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The Citroën Guide Air Conditioning: Air conditioning 50
Air conditioning
Once considered pure luxury, air conditioning and
other forms of climate control have became stan
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dard items on today’s car. After all, creating an
acceptable environment for the driver is more
than a mere question of comfort, it contributes to
safety to a great extent.
There were several climate control systems fitted to our
Citroëns, offering various degrees of automation of keep
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ing the climatic conditions inside the car. The system can be
manual, semi-automatic or automatic. The manual ver
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sion also came with separate settings for driver and passen
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ger.
The semi-automatic system is rather similar to the man
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ual one, the visible difference is that the operating knob on
the dashboard is marked in degrees instead of just blue and
red. The direction and recirculation controls are indentical
to the manual system. The automatic climate control looks
radically different, with a controlling panel using buttons
and a digital temperature display.
The AC system in the XM is fairly simple. If it is on, the air
is always cooled to about 8–10 °C on the inlet side (this is
varied between the air intake from outside and
recirculation from the inside) and then if you set a higher
temperature, it's reheated. The heater also always works, its
effect is only regulated by allowing air to flow or not to flow
through it (this is what the flap valve does). The air always
flows through the AC heat exchanger. As a result, the AC
also dries out the air whenever it is on. Once the air passes
out of the temperature regulating flap valve, another flap
valve regulates where it goes inside the cabin. That's really
all there is to it.
The AC system itself is almost self-sufficient. It has a radia
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tor, compressor, heat exchanger with evaporator, and a con
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denser—and the connecting pipes. The climate control ECU
actually only provides a signal to a relay that switches the
AC system on by operating the electric clutch on the com
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pressor. This same signal switches the radiator fans on to
the low speed. The AC system in turn sends a 'fans to full
speed' signal to the fan controller, when the coolant temper
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ature reaches a trip point (this is handled by a different
switch section in the same pressure sensitive switch that pre
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vents the AC going on without any coolant in the system,
described above).
As far as I know (unless it changed in later versions), the
AC itself (as oposed to climate control) never had an ECU.
The evaporator has an integrated pressure/tempera
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ture valve, opening up the pressure line to the return line.
After coming through the evaporator, the temperature
of the fluid (more precisely, a mixture of liquid and vapor)
suddenly drops because of the drop in pressure. It enters
the heat exchanger which operates like a radiator, cool
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ing the air and heating itself up. The fluid then goes back to
the drier-radiator-compressor end of the loop. The con
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densed moisture is collected from the heat exchanger and
let out through to floor of the cabin via a plastic tube.
As the air always enters through the heat exchanger, and
whether it gets cooled at this point, depends only on
whether the compressor is working or not. The tempera
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ture flap only decides which part of the air is going to be
taken before or after the heater radiator. This is how the
temperature is regulated.
When the compressor is on, is to condense the mois
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ture out of the air, and then re-heat it as necessary to the
temperature set on the controls. Since the temperature is
regulated by the temperature flap, it has really nothing to
do with the compressor at all—the only consequence of the
compressor not working (for any reason) is that the system
will obviously not be able to produce a temperature lower
than ambient.
There are four sensors providing input. The first one is at
the entrance of the air, before the heat exchanger, the sec
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ond one after the temperature flap, the third one on the
roof, and the last one in the heat exchanger. They have very
different but sometimes overlapping roles.
The first three collectively influence temperature regula
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tion. In particular, the sensors after the temperature flap
and on the roof determine what the actual temperature is.
The sensors before the heat exchanger and after the temper
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ature flap decide how fast the temperature flap will be
moved to prevent extremely fast changes in temperature in
the cabin. This does not alway work very well, which is why
you get a blast of air when the system is set on auto and
you leave the car in the sun in summer. Both of these param
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eters (temperature and temperature difference) influence
the fan speed.
compressor
condenser
pressure
release
valve
evaporator
receiver
dryer
a
mbient air
cooled air
refrig. fluid, cool
refrig. fluid, warm
