Part 2

Air conditioning system (Continued)

Control of the temperature in the passenger compartment
The air conditioning system has only two positions, off and on.
If it is too cold in the passenger compartment, the temperature can be controlled with the normal heat and blower fan controls.

Service socket
On systems with an expansion pipe, there is often only one nipple (on the low pressure side) to which the A/C station or pressure gauge can be connected.

System with TEV-valve (Thermostatic Expansion Valve)





TEV valve
Active flow regulator (= variable choke valve).
Consists of:
- a valve which is affected by a spring and a membrane.
- A capillary pipe with a resistor positioned on the evaporator outlet pipe.
- The position of the resistor has been carefully tested.
- The capillary pipe is filled with refrigerant.
- a pressure equalization pipe is connected to the evaporator outlet pipe.
Governs the volume of refrigerant which is released from the evaporator.
Too little refrigerant = the temperature at the outlet pipe increases ⇒ the refrigerant in the capillary pipe expands and exerts pressure on the membrane ⇒ the valve is opened and releases more refrigerant to the evaporator.
Too much refrigerant = the temperature at the outlet pipe falls ⇒ the refrigerant in the capillary pipe reduces in volume ⇒ the spring presses the valve back ⇒ the valve is shut and releases less refrigerant to the evaporator.
The temperature and pressure in the system are related to each other. Assumes that there is a drop in pressure in the evaporator, in which case the temperature at the outlet pipe also falls. This means that the valve is closed and there will be too little refrigerant - the capacity reduces. The pressure in the outlet pipe is led via a pressure equalization pipe to the underside of the membrane and compensates for any fall in pressure.
The TEV valve is carefully adapted for its system and for the type of refrigerant that is used.
The valve is calibrated to release through exactly the correct amount of refrigerant which can be evaporated completely and to keep the amount of super heating to a minimum (super heating is normally approximately 6-10°C (43-50°F). This is to obtain maximum cooling capacity and to prevent refrigerant in liquid form leaving the evaporator and reaching the compressor.

Freeze protection
Thermostat with capillary pipes or a temperature sensor depending on the car and model year.
Positioned where the evaporator is coldest (where frost forms first) The location is extremely important and is often tested manually.
The thermostat/pressure sensor interrupts a current circuit and disengages the compressor when the temperature becomes too low. This is so that the water that condenses on the evaporator does not freeze to ice and obstruct the airflow through the evaporator. Heavy build up of ice can also cause damage to the evaporator.
When the temperature increases again, the thermostat/temperature sensor closes the current circuit and the compressor is engaged.
The thermostat/temperature normally opens at approximately 2°C (36°F) and closes at approximately 6°C (43°F).

Receiver drier
The function of the TEV valve can be interfered with by gas bubbles in the refrigerant. Therefore the receiver drier must be positioned on the high pressure side and be able to separate gas from liquid.
The receiver drier outlet is located so that only refrigerant in liquid form can leave the receiver drier.
Refrigerant in gas form is collected in the top of the receiver drier. When the heat is transferred, the refrigerant will also condense.
There is a filter in the receiver drier which collects any impurities in the system.

Control of the temperature in the passenger compartment
Certain cars have an adjustable thermostat which can be adjusted for the evaporator temperature at which the compressor will be disconnected.
The maximum position (marked red) should however only be used briefly after start to rapidly lower the temperature in the passenger compartment, and if applicable in dry desert climates. Otherwise there is a risk that the condensation will freeze to is.
On most cars the air conditioning has only two positions, off and on.
If it is too cold in the passenger compartment, the temperature can be controlled with the normal heat and blower fan controls.

Service socket
On systems with a TEV valve, there is two nipples (one on the high pressure side and one on the low pressure side ) to which the A/C station or pressure gauge can be connected.

Monitoring functions, - components




Which functions/components there are, how they are designed and where they are located depends on the type of system, car and model year and the engine variant.

Pressure switches
Are located on the high pressure side and in principle consist of switches which open/close a current circuit or a pressure sensor which transmits a signal to the engine control module (ECM).

These can be:
- A: A linear pressure sensor. In this case, all the functions are always governed via the engine control module (ECM) and the current pressure can be read off using the Volvo Scan Tool (ST).
- B: Separate units.
- C: A combined unit with multiple integrated pressure switches.

