A/C Question
Thread Starter
Safety Car
Joined: Feb 2000
Posts: 3,771
Likes: 0
From: MANDEVILLE, LA La
A/C Question
After charging my A/c on my 95' by the book,now my low side lines from the rec. drier to the evaporator ices up.The Temp. at middle A/C vent is approx. 42F Degrees. Really Cold.Is this normal? Could I have a problem?
Le Mans Master
Joined: Jul 2007
Posts: 5,043
Likes: 8
From: Tampa Florida
That's a little on the cold side if the system has the proper charge level. I suspect that you have a blockage on the evaporator coil inside the AC/heater core box. A blockage causes slow/low air flow across the coils. Clogged coils fins can result in ice forming in and across the coils which can produce the low 40 degree temps your seeing. Remove the fan motor and take a look inside the box and clean the coils as necessary.
Melting Slicks
Joined: Oct 2004
Posts: 2,510
Likes: 4
From: Austin Texas
You really need to get an AC gauge set and check the high and low pressures. Your high side should be ~200psi and your low side 25-28psi. If you have a clogged Oriface tube, your High side pressure will be higher than normal and your Low side pressure will read lower. If your compressor is cycling more than 8 times per minute during idle, this could indicate a pressure that is too low on your low side. Which could be a clogged oriface tube.
SunCr should chime in as he is the AC expert on this Forum...
Melting Slicks
Joined: Oct 2004
Posts: 2,510
Likes: 4
From: Austin Texas
Well, Here are many excerpts from SunCr. He speaks specifically on icing, pressures, blockage etc. I have collected a lot of his
comments over time, and below is what I have so far.......daunting...isn't it......
A/C seems to be as much art as it is science.
QUOTE:
From SunCR : CF
The switches on the High Pressure Line are the High Pressure Cutoff and Fan Switch and the one, when disconnected, that makes the Main Fan run is the Fan Switch.
The High Pressure Cutoff should be closed when the system isn't running so you can check it with a DVM. It really shouldn't be opening if the system is right and from what you've described - initial cycling, but then it runs, I doubt it's causing a problem. Pressure builds as the system runs, not while it's sitting still (well, maybe a little bit, but that has to do with the outside air temp, not because it's being compressed into a high pressure gas).
A/c systems work by turning the refrigerant into a high pressure gas (job of the compressor) and then turning it into a high pressure liquid (at the condensor) which is then metered into the Evaporator through a restriction (the orifice) making it a low pressure liquid. Air, flowing over the Evaporator coils causes the liquid to boil and in the process, the air gives up it's heat. The refrigerants used in automotive systems (and most home units) do not change temperature while vaporizing or condensing.
Automotive systems are designed to maintain Evaporator temperatures as close to 32 degrees as possible. R12, at 30 PSI, will boil at this temperature. R134 at 28 PSI. Those are numbers to keep in mind when you look at the Low Side on the Manifold Gage Set.
After the gas has boiled and changed into a low pressure gas, the system is designed to add heat - called superheat in a/c and engineering speak - to the low pressure gas that has been made. A superheated gas cannot change back into it's liquid state until it gives up that extra heat and that does two things: 1. It maintains an Evaporator Temp a tad above freezing when the boiling has occurred at a temperature below 32 degrees; and 2, Lessens the chance of any liquid getting back to the compressor which would immediately destroy it - the same as a blown head gasket flooding a cylinder with coolant. (The opposite is also designed into what's taking place in the condensor - that's called subcooling which helps to keep the liquid from turning back into a high pressure gas). A superheated or subcooled gas will still have it's saturation pressure. You determine the amount of superheating or sub cooling by taking the temperature of the Evaporator Coil or Condensor and doing a little bit of math - most automotive techs ignore it and simply look at pressures.
