Volvo Air Conditioning

Bigblue · Post by **Bigblue** » 09 Jun 2012, 20:55

I have been reading “The Big Air Conditioning Master topic” and I was surprised at some of the misconceptions I found in this thread…I am not an AC engineer and have not worked in the automotive field since the 1960’s, but in my time I serviced a lot of Freon AC systems…While the refrigerants and oils used today are different, everything else is pretty much the same…I thought maybe some of these misconceptions could be put to bed if a little understanding of how refrigerants and AC systems work were forwarded…
Someone said that the pressure in a can of R-134 was 50 pounds…Not!...The pressure in the can depends on the outside air temperature and can be 28 psi at 32 degrees, 126 psi at 100 degrees, or anything in between…If the can is connected to a running system that is not correctly charged, there is no telling what the pressure might be, but if you know the ambient temperature and have a temperature/pressure table, you can correctly charge the system …Temperature/pressure tables are available on the internet…Google will find them for you…
The static pressure in the AC system (or can) depends on the ambient air temperature and the temperature of the system…If the air temperature is 75 degrees, the entire car will be 75 degrees and the pressure in a correctly charged AC system should be 79 psi with the system cold and not running…Refrigerant in a liquid state boils off to a vapor in a sealed container (or system) until a certain temperature/pressure relationship is reached, after which boiling will stop…This boil-off/cut-off pressure depends on what the temperature is…If the temperature is raised, more boil-off and a higher pressure will result…If the temperature is lowered, the refrigerant vapor will condense back into a liquid and the pressure will drop…
Once the AC is started in a correctly charged system, the compressor suction will pull the low side down to around 32 psi…This is ideal because that at that pressure the temperature of the evaporator will be 36 degrees, just high enough to keep the evaporator from icing up…The high side will be pressurized to around 200 to 250 psi…A ball park figure for the high side pressure is 2.5 times the ambient temperature…For example, if the outside air temperature is 100 degrees, the high side pressure will be about 250 psi…These figures will vary depending on the outside air temperature, the relative humidity, and the state of charge…
It has been pointed out that Vadis advises charging through the high side…We cannot do that with a Volvo 850 because it does not have a high side port…Vadis was written for the professional Volvo mechanic who will be using a bulk AC service station…This station will have a vacuum pump to evacuate the AC system, a reclamation tank to collect the old refrigerant, a heated bulk tank of refrigerant mounted on a weighing scale, and a set of pressure gauges…They will pump down (evacuate) the system and then fill it by weight, not pressure… For instance, my Volvo calls for 2.17 lbs of R-134… The advantage to this method is that the confusing temperature/pressure thing is not a factor…The pressure gauges are used for trouble shooting and checking for proper operation after filling…It would not really matter whether the high side or the low side is used during charging, but the pressure throughout the system will stabilize more quickly if the high side is used…Evacuation and recharging by weight is the only really reliable way of recharging the AC system, especially in systems like the Volvo 850 that do not have a high pressure port…Evacuation gets the moisture out which can cause a raft of problems, from corrosion of valves and orifices, to freezing and blocking of said orifices…Moisture can get in when folks do a shade tree recharge and do not purge the recharge hoses properly…
The cut-off switch that we are advised to jumper out near the firewall in the Volvo 850 is not the high pressure cut-off, it is the low pressure cut-off set to cut out at 25 psi and it should not be jumpered… The purpose of the low pressure cut-off is to protect the compressor from damage if the system loses it’s charge…When a system is low on refrigerant but is forced to run by jumpering this switch, the compressor is running without enough lubrication…The oil circulates through the system along with the refrigerant and if there is not enough refrigerant, there is not enough oil…This could damage the compressor…The right thing to do is to keep adding refrigerant until the switch cuts in and stays “cut in”…A poorly performing system could be because the compressor is worn from being forced to run with a jumper installed…The Volvo 850 is best serviced by evacuating the system and then recharging it by weight…
Two systems are used in various Volvo models…One uses a thermostatic expansion VALVE and a receiver/dryer…The other uses an expansion TUBE and an accumulator…A thermostatic expansion valve is never used with an accumulator and an expansion tube is never used with a receiver dryer…The receiver/dryer is in the high pressure side at the output of the condenser and stores the liquid refrigerant as it comes out of the condenser…The liquid refrigerant is released from the receiver dryer to the expansion valve, which in turn releases it as a fine spray into the evaporator where it quickly vaporizes…It is now a low pressure vapor and it exits the evaporator, going directly to the compressor…
In a system with an expansion tube and an accumulator the accumulator is in the low pressure side at the output of the evaporator…This system operates by flooding the evaporator with liquid refrigerant…It should be a vapor by the time it leaves the evaporator, but sometimes it isn’t…If this liquid were to reach the compressor, it would destroy it…The accumulator “accumulates” the liquid and then releases it as a vapor to the compressor…Receiver/dryers are located in the high pressure side of the system and are usually robust…Accumulators are located on the low pressure side and might be a deep-drawn sheet aluminium can…
The expansion tube/accumulator system is used in the Volvo 850…Expansion tubes are prone to clogging and can cause all kinds of problems…When stubborn, difficult to diagnose problems with odd-ball pressures appear, it is a good idea to change the expansion tube…In the Volvo 850 it is located in the high pressure line near the serpentine belt on the lower right side…

