[xj40] '91 A/C system, heater bypass valve and air recirc

On the MY91 cars, what conditions must be met for the ECU to
open the recirc flaps and or close the heater bypass valve?
(if a heater bypass valve has been installed)

I know a few years ago, one poster built a separate switch
and control system to be able to deliberately command one or
both of those vacuum solenoids and I’ve thought of doing the
same.

Still, if I knew when those systems were called for, I might
find a manual control redundant or not adding value to the
controls.

Thanks!–
91 XJ6, 93 Sov, 97 Sov – Driveway’s crowded!
Tampa Fl, United States
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In reply to a message from Win sent Fri 8 Aug 2008:

Win,

I’ve never understood under which circumstance the A/C ECU
will automatically close the flaps on the blower casings to
switch to recirculation mode (except that I know it does
that when you turn the A/C system off).

I think you would need to connect a voltmeter to the
Red/blue wire on pin 3 from the upper 15-way white connector
on the A/C ECU to the recirculation solenoid valve to see
when the ECU puts 12 volts on that wire to put vacuum on the
flaps to close them to outside air.

If your car was fitted with a coolant shut-off valve, the
ECU would put 12 volts from pin 2 of the centre 15-way white
connector on the ECU on a Red wire to operate the coolant
valve solenoid to put vacuum on the valve to close it
whenever ‘Full Cool’ is demanded / selected. Obviously it
would have automatically have shut that valve whenever A/C
was called for.

Check the posts from Brett Gazdzinski in the Modern list
archives where he describes the fitting of a three position
switch so that he had recirc air only, outside air only, and
could also revert to the normal Jaguar automatic function -
whatever that is! :slight_smile:

Brett said that a major benefit of being able to select
‘recirc only’ was when stuck in traffic on hot and humid
days so that he could get ice cold air without drawing in
traffic fumes. Brett did publish somewhere a wiring diagram
of his modification - but I never understood it, so haven’t
done it on my car.–
The original message included these comments:

On the MY91 cars, what conditions must be met for the ECU to
open the recirc flaps and or close the heater bypass valve?
(if a heater bypass valve has been installed)
I know a few years ago, one poster built a separate switch
and control system to be able to deliberately command one or
both of those vacuum solenoids and I’ve thought of doing the
same.


Bryan N, '91 Sovereign 4.0 L, RHD
Cambridgeshire, United Kingdom
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In reply to a message from Bryan N sent Fri 8 Aug 2008:

Bryan,

I was not sure if Brett had done the modification to the
recirc system or not, will do some more digging in the
archives. For some reason, had remembered Dave Lokensgard
as being the man behind the idea.

If I remember correctly, the biggest quirk to his system was
the inclusion of some resistors to mimic the solenoid coils
to the A/C ECU when the switch was being used to bypass the
functions.

I’ve got to admit, I’m really growing into the idea of my
own controls for both, not sure if I really trust the logic
embedded in the ECU. Making/installing the controls is
easy, it’s making the switch look like it belongs is the
challenge.–
The original message included these comments:

Win,
I’ve never understood under which circumstance the A/C ECU
will automatically close the flaps on the blower casings to
switch to recirculation mode (except that I know it does
that when you turn the A/C system off).
Check the posts from Brett Gazdzinski in the Modern list
archives where he describes the fitting of a three position
switch so that he had recirc air only, outside air only, and
could also revert to the normal Jaguar automatic function -
whatever that is! :slight_smile:
Brett said that a major benefit of being able to select


91 XJ6, 93 Sov, 97 Sov – Driveway’s crowded!
Tampa Fl, United States
–Posted using Jag-lovers JagFORUM [forums.jag-lovers.org]–
–Support Jag-lovers - Donate at http://www.jag-lovers.org/donate04.php

Visit the Jag Lovers homepage at http://www.jag-lovers.org for exciting services and resources including Photo Albums, Event Diary / Calendar, On Line Books and more !