To get it all working (and to keep it running), your system uses a low pressure switch, mounted on the Evaporator Outlet, as an indication of the static charge (compressor not engaged) and evaporator temperature (compressor engaged). That switch simply opens and closes based on the pressure of the refrigerant in the system. The switch contacts are closed at 50 Psi and as long as it has that much, the a/c clutch should engage when you request a/c at the Controls. Those contacts remain closed until the gas leaving the Evaporator drops to 25 psi. At that number, they open up and power is cut to the compressor. The high side continues to flow into the Evaporator and any remaining liquid boils and returns to it's saturation pressure for the outside air temp. That extra volume causes the pressure to rise and when it reaches 50 psi, the whole thing starts over.
A fully charged and functioning system, as mentioned earlier, shouldn't experience this more than 8 times in one minute. A low charge, or one that is severely restricted somewhere on the high side will cycle very rapidly and easily exceed this threshold. The gas that makes it into the Evaporator flashes off immediately and there's nothing left when it reaches the Low Pressure Switch so the compressor cuts out. What's left immediately returns to it's static pressure and in a nano second, it's back to 50 psi. An empty system doesn't do anything. If it's below 50 degrees, some engine heat is usually necessary to get the gas up to 50 psi and the system may cycle off and remain off for longer periods of time. If it's just starting up, it can take a couple of minutes to get everything up to speed.
In order to keep things constant, systems are designed for a certain volume of gas - usually measured in pounds - so that under most temperatures, the same air temp comes out of the vent. It's not exact because the temp of the high pressure liquid (and thereby it's pressure) coming from the Condensor is going to be determined by the air temp flowing over the condensor coils and the efficiency of those coils. But at least almost everything is fixed: Volume, Compressor Efficiency, and the restriction or metering device at the Evaporator. And to keep a constant temp (pressure) at the Condensor (when idling or at low speeds), the system relies on the Main Fan to start spinning once a designed parameter has been met. For your Vette, the Engineers decided that 200 psi was a good average number and the fan switch opens at 230 psi signaling the ECM to crank on the fan. That air flow cools the charge (reduces it's pressure), and the fan switch contacts reclose at anywhere from 170 to 190 psi turning the fan off (your FSM should give the specs). All of this keeps what's going to the Evaporator at around the 200 psi average.
R134 has a smaller molecule than R12 and as you can see, it freezes a tad lower than R12. That smaller molecule means you can cram more into a system, but that extra volume will make more pressure in a system designed to work with R12, making the 32 degree bogey at the Evaporator a tad difficult (if not impossible) to reach. That smaller molecule also allows R134 to reach much higher operating pressures so systems designed to use it crank on the fans at much lower pressures. That keeps the high side at the design number (unfortunately, impossible to do on your Vette - no one makes a different fan switch). Conversions try to overcome these deficiencies by using less gas and because of the lower freezing temp, reducing the threshold for compressor cutout to 22.5 psi (the Low Pressure Switch can be adjusted).
I might add that refrigerants, confined and doing nothing exist in a saturated state - some liquid; some gas - and have a pressure within that confinement that rises and falls with temperature. The Accumulator, which sits at the lowest point in the system, traps any liquid so that only gas reaches the compressor, but you will see pressure variations depending on the outside air temp. If you want to see how that static state translates into temperature or the temperature of that gas translated into pressure, you can use the Pressure/Temp Chart for the gas in your car.
I have never seen a pressure switch installed where you loose refrigerant when you remove the switch. All I have seen are isolated by a shraider valve under the switch, inside the A/C line. When you unscrew the pressure switch, the shader valve closes and cuts off flow.
Most automotive a/c systems need that 200 psi average to make cold air and to keep the Low Pressure contacts closed (so it doesn't cycle excessively). The magic number should be in your Shop Manual and a manifold gage set will tell you whether it's working right or not. On the otherhand, high pressure cutoffs cut power to the compressor clutch at 400 psi and high pressure relief valves spew refrigerant at about 600 psi. Newer stuff simply uses a thermistor like device to relay an electrical signal to the PCM proportional to high side pressure and the PCM uses that to control fan(s), high cutoff, idle, WOT and a/c (powertrain) diagnostics.
