should be heated air previously dried by passing
through the aircon evaporator.–
The original message included these comments:

Surely that vent has to be open to allow air to hit the
screen for demisting? The condensation on the outside is
just because the glass is cold and the outside air is damp -
every a/c equipped car I’ve ever owned does it.


Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from Mike Stone sent Sun 23 Aug 2009:

Also check the sheet metal screws that hold the demister vents to
the facia top. You will need remove the solar sensor in the center
to gain access to the screws. I have had success putting my fingers
down into the opening, lifting the vent into place and resecuring
the fasteners with a very short screwdriver.

bob gauff–
The original message included these comments:

The vent on top of the dash below the windscreen is supposed to be closed
when running the A/C. Either the mechanism that opens and closes the vent
is broken (very common problem) or you have a vacuum line leaking / off.
Repairing / replacing the vent requires removal of the dash. I don’t know
if replacements are available. I had to repair the right side vent on my
car but now the left side is bad. Removing the dash is, as you might
imagine, a bit of a project. Very do-able but you have to be sure and label
every electrical connection and vacuum line that has to be removed to insure
you get them reconnected properly. I used bits of masking tape to label
both sides of the connections (A, B, C, etc.) and made a list that I then
checked off as I reconnected things upon reassembly. That way I made sure


Bob Gauff Owning and Repairing Jaguars Since the Early 70’s
Decatur, TX., United States
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In reply to a message from BigE 53 sent Sun 23 Aug 2009:

Vent flap problem. I’ve had the problem on my drivers side
for many years. In certain climate conditions, it condenses
on the outside and I need to use the wiper to remove the
condensation.

I’ve lived with the problem/reward ratio. The problem is
not big enough to venture into the reward of going through
the labor to remove the dash.–
John 90 XJ40
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In reply to a message from Mike Stone sent Sun 23 Aug 2009:

Mike,
Thanks, now I know what the cause was. The mechanism that closes
the vents is most likely broken. Sounds like the easiest remedy is
to simply cover up the vents on top of the dashboard when the A/C
is on. I drive the car so infrequently, and live where I only
need the A/C three months out of the year to hassle with removing
the dashboard and HOPING to repair or replace the vent mechansism.–
'88 Vanden Plas
Queens, NY, United States
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In reply to a message from BigE 53 sent Sun 23 Aug 2009:

Had same problem and resorted to a bit of removeable black
carpeting to cover the vent, untidy but beats pulling out the whole
dash etc. richard–
ric10
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In reply to a message from ric10 sent Tue 25 Aug 2009:

I think you’re right on, Richard, carpet cover is the way to
go. This issue was the number ONE problem in the early cars
and I clearly remember doing dash flaps (I got most of them
as I was 19 and the only hourly guy in the place :)) in
EVERY car that was new for sale. We worked and worked to
find an alternative but the flap fit problem simply wasn’t
solveable other than replacement. Interestingly enough, it’s
the first update topic in the service update schedule :slight_smile: … Ned–
jibsheet
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