Post by **matthew1** » 09 Jun 2012, 21:42

Thanks Bigblue. I'll add this to the master AC topic.

Everyone else: please note I'm not endorsing this information -- nor any opinion anywhere on these forums unless I specifically write so*. Like anything on the Web, I suggest doing your own independent research to corroborate viewpoints/opinion/data.

* and even when I do it's no great shakes because I'm neither an engineer nor a mechanic

Bigblue · Post by **Bigblue** » 09 Jun 2012, 21:55

AMEN!

Post by **MoVolvos** » 09 Jun 2012, 22:00

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From what I remember studying the 2 year HVAC program at the local J.C. in the mid 80's and doing a (very) little Commercial and Automotive work the write up is good.

Blessings,
BKM
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Post by **jimmy57** » 09 Jun 2012, 22:31

Good but there is a point that needs correction. The low side pressure switch is there for a few reasons, compressor cut off for low charge is a side effect and not the intent.
The pressure cycling switch on orifice tube a/c units without variable displacement compressors is there because the system is not regulated. The orifice provides no throttling of refrigerant to match the heat load. The pressure cycling switch is there to prevent freezing when the heat load is low by shutting off the compressor until pressure rises to 42-45 psi which is not so high as to let evap temp increase too much but is enough to let the evaporator thaw if it has a bit of localized icing. The cut off when low on charge or when it is just too cold outside are nice side effects.
On many late model systems with expansion valves the reasoning for TXV is changed a bit. TXV allows better cooling at low engine speeds when the compressor efficiency is low on fixed displacement compressor systems. The TXV will keep the pressure drop in evaporator more consistent and keep the low side pressure in the range to enable evaporation more readily.
TXV systems use evaporator temp probes and still cycle the compressor when heat load is low.

There is something that is not done regularly when servicing a/c systems: operating the system at the recommended conditions for diagnosing: 1200-1500 rpm, fan speed at a click below high, recirculate selected, windows and doors closed, and hood closed on any where there is a seal used between hood and core support or any part of the cooling system "stack".
The 32 psi is regularly not attainable with engine idling unless the car uses a variable displacement compressor.
Volvo uses VD compressors on3.0 turbo and 3.2 litre short 6, V8, and the new S60 with 5 cylinder. Other usage in other markets outside North America. The VD compressors have higher capacity and when regulated to maximum displacement they have almost the full capacity needed.

Post by **MoVolvos** » 09 Jun 2012, 23:46

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Not to make things confusing or complicated for the Average DIY guys, as I do not work or like to work on my own AC system, but the Low Side Pressure Cutoff is not to be override for Charging purposes. It is first and foremost Designed and Used for Compressor Protection and Secondarily as a Control Device (not Thermostat) as in fixed orifice tube systems in autos or simple commercial systems. This can only occur with no leak and the correct amount of Refrigerant in the system to operate properly under designed operating pressures.