The correct setting is about 25 to 28 psi. This corresponds to the freezing temperature for R134a, in the evaporator. Any lower and ice may form at low loads.
Run the engine at about 2000 rpm to simulate cruising speed and the compressor shoud bring the pressure slowly down to the cut off and re-engage as the pressure rises. It may not cycle off at all if the car is very hot inside or you may have to run the engine a little faster.
Compressor operation is optimized for about 1500 to 2000 rpm. At idle the efficency is way down and the compressor shouldn't cycle unless the charge is low.
Cycling, orifice HVAC systems are designed to operate at head pressures that average 200 psi. Static, non operating pressures, are dependent on the outside air temp and rise with air temp. Once the compressor engages, the pressure immediately rises and that also creates heat - about 100 degrees at the condensor for this pressure. Cooling is provided by fan(s) which for most applications are turned on and off by the ECM and the ECM needs to know what the head pressure is in order to maintain the ideal pressure. This provides the best vent temps and by turning the fans on and off (which it should be doing if it's much below 50 degrees out), minimizes cycling of the compressor clutch. Since 1990 - for those vehicles not using a mechanical fan - information is provided to the ECM by a pressure sensitive sensor or thermistor mounted on the High Pressure line. It's fed a 5 volt reference from the ECM and each volt returned is approximately 100 psi. The ECM, based on this signal, will then ground the fan relay(s). Exact operating parameters should be outlined in the FSM - try looking in the electrical schematics under Fans or Compressor if it isn't in Driveability and Emissions. The easiest way to troubleshoot a problem is with a scanner. That will show you the a/c pressure sensor signal in volts (better ones also translate this to pressure) and you can observe the fan(s) to make sure that they are operating as designed. A couple of notes: Below 50 degrees, it may take some engine heat to raise system pressure to the threshold for fan operation - best to test when it's 70 degrees or better. Extremely high pressures - much above 260 psi for R134, might be indicitve of a restricted system - even though both fans are cranking. Hook up a manifold gage set so that you can observe both the high and low side pressures.
The only way to tell if anything is wrong is with a manifold gage set - I don't know how something with a "blue" area inscribed into the dial can compensate or tell you anything when pressure is dependent upon the temp of the air blowing across the condensor. Seems to me that's a good way to blow something up - especially if that blue turns red when the temp climbs.
Jumpering the low pressure switch harness contacts verifies that the circuit is intact but without knowing pressures, you could damage something - especially if there's not enough gas in it to carry the oil charge.
If the orifice is plugged up, your high will be too high; and the low too low. There are plenty of charts available to give you where they should be. Most restrictions, if severe enough, simply cause the compressor to cycle excessively (more than 8 times per minute at 70 degrees). Since the evaporator isn't getting a full shot of gas, what gets in flashes off immediately and there's no oomph left when it reaches the low pressure switch.
The low pressure switch should cut power to the compressor when the pressure is low enough to cause the Evaporator and suction line to ice over. Ice is created when the liquid refrigerant boils below freezing. If you're seeing ice, then something is probably wrong with the switch. Check it out by putting the system on high and pulling the blower motor connector. The low side will drop and the low pressure contacts should open cutting power. Note that pressure on your Low Pressure Gage. For R134, it should be 22.5 psi; for R12, 25 psi. Some switches are adjustable. Turn the screw between the terminals (if they exist) counterclockwise to lower the threshold; clockwise to raise it. Make your adjustments 1/2 turn at a time and retest.
If the pressures look normal, but it's making ice, something else is going on - probably a combo of refigerants. Those unlike mixtures fractionate and cause a host of funky problems. Hard to guess without knowing what the pressures are but if you simply dumped in some R134 or anything that is different on top of the original charge, it's time to start over.
comments over time, and below is what I have so far.......daunting...isn't it......
