In reply to a message from Eliot Brenner sent Thu 6 Jul 2006:
I’m not sure I fully understand your text - seems to be the
odd contradiction in there somewhere!
I believe that when conducting this type of test, you should
do it whilst the car is at rest (to avoid an accident!) and
recognise that you can get weird results if you have the
system in ‘Auto’ mode. This is because in ‘Auto’, the main
influence on the temperature output from the system is the
cabin temperature sensor. Since that component is on the
footwell ducting, it is greatly affected by the temperature
of the air exiting the footwell vents, less so the air
exiting the fascia vents, so sticking your hand in front of
the vents doesn’t mean an awful lot - the system is trying
to control the overall cabin temperature.
If you want to test the system and avoid weird temperature
fluctuations at the vents (and varying blower speeds), set
the control to ‘Man’ mode so that the ECU ignores the cabin
temperature and controls the temperature of the air exiting
the vents in direct relationship to the setting on the
temperature control knob and, in respect of the fascia
vents, as modified by the position of the face level slider
In essence, in manual mode, whatever the temperature you set
on the control knob on the panel, with the slider control in
the ‘hot’ position, you should get air from both the
footwell (front and rear) vents AND the fascia vents at a
similar temperature. If you then swing the slider control to
the ‘cold’ position, the temperature of the air from the
footwell (front and rear) vents should remain the same, but
the temperasture of the air exiting the fascia vents should
be lower than that at the footwell vents.
In all of this of course, dependant upon the temperature you
set on the control knob, the air exiting the fascia vents
may do so entirely through the side vents (control set
towards heat) because the centre vents will only open when
the system thinks that cooler air is required (control set
towards cool - or A/C is brought in to play).
It is possible on certain temperature settings (‘on the
cusp’) to play with the slider control and watch the centre
vent flaps opening and closing (after a suitable built in
delay) as that control is slid from hot to cold and vice-versa.
Try it again and let us know what you get.–
The original message included these comments:
OK, I just came across one I’ve not seen before, or at least have slept
89 xj40 with delanair unit. Temp in mid '70s, but very humid. A/C on low
(two drops) just to get humidity out. Temp dial set to hot because I did not
need much A/C. Fascia switch set to coldest.
Anyone see anything funny here? Am I missing something?
Bryan N, '91 Sovereign 4.0 L, (RHD)
Cambridge, United Kingdom
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