Description of Basic Operation
 

The compressor runs until the temperature becomes so low there is a danger of ice forming in the evaporator core, at this point the compressor is switched off by the low pressure switch (pressostat).  The compressor remains off until the temperature rises to a preset temperature, when the switch turns on the compressor again.  This is known as “cycling” and maintains the air conditioning at the optimum operating temperature.

The compressor feeds hot compressed coolant gas to the top of the condenser (located in front of the radiator) where it is cooled and becomes a liquid. It is then fed from the bottom of the condenser to the “orifice tube” just before the input tube inters the cabin through the firewall, where it expands and boils to become a cold gas which then cools the evaporator core inside the cabin HVAC (Heater, Ventilation, Air Conditioning) unit.

Heated coolant from the evaporator now feeds back to an “accumulator” bottle (also known as a “receiver/drier”) at the front right of the engine compartment. This accumulator is used primarily to collect any liquid refrigerant before it enters the compressor, where it would cause damage. The accumulator also contains a drying agent or desiccant to remove any moisture from the system, which can cause severe damage.

The compressor is driven by a pulley from the engine crankshaft, and is activated by a magnetic clutch on the pulley. Power to the clutch is supplied from the Air Conditioning Relay in the passenger cabin. The relay, and hence the air conditioning, is also controlled by a number of switches and controls to ensure safe operation of the air conditioning system.

12v power is routed from the a/c ON switch through the a/c relay switched contacts, through the high pressure switch “pressure OK” output, through the low pressure switch contacts and finally the compressor overheat protection switch (mounted on the compressor housing) to the compressor clutch solenoid.

The Air Conditioning Relay is mounted behind the passenger glovebox, and is actually operated under the control of the ECU (Engine Control Unit). Power is applied to one side of the relay from the cabin fan speed switch or from the ECU. The other side is grounded by the ECU to operate the relay.

The High Pressure switch protects the system should the operating pressure rise too high. Both “Excessive Pressure” and “Normal Pressure”  outputs from the switch are applied to the ECU.  The High Pressure switch will be found in the high-pressure refrigerant line next to the accumulator bottle in the engine compartment – the actual position may vary depending on the model and year.

The Low Pressure switch (Volvo call this a Pressostat) is located in the refrigerant line near the firewall at the back of the engine compartment. If there is insufficient gas pressure in the system this will prevent the compressor from starting to avoid damage due to no lubrication. Low pressure will cause rapid on/off cycling of the a/c system.

For the A/C to operate, the dash A/C switch, cabin fan motor switch, high pressure switch, and the low pressure switch must all be closed and the ECU functioning correctly – if all is OK then the a/c relay will be operated by the ECU and the compressor clutch will then engage.

While the a/c is switched on the radiator cooling fan will run continuously under control of the ECU.

The air conditioning can also be cut off by the ECU under conditions of maximum throttle, for maximum engine performance.
 

 

Temperature regulation and controls

The Volvo 850 uses a system known as a “Cycling Clutch system with Orifice Tube” (CCOT).

There are two types of HVAC control fitted to the 850. The base and early models have manual damper controls activated by cables, except for the recirculation damper which is motor controlled and has a 4 pin connector.

The ECC (Electronic Climate Control) is capable of automatically maintaining the cabin temperature constant. This version operates all dampers in the HVAC box and the recirculation damper by electric motors, which also have positional feedback to the ECC/ECU and use a bigger 6 pin connector (when replacing the RECIRC motor you must specify whether manual or auto a/c). Other than this, both systems are broadly similar.  The information in this article refers to the manual system.

The air input to the HVAC unit can be either fresh air from outside the car or recirculated air from the passenger cabin.  The selection is made by a motor controlled damper which is located behind the glove box. The motor is activated by the dash RECIRC switch.

There is a common blower fan and duct work for both heating and air-conditioning purposes. Air passes from the fan through the evaporator into the heater and control box. If heat only is required the air conditioning compressor can be switched off by the dash A/C switch.

Cabin temperature control is accomplished in the 850 primarily by reheating part of the cooled air in the heater core. Hence if the heater is faulty or has been removed there will be no control of air temperature inside the car - other than by opening or closing panel and cabin recirculation vents, and by changing the speed of the fan motor.  Under these conditions any real temperature control is difficult to achieve.