There tasks are:
- to start the engine cooling fan (FC) at 1st or 2nd speeds at high pressure. The air flow through the condenser is then increased. More heat can be given off by the refrigerant to the air, the temperature/pressure drops and the capacity is maintained.
- to disengage the compressor if the pressure falls too low (= safety switch), in the event of leakage for example.
- to disengage the compressor if the pressure rises too much (= safety switch), if the pressure switch or engine cooling fan (FC) is not functioning for example.
On the majority of pressure switches from and including 1992 model years there is a valve case (Schraeder valve) in the connection. This enables the pressure switches to be removed without needing to drain the system.

Note! If the connection has internal threads (as at C in the illustration) there is no valve casing.

Safety valve
Is located on the high pressure side, on the compressor or on a pipe.
The valve opens and releases refrigerant if the pressure is too high in the system. It then shuts again when the pressure has returned to normal.
This function protects the system from exploding if, for example, the safety switch is not functioning.

Temperature switch
Is located on and gauges the compressor temperature.
Opens the compressor current circuit at approximately 105°C (221°F). Closes the circuit when the temperature has fallen to approximately 90°C(194°F).
This function protects the compressor from overheating/breakdown in the event of, for example, refrigerant leakage (= low quantity).

Other functions that may be present
The compressor can be engaged 10 seconds after the engine is started at the earliest.
Often governed by a timer circuit in the A/C relay or in the engine control module (ECM).
This function prevents the engine stopping immediately after start. The engine is subjected to a heavy load when the compressor engages.
Disengagement of the compressor at:
- wide open throttle (WOT) (often time governed disengagement).
- heavy acceleration and low speed (usually occurs on diesel engines).
- high engine coolant temperature.
Usually governed via the engine control module (ECM).
These functions are to provide optimal acceleration and to prevent the engine from overheating.

Values for pressure switches/safety switch/safety valve
The values vary depending on, amongst other things, the type of system, refrigerant and compressor.
The following values are only an example. They apply to a Volvo 900 with R134a refrigerant and Sanden SD-7H15 compressor.
- The engine cooling fan (FC) low speed is engaged at 18 bar and disengages at 14 bar.
- The engine cooling fan (FC) high speed is engaged at 23 bar and disengages at 19 bar.
- The compressor disengages at 30 bar and engages at 20 bar.
- The safety valve opens at 38 bar.

Compressor control




The current circuit for the compressor (engagement/disengagement) varies greatly depending on the type of system, engine variant and the car and model year.
The circuit may be very simple with few components or more complex with many components.

Example 1
The example shows the circuit on a 1977 Volvo 240.
The circuit includes:
- a switch to manually engage/disengage the compressor.
- a thermostat governed switch.
The thermostat senses the temperature at the evaporator via a resistor.
At too low temperatures (= risk of ice formation) the switch opens and the compressor is disengaged
When the temperature increases the switch closes again and the compressor is engaged.




1. Compressor
2. Air conditioning (A/C) relay
3. Engine control module (ECM)
4. Air conditioning (A/C) pressure sensor
5. Engine coolant temperature sensor
6. Throttle position (TP) sensor
7. Air conditioning (A/C) pressure switch (Pressostat)
8. Control module ECC
9. Passenger compartment temperature sensor
10. Outside pressure sensor
11. Engine coolant temperature sensor ECC
12. Sun sensor

Example 2
The example shows the circuit on a 1996 Volvo 960 with Motronic 4.4 and ECC.
The compressor is governed via a relay by the engine control module (ECM) depending on a number of input signals. With this connection the engine control module (ECM) can quickly (in advance) compensate for the changes in load that occur when the compressor is engaged and disengaged (= less jerkiness).

Function
The ECC control module calculates whether the compressor needs to be engaged to obtain the correct temperature in the passenger compartment. The calculations are based on the drivers requirements (= the settings of the control) and signals from a number of sensors.
If necessary, the control module transmits a signal (= requests engagement of the compressor).
The signal is sent to the engine control module (ECM) via the pressure switch (Pressostat).
The pressure switch (Pressostat) breaks the circuit if the pressure and therefore the temperature in the system (on the low pressure side) falls too low.
The engine control module (ECM) activates the A/C relay and the compressor is engaged.
(10 seconds after the start of the engine at the earliest - timer circuit in the control module).
The engine control module (ECM) disengages the compressor:
- when the request to engage from the ECC control module/pressure switch (Pressostat) ceases.
- at wide open throttle (WOT), for a maximum of 15 seconds (signal from the throttle position (TP) sensor).
- at high engine coolant temperature (ECT), above approximately 125°C(257°F) signal from the temperature sensor).
- at too high-pressure on the high pressure side, above approximately 30 bar (signal from pressure sensor).
The signals from the engine coolant temperature sensor and the pressure sensor are also used by the engine control module (ECM) to govern the engine cooling fan (FC).