Designed as a safety device to help protect the compressor from catastrophic failure, the low pressure cut out switch typically senses pressures on the low side of the system.
http://youracauthority.com/acterms/Low_ ... itch.shtml

A/C CYCLING PRESSURE SWITCH Used with fixed orifice systems, this switch is used in place of a thermostatic cycling switch to prevent evaporator icing. It is often located in the accumulator/drier, although it may be located anywhere in the low-pressure side of the system. The switch compares low-side pressure to atmospheric pressure, and may be used, not only to prevent evaporator icing, but also as a low-pressure cut-off in the event that the system loses its refrigerant. This prevents damage to the compressor that results by running it without lubrication
http://autoacrepairs.com/gpage2.html

Their primary function is protection. When abnormal low pressure occurs, they stop the compressor.
http://www.real-world-refrigeration.com ... rol-4.html

Cutoff used as Temperature Control
http://www.real-world-refrigeration.com ... tem-2.html

Blessings,
BKM
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Bigblue · Post by **Bigblue** » 10 Jun 2012, 07:57

Admittedly the low pressure cutoff has more than one function...The point I was trying to make was peventing compressor damage during recharge, which is what we should be most concerned about as a back yard mechanic...The technicalities of TXV's, either internally or externally regulated, VIR's, etc, is beyond the scope and need to know for the average guy...

Expansion tube systems suffer from the low speed heat load phenomenen and I am sure there are many who curse their AC when they get stuck in bumper-to-bumper on a 95 degree day...A replacement for the expansion tube, called a variable orifice valve (VOV) can mimic the thermostatic expansion valve to some extent and are available from Groton...They are a lot more expensive than a conventional expansion tube, and while I have no first hand experience with them, they might be well worthwhile to some folks...

Post by **plabas** » 05 Jul 2012, 21:11

I have a 2005 s60 r that was diagnosed with locked compressor and melted clutch. Dealer replaced it and I drove short way home with nice cold air. Later on it started blowing hot air again and I took it back. They took day and a half to tell me txv was bad. They took that out and found non metallic debris inside which is from filter drier I am assuming. They are saying new txv and condenser are needed because condenser could be clogged with the debris, but all the debris got stuck in the txv. Does replacing the condenser that does not have a hole in it sound right? Any insight is welcomed.

Bigblue · Post by **Bigblue** » 11 Jul 2012, 14:50

Your instincts are correct, a non-leaking condenser does not need replacing...You are also right in assuming that the txv trapped the debris in it's screen and most likely none got into the condenser...I think the debris came from the original damaged compressor...Piston ring and seal material will be thrown off by the compressor when it fails...The receiver-dryer will trap a lot of this debris and should have been replaced, and out the system should have been flushed...The txv might be OK if it is cleaned, but since it is out anyway you might as well replace it...

Post by **jimmy57** » 11 Jul 2012, 18:12

Condensers have changed to parallel flow on everything I have touched over the last dozen years. PF condensers have huge efficiency gains and keep high side pressure down. The downside is they can't be flushed if compressor bits or dessicant gets in them from a failure. The paths are parallel so you blow though it and the flow from your blower or liquid flusher goes through the open flues but does not build pressure to blow debris forward or backward out of the plugged ones. Later on the refrigerant flow can dislodge the debris and you can have repeated orifice or TXV clogging.
Most A/C parts wholesalers have a brief compressor warranty if PF condenser not changed and a 1 year or 2 year warranty if you change condenser.
I had this issue bite me in the butt repeatedly when PF condensers went into use. They are right in wanting to do this. The rec/drier dessicant getting loose and stopping refrigerant and thus oil flow may have taken out the compressor in the first place. Their mistake was not quoting the condenser first time around. I would have a talk on that error and how the responsibility for anything other than the condenser and the labor for condenser should not be yours.

Grabbed from one of many articles on this:
When a compressor fails, it may spit metallic debris into the A/C system. Most of this debris ends up in the condenser where it can block tubes and interfere with efficient cooling. Some of the debris may be carried to the orifice tube or expansion valve and create a blockage. Debris can even be blown back into the suction tube. If not removed by flushing, it can be sucked back into a new compressor and cause it to fail.

Flushing the hoses is always recommended following a compressor failure. Flushing the condenser is also recommended. But with many condensers, replacement is the only sure-fire way to get rid of contaminants. Older serpentine-style tube-and-fin condensers can often be flushed successfully, but parallel flow condensers are very difficult to clean. So too are newer style condensers with extremely small extruded tubes. For these kinds of applications, the condenser should be replaced. It is expensive, but not as expensive as ruining a new compressor because of residual debris or sludge in the old condenser.