A/C seems to be as much art as it is science.
QUOTE:
From SunCR : CF
The switches on the High Pressure Line are the High Pressure Cutoff and Fan Switch and the one, when disconnected, that makes the Main Fan run is the Fan Switch.
The High Pressure Cutoff should be closed when the system isn't running so you can check it with a DVM. It really shouldn't be opening if the system is right and from what you've described - initial cycling, but then it runs, I doubt it's causing a problem. Pressure builds as the system runs, not while it's sitting still (well, maybe a little bit, but that has to do with the outside air temp, not because it's being compressed into a high pressure gas).
A/c systems work by turning the refrigerant into a high pressure gas (job of the compressor) and then turning it into a high pressure liquid (at the condensor) which is then metered into the Evaporator through a restriction (the orifice) making it a low pressure liquid. Air, flowing over the Evaporator coils causes the liquid to boil and in the process, the air gives up it's heat. The refrigerants used in automotive systems (and most home units) do not change temperature while vaporizing or condensing.
Automotive systems are designed to maintain Evaporator temperatures as close to 32 degrees as possible. R12, at 30 PSI, will boil at this temperature. R134 at 28 PSI. Those are numbers to keep in mind when you look at the Low Side on the Manifold Gage Set.
After the gas has boiled and changed into a low pressure gas, the system is designed to add heat - called superheat in a/c and engineering speak - to the low pressure gas that has been made. A superheated gas cannot change back into it's liquid state until it gives up that extra heat and that does two things: 1. It maintains an Evaporator Temp a tad above freezing when the boiling has occurred at a temperature below 32 degrees; and 2, Lessens the chance of any liquid getting back to the compressor which would immediately destroy it - the same as a blown head gasket flooding a cylinder with coolant. (The opposite is also designed into what's taking place in the condensor - that's called subcooling which helps to keep the liquid from turning back into a high pressure gas). A superheated or subcooled gas will still have it's saturation pressure. You determine the amount of superheating or sub cooling by taking the temperature of the Evaporator Coil or Condensor and doing a little bit of math - most automotive techs ignore it and simply look at pressures.
To get it all working (and to keep it running), your system uses a low pressure switch, mounted on the Evaporator Outlet, as an indication of the static charge (compressor not engaged) and evaporator temperature (compressor engaged). That switch simply opens and closes based on the pressure of the refrigerant in the system. The switch contacts are closed at 50 Psi and as long as it has that much, the a/c clutch should engage when you request a/c at the Controls. Those contacts remain closed until the gas leaving the Evaporator drops to 25 psi. At that number, they open up and power is cut to the compressor. The high side continues to flow into the Evaporator and any remaining liquid boils and returns to it's saturation pressure for the outside air temp. That extra volume causes the pressure to rise and when it reaches 50 psi, the whole thing starts over.
A fully charged and functioning system, as mentioned earlier, shouldn't experience this more than 8 times in one minute. A low charge, or one that is severely restricted somewhere on the high side will cycle very rapidly and easily exceed this threshold. The gas that makes it into the Evaporator flashes off immediately and there's nothing left when it reaches the Low Pressure Switch so the compressor cuts out. What's left immediately returns to it's static pressure and in a nano second, it's back to 50 psi. An empty system doesn't do anything. If it's below 50 degrees, some engine heat is usually necessary to get the gas up to 50 psi and the system may cycle off and remain off for longer periods of time. If it's just starting up, it can take a couple of minutes to get everything up to speed.