There is a variable “air mix” control which takes ambient or cooled air from the chamber after the air conditioning evaporator and feeds it to the center eye level vents, where it mixes with the other processed air from the HVAC. The purpose of this is to provide the driver with cool air if required to prevent drowsiness.

Ambient air from outside the car will only be provided if both air conditioning and recirculation switches are OFF (and the cabin fan operating), as the air is taken from after the aircon evaporator.
 
 
 

Air Conditioning Performance
 

Most air conditioning problems are due to a loss of refrigerant. In systems that are in good condition, some refrigerant loss every year is considered normal. Owners should therefore not worry unduly if they have to top up with refrigerant periodically. However, high loss rates indicate a leak which needs to located and repaired.

Quick Test for Low Refrigerant Charge

Most problems with the 850 are due to a low refrigerant charge. If your aircon isn’t working at all try the following procedure to determine whether this is the fault, before you start on circuit tracing and testing.
Start the engine with AC on and set to coldest setting, RECIRC ON, Fan on 4. If the compressor does not start to run, the amount of refrigerant left in the system - and hence the system pressure - is probably too low to operate the Low Pressure (warning) switch.  Locate the switch in the line at the back of the engine compartment next to the firewall.  Remove the connector and jumper the two leads (green and gray). The compressor should now run.  Do not run it for long periods without a refrigerant charge or damage will occur to the compressor.
 

Most air conditioning problems are due to a loss of refrigerant. In systems that are in good condition, some refrigerant loss every year is considered normal. Owners should therefore not worry unduly if they have to top up with refrigerant periodically. However, high loss rates indicate a leak which needs to located and repaired.

So how can you tell if your aircon is working normally?

Run the car around for 10 minutes at the coolest setting of the HVAC and the fan at high speed, close all vents except for the center panel vents. RECIRC switch ON, engine speed 1500 rpm.

The radiator electric fan should run continuously when the aircon is switched on.

Measure the temperature by placing a thermometer in a center dash vent. The temperature will vary depending on outside air temperature and humidity. Because of this, it is difficult to say exactly what evaporator outlet air temperature should be. Typically, with an outside air temperature of 70 degrees and 20% humidity, the evaporator outlet air temperature should be perhaps 35-45 degrees F. With 80+ degrees temperatures and 90% humidity, the temperature might be as high as 55-60 degrees F.  On most systems, there should be an approx. 35 degree F drop compared with outside temperature.

Check compressor cycling. The compressor will cycle periodically on cold days, or when the fan speed is on low and the cabin is cool.  In other words - whenever the evaporator core is in danger of freezing.  On very hot and humid days it may not cycle at all.  Fast cycling, every 5 seconds or so, indicates loss of coolant.  [Closing all vents, and setting fan speed 1 will usually cause the system to cycle].

Next, feel the output line from the compressor to the condenser. This line will be hot and can burn.

Check the condenser (this is the cooling core at the front of the radiator (open the hood and you’re looking at it) - by feeling up and down along the return bends for a temperature change. There should be a gradual change from hot to warm as you go from the top to the bottom. Any abrupt change indicates a restriction, and the condenser needs to be flushed out or replaced. Look for leaks on the face of the cooling fins – they will show up as oily patches on the core.  Most important – keep the condenser clean and free from debris and bugs by carefully cleaning or blowing through the core.

Feel the line from the condenser outlet to the evaporator. It will be warm until it connects to the Orifice Tube (you can see a visible restriction in the pipe) just before it enters the passenger cabin. At this point the tube will change to be cold and will sweat condensation.

The line from the evaporator outlet to the accumulator bottle should be cold and sweating with condensation.  If it is covered with thick frost, this might indicate that the evaporator is being flooded. This can be caused by an excess charge of refrigerant. If this occurs the high pressure switch will usually operate to protect the compressor.

Look at the accumulator bottle. The inlet and outlet lines should also be cold and both the same temperature. Any temperature difference or frost on the lines or bottle are signs of a restriction. The bottle should however be cold and sweating condensation.

The line to the compressor should also be cold to the touch, but the compressor itself should NOT sweat.

If all of these tests appear normal but the system still does not seem cool enough, it is probable that the system needs topping up with refrigerant. Your system may have a leak.
 
