CITROEN DS series Technical Manual page 51

U
The Citroën Guide
The sensor before and in the heat exchanger as well as
the teperature selection, influence the AC part, i.e. the oper-
ation of the compressor. For instance, the compressor will
not operate below a certain external temperature. Also, it
will not operate if the temperature is set to maximum.
When the system is cooling the incoming air, it needs to
have the exchanger at a temperature which is lower than
the ambient air temperature, obviously. As the compressor
either runs or not, it cannot cool just a little bit—it always ei-
ther runs on full or does not run. When it starts, it will start
cooling the heat exchanger. How cold it will get, depends
on how hot the incoming air is and how much air is coming
in. In any case, when it gets significantly colder than the in-
coming air, the moisture from the air will start to condense
on the heat exchanger, which is why there is a collector
underneath it and a drip outlet. If the compressor keeps on
working, while the heat that needs to be taken from the air
is lower from the heat transfer ability of the whole system,
the heat exchanger will continue to progressively get
colder. If nothing is done, it will get well below freezing (it
can go as low as –40 °C given proper fluid, and of course
construction designed for this). What will happen then is
that the condensed water from the air will start freezing on
the heat exchanger fins, and eventually, the whole thing
will become a solid block of ice (usually there will be a crack-
ling noise to acompany the event), preventing actual air
flow. If the condition persists, the pressure in the system
will build up until the valve in the evaporator opens, and by
this time it is possible that the fluid actually gets heated up
enough that the remaining part going through the heat
exchanger will actually melt the ice producing a fog (I've
seen it happen!). All of this will be the lucky turn of events,
asuming the ice has not cracked the heat exchanger and
that there is no fluid leak.
So, obviously, there is a sensor, and that's the fourth one
in this story, which detects the temperature of the heat
exchanger becoming too low. When that happens, the com-
pressor is cut out, until the heat exchanger temperature
rises to an acceptable level. The thermal inertia and differ-
ent cut out and cut in temperatures insure that the compres-
sor doesn't keep switching on and off too quickly, which
would place an undue strain on the electromagnetic clutch.
The logic in the ECU is done very simply, if the fourth sen-
sor detects that the heat exchanger is too cold, the compres-
sor will switch off, regardless of the AC switch and tempera-
ture set. The only thing it will do, as I said in the earlier mail,
is that it will switch on for about 1 second whenever the AC
switch is turned on, this is probably some ECU feature. The
compressor will never turn on if the gas pressure is insuffi-
cient, and this part is handled by the pressure switch on the
drier, and has nothing to do with the ECU. In fact, the ECU
only gives the whole system a 'go-ahead'.
Air Conditioning: Air conditioning 51