In order to keep things constant, systems are designed for a certain volume of gas - usually measured in pounds - so that under most temperatures, the same air temp comes out of the vent. It's not exact because the temp of the high pressure liquid (and thereby it's pressure) coming from the Condensor is going to be determined by the air temp flowing over the condensor coils and the efficiency of those coils. But at least almost everything is fixed: Volume, Compressor Efficiency, and the restriction or metering device at the Evaporator. And to keep a constant temp (pressure) at the Condensor (when idling or at low speeds), the system relies on the Main Fan to start spinning once a designed parameter has been met. For your Vette, the Engineers decided that 200 psi was a good average number and the fan switch opens at 230 psi signaling the ECM to crank on the fan. That air flow cools the charge (reduces it's pressure), and the fan switch contacts reclose at anywhere from 170 to 190 psi turning the fan off (your FSM should give the specs). All of this keeps what's going to the Evaporator at around the 200 psi average.
R134 has a smaller molecule than R12 and as you can see, it freezes a tad lower than R12. That smaller molecule means you can cram more into a system, but that extra volume will make more pressure in a system designed to work with R12, making the 32 degree bogey at the Evaporator a tad difficult (if not impossible) to reach. That smaller molecule also allows R134 to reach much higher operating pressures so systems designed to use it crank on the fans at much lower pressures. That keeps the high side at the design number (unfortunately, impossible to do on your Vette - no one makes a different fan switch). Conversions try to overcome these deficiencies by using less gas and because of the lower freezing temp, reducing the threshold for compressor cutout to 22.5 psi (the Low Pressure Switch can be adjusted).
I might add that refrigerants, confined and doing nothing exist in a saturated state - some liquid; some gas - and have a pressure within that confinement that rises and falls with temperature. The Accumulator, which sits at the lowest point in the system, traps any liquid so that only gas reaches the compressor, but you will see pressure variations depending on the outside air temp. If you want to see how that static state translates into temperature or the temperature of that gas translated into pressure, you can use the Pressure/Temp Chart for the gas in your car.
I have never seen a pressure switch installed where you loose refrigerant when you remove the switch. All I have seen are isolated by a shraider valve under the switch, inside the A/C line. When you unscrew the pressure switch, the shader valve closes and cuts off flow.
Most automotive a/c systems need that 200 psi average to make cold air and to keep the Low Pressure contacts closed (so it doesn't cycle excessively). The magic number should be in your Shop Manual and a manifold gage set will tell you whether it's working right or not. On the otherhand, high pressure cutoffs cut power to the compressor clutch at 400 psi and high pressure relief valves spew refrigerant at about 600 psi. Newer stuff simply uses a thermistor like device to relay an electrical signal to the PCM proportional to high side pressure and the PCM uses that to control fan(s), high cutoff, idle, WOT and a/c (powertrain) diagnostics.
The correct setting is about 25 to 28 psi. This corresponds to the freezing temperature for R134a, in the evaporator. Any lower and ice may form at low loads.
Run the engine at about 2000 rpm to simulate cruising speed and the compressor shoud bring the pressure slowly down to the cut off and re-engage as the pressure rises. It may not cycle off at all if the car is very hot inside or you may have to run the engine a little faster.
Compressor operation is optimized for about 1500 to 2000 rpm. At idle the efficency is way down and the compressor shouldn't cycle unless the charge is low.
Cycling, orifice HVAC systems are designed to operate at head pressures that average 200 psi. Static, non operating pressures, are dependent on the outside air temp and rise with air temp. Once the compressor engages, the pressure immediately rises and that also creates heat - about 100 degrees at the condensor for this pressure. Cooling is provided by fan(s) which for most applications are turned on and off by the ECM and the ECM needs to know what the head pressure is in order to maintain the ideal pressure. This provides the best vent temps and by turning the fans on and off (which it should be doing if it's much below 50 degrees out), minimizes cycling of the compressor clutch. Since 1990 - for those vehicles not using a mechanical fan - information is provided to the ECM by a pressure sensitive sensor or thermistor mounted on the High Pressure line. It's fed a 5 volt reference from the ECM and each volt returned is approximately 100 psi. The ECM, based on this signal, will then ground the fan relay(s). Exact operating parameters should be outlined in the FSM - try looking in the electrical schematics under Fans or Compressor if it isn't in Driveability and Emissions. The easiest way to troubleshoot a problem is with a scanner. That will show you the a/c pressure sensor signal in volts (better ones also translate this to pressure) and you can observe the fan(s) to make sure that they are operating as designed. A couple of notes: Below 50 degrees, it may take some engine heat to raise system pressure to the threshold for fan operation - best to test when it's 70 degrees or better. Extremely high pressures - much above 260 psi for R134, might be indicitve of a restricted system - even though both fans are cranking. Hook up a manifold gage set so that you can observe both the high and low side pressures.
