SSP_208_d2.pdf

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Self-Study Programme 208 Air Conditioner in the Motor Vehicle
System control
At high (increasing) pressure
At high (increasing) pressure, the crystal is
“deformed“ more, so the change of resistance is
larger. The test voltage decreases
proportionately.
Pulse-width
modulated
signal
Microprocessor
Voltage
Test voltage
Silicon crystal
(resistor)
208_065
t A p
Pulse width signal
The pulse width increases in proportion to the
increasing pressure.
Period duration 20 ms
At a high pressure of 3.7 MPa (37 bar), the pulse
width is 18 ms. This is equivalent to 90% of the
period duration.
Pulse width 18 ms
208_066
Using the digital memory oscilloscope
of the new vehicle diagnosis system
VAS 5051, it is possible to visualise the
pulse width signal.
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Disconnected safety switch in the
refrigerant circuit with restrictor
In the refrigerant circuit with restrictor, the low
pressure and the high pressure are often
monitored by two separate safety switches.
F73
Low pressure
Air conditioner low-pressure switch F73 switches
off the compressor when the pressure drops
below approx. 0.17 MPa (1.7 bar) in the
refrigerant circuit, for example.
(This pressure drop can occur if the refrigerant
level in the circuit is too low.
The compressor is protected.)
High pressure
Magnetic clutch high-pressure switch F118
switches off the compressor when the pressure
exceeds approx. 3.0 MPa (30 bar) for example.
F118
The absolute values should always be regarded
as being system-specific.
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Coolant temperature switch with pilot lamp
The compressor constitutes an additional load
for the engine.
t A p
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To avoid overheating the coolant when the
engine is under heavy load, e.g. when travelling
uphill, the additional compressor load is
switched off.
For this purpose, the coolant temperature is
monitored additionally by a coolant temperature
switch with a pilot lamp.
(The primary monitoring device is the coolant
temperature sender with indicator lamp in the
dash panel insert.)
The compressor cuts out at approx. 119 °C and
cuts in at approx. 112 °C.
In vehicles with extended electronic
sensor evaluation via the
control unit combination of the vehicle,
this additional check is no longer
necessary. The signal generated by the
primary monitoring device is utilized.
Various switches with pilot lamp are used
depending on vehicle type, e.g.
F18 - Radiator fan thermo switch
F163 - Air conditioner
cut-off thermo switch.
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Cooling fan circuit
Circuit connecting fan to engine/
condenser cooling system
shown using the VW Golf/Audi A3 as an
example
Fan operation is a basic condition for proper
functioning of an air conditioner (refrigerant circuit)
and the engine (coolant circuit). Condenser
performance will be impaired if there is no cooling.
Proper functioning of the air conditioner is no
longer assured. In air conditioning, a second or
third fan is often also used.
These fans provide the necessary fresh air flow
through the radiator and condenser.
The fan control regulates radiator fan control unit
J293
depending on the temperature of the coolant and
the pressure in the refrigerant circuit.
The absolute values are always vehicle-specific!
Cooling fan
1st auxiliary fan
2nd auxiliary fan
Radiator fan control unit J293
Thermo switch F18
Air conditioner pressure
switch F129
Example: Check functions
Example: 2-fan combination
Coo ll lan tt t tt tempera tt ture
The signal generator is radiator
fan thermo switch F18.
The thermo switch is located in the vehicle
radiator.
Speed 1
– The air conditioner is switched on, therefore
the compressor is switched on and the
pressure in the refrigerant circuit is greater
than 0.2 MPa (2 bar).
= both fans run at speed 1
ON
92 .. 97 o C
– High pressure in the refrigerant circuit is
greater than 1.6 MPa (16 bar) and/or coolant
temperature is above 99 o C
= both fans run at speed 2
OFF
84 .. 91 o C
Speed 2
ON
99 .. 105 o C
OFF
91 .. 98 o C
Pressure ii in refr ii igeran tt t c ii ircu ii i tt t
The signal generator is
air conditioner pressure switch F129 or the high
pressure sender G65.
F129 switches the fan(s) to speed 2at a pressure
of approx. 1.6 MPa (16 bar) (also refer to
page 35).
– If the pressure in the refrigerant circuit drops
below 1.6 MPa (16 bar) and the coolant
temperature is below 99 o C
= both fans will again run at speed 1
– When the engine is being operated without
the air conditioner, only the cooling fan will be
running. The fan will be running at speed 1 or
2 depending on coolant temperature.
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Radiator fan control unit J293
The radiator fan control unit is integrated in the
vehicle control system.
Incoming signals in the basic version:
– from thermo switch F18
– from pressure switch F129
– from operating and display unit E87 (with
automatic air conditioner)
Tasks
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To convert the incoming signals
– To switch the cooling fan on and off
– To switch the compressor magnetic clutch on
and off.
Expanded functions of a new generation:
Radiator fan control unit J293 has been
developed technically and adapted functionally
to the new high pressure sender G65.
New generation
It is fitted together with the high pressure sender
and, as a distinguishing feature, has modified
plug connections.
The control unit evaluates the pulse-width
modulated signal from the high pressure sender.
The overall pressure range of the refrigerant
pressure is monitored continously in this way.
Functions
There are also circuit variants where the
functions of the control units are
assumed by an air conditioner control
unit.
Integration in their control system is
always vehicle-specific.
For details, please refer to the current
flow diagram.
– To switch the radiator fan speeds and the
magnetic clutch of the air conditioner com-
pressor on and off
– Bidirectional signal interchange with the
engine and gearbox control unit
– Monitoring the coolant temperature
– with timer module for activating coolant run-
on pump V51 (e.g. 1.8-ltr. 5 V engine 165 kW)
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Temperature control
Manual control
Evaporator
Fresh-air flow
Fresh air blower
Refrigerant circuit
Temperature-
adjusted
interior air
Condenser
Heat exchanger
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Why temperature control?
What is regulated?
– The fresh-air flow cooled down at the
evaporator is pumped into the passenger
cabin by means of the fresh air blower.
– This air is usually cooler than necessary
(blower capacity is designed for maximum
cooling, however the prevailing ambient
temperatures are usually moderate).
– To attain a pleasant interior temperature, a
portion of the cold fresh air flow is therefore
ducted over the heat exchanger the heating
system and heated up.
– Temperature fluctuations can also be caused
by different ambient temperatures, road
speeds, coolant temperatures, fresh air
supplies etc.
– In the case of simple manual air conditioners,
the driver has to regulate the temperature.
– Registration of actual values, i.e. temperature
sensing.
– Setpoint/actual value comparison, i.e. the
driver performs an individual evaluation. The
driver defines the comfort temperature, i.e.
whether too warm or too cold.
– Based on the evaluation, the driver decides
whether
• the temperature needs to be adjusted
• in what direction
• by how much
and makes this adjustment manually.
The driver is, in the figurative sense, both the
controller and the actuator.
The driver adjusts the temperature flap.
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