 

Leak procedure

The correct way to perform a refrigerant recharge is to extract and recover the refrigerant under a vacuum at a specialist shop. In some countries a shop aircon recharge costs only 5 dollars or so, it is not worth using a DIY kit.  However, in countries where a recharge is expensive, cans of refrigerant and parts to top up the system are often available as a kit. Whether it is advisable to attempt a DIY recharge is best left to your own judgment after reading this article.

If the charge has completely bled away then moisture may have entered the system – this may cause trouble when recharging as the water can freeze inside the system and block it. However, if the system has only just lost charge, it may be worth trying to recharge it yourself as the desiccant in the accumulator bottle may have removed the moisture. If it has been empty for a long period then probably it is better to have the a/c professionally evacuated and recharged and a new accumulator bottle fitted.

WalMart sell an R-134a conversion kit which contains 3 lbs of R-134a refrigerant, l2oz of oil, can tap, hose, and also adapters to fit and convert old R-12 systems.  No pressure gauge is included, so judging when the system has enough refrigerant is the key to success.  If the system already has some remaining charge, this is not easy to judge. Other companies around the world doubtless sell R134a but their kit contents will vary. Often in countries where R134a is unavailable in cans it is because the cost of having it done professionally is very low – check around for prices.

Connect a can of refrigerant to the hose, then connect the hose to the recharge point, which is located in the engine compartment, under the diagnostic port.  Place a thermometer in the outlet vent as described above. Start the engine (try to maintain at 1500 rpm), with AC on and set to coldest setting, RECIRC ON, Fan on 4. If the compressor does not start to run, the amount of refrigerant left in the system - and hence the system pressure - is probably too low to operate the Low Pressure switch.  Locate the switch in the line at the back of the engine compartment next to the firewall.  Remove the connector and jumper the two leads (green and gray). The compressor should now run.  [Do not run it for long periods without a refrigerant charge or damage will occur to the compressor].

Keeping the can upright, open the valve on can to start the recharge. Placing the can in a bowl of hot water will speed up the process. The cycle time of the compressor on and off will get longer till it reaches its optimum point. Once the can is empty, stop the engine and close the valve to the can. Remove the hose from the AC connector on car and if necessary, replace the empty can with a new one. Add refrigerant until you believe the system is working correctly as described above, then stop. Too much refrigerant will cause the temperatures to drop too low, and the system will shut down under control of the high pressure switch for some minutes, then start up again. Be careful not to overfill the system as this can cause damage. If this is suspected use a test meter to see if the High Pressure switch has opened.

Some recharge kits provide a gauge to more accurately establish the state of charge of the system. To do this it is, strictly speaking, necessary to monitor both the high pressure part of the system (before the orifice tube) and the low pressure side, on the return from the evaporator to the accumulator.  Unfortunately, the 850 has only the R134a refill connector, which is on the low pressure side, where the system pressure can be monitored.  Normal operating pressure will be from about 40 psi down to 20 psi when the system is at it’s coldest, when the low pressure switch will operate and cut off the compressor.

Tips:

The A/C dash switch contains a small lamp which often fails. This may be replaced with a little care at negligible cost by carefully dismantling the switch. The RECIRC switch light does not usually fail as it instead uses a semiconductor LED. (Such an LED could also be used as a replacement for the A/C lamp).

In very hot and humid countries there have been some reports of the compressor’s overheat switch operating to close off the air conditioning, when in fact there is no reason to do so.  This switch is often taken out of service to rectify the problem.

A climate control manual is available from Volvo. The manual can be ordered from dealers or online.  This has complete details of the air conditioning system and is useful, if expensive.

The Haynes workshop manual is not very helpful and the schematic diagrams quite confusing unless you are familiar with the operation of the 850 aircon. For example few details of the ignition system and the EZK ignition module and ECC operation are shown. So it isn’t easy to see what is happening in order to trace a suspected fault.

System pressure is directly related to coolant temperature, the following chart may be helpful.

   
 
 

Degrees Fahrenheit	R-134a Pressure (PSIG)
-10	1.9
0	6.5
10	12.0
20	18.4
24	21.4
25	22.1
26	22.9
28	24.5
30	26.1
40	35.0
50	45.3
60	57.3
70	70.9
80	86.4
90	103.9
100	123.6
110	145.6
120	170.3
130	197.6
140	227.9
150	261.4
160	298.4
170	338