The only way to tell if anything is wrong is with a manifold gage set - I don't know how something with a "blue" area inscribed into the dial can compensate or tell you anything when pressure is dependent upon the temp of the air blowing across the condensor. Seems to me that's a good way to blow something up - especially if that blue turns red when the temp climbs.
Jumpering the low pressure switch harness contacts verifies that the circuit is intact but without knowing pressures, you could damage something - especially if there's not enough gas in it to carry the oil charge.
If the orifice is plugged up, your high will be too high; and the low too low. There are plenty of charts available to give you where they should be. Most restrictions, if severe enough, simply cause the compressor to cycle excessively (more than 8 times per minute at 70 degrees). Since the evaporator isn't getting a full shot of gas, what gets in flashes off immediately and there's no oomph left when it reaches the low pressure switch.
The low pressure switch should cut power to the compressor when the pressure is low enough to cause the Evaporator and suction line to ice over. Ice is created when the liquid refrigerant boils below freezing. If you're seeing ice, then something is probably wrong with the switch. Check it out by putting the system on high and pulling the blower motor connector. The low side will drop and the low pressure contacts should open cutting power. Note that pressure on your Low Pressure Gage. For R134, it should be 22.5 psi; for R12, 25 psi. Some switches are adjustable. Turn the screw between the terminals (if they exist) counterclockwise to lower the threshold; clockwise to raise it. Make your adjustments 1/2 turn at a time and retest.
If the pressures look normal, but it's making ice, something else is going on - probably a combo of refigerants. Those unlike mixtures fractionate and cause a host of funky problems. Hard to guess without knowing what the pressures are but if you simply dumped in some R134 or anything that is different on top of the original charge, it's time to start over.
Safety Car
Joined: Jul 2004
Posts: 3,845
Likes: 30
..i suspect either low charge or faulty exterior fan (w/o reading the rest of this post)....
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Just look at (post) your pressures - you charged by the book, the pressures should be within whatever that book says for the temp of the air flowing over the evaporator - they won't be if it's making ice.
Thread Starter
Safety Car
Joined: Feb 2000
Posts: 3,771
Likes: 0
From: MANDEVILLE, LA La
As Mike C4 Stated:
"I also get icing on the aluminum line from compressor to the receiver dryer. But this only happens when the hood is up and the engine is not at normal operating temperature. With your hood closed, the heat from the engine should stop this line icing".
He was right. It works great,No more Icing,.Pulls down to 40F at Center Vent with 80F Ambient Temp. in S.E. Louisiana.
Thanx Guys
"I also get icing on the aluminum line from compressor to the receiver dryer. But this only happens when the hood is up and the engine is not at normal operating temperature. With your hood closed, the heat from the engine should stop this line icing".
He was right. It works great,No more Icing,.Pulls down to 40F at Center Vent with 80F Ambient Temp. in S.E. Louisiana.
Thanx Guys
Le Mans Master
Joined: Dec 2000
Posts: 7,839
Likes: 22
From: San Diego, Ca
Ice only forms if refrigerant is boiling below freezing and if you're seeing it on the outlet, good probability it's sucking liquid back to the Accumulater - make it pass that and it'll lock up the compressor. Engine heat, though it raises pressure, doesn't do much good when it's cold. Your pressure switch is either misadusted - on the fritz - the charge is too low - and/or you've got a mix of refrigerants that doesn't follow the charts.
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