Jimmie Jazz
07-30-2006, 02:01 AM
I'm writing this DIY to help folks understand the operation of their car's Air Conditioning System, so that basic troubleshooting can take place. I've included some definitions, simple testing and charging procedures using a standard R-134a gauge set, available for rental at most major auto parts chains. As always, exercise common sense and good safety practices when working on your car, and keep in mind I'm not an A/C expert. If you aren't comfortable with the amount (or lack) of detail in this DIY, then please do not attempt any of these procedures. If I run across some good pictures I'll edit this post with them at a later date.
A basic A/C system is a pretty simple mechanism; there aren't a lot of moving parts to worry about. These are the basic pieces in order of operation:
Low Pressure Port (Smaller Connector)
Compressor - Compresses refrigerant into near-liquid state
High Pressure Port (Larger Connector)
Expansion Valve - releases/sprays compressed refrigerant into Evaporator Coil
Evaporator Coil - allows heat contained in cabin air to be absorbed by refrigerant
Condenser Coil - Acts as a radiator for cooling the refrigerant
Filter/Drier - Traps any H20 or debris and keeps it from entering the compressor
The variables include (but are not limited to):
How well the compressor is working (a moving part)
How much refrigerant is in the system loop
How well the Expansion Valve is working (on a BMW, a moving part)
Outside Air temperature
How well the air is circulated past the Evaporator Coil (Inside the cabin),
How well the air is circulated past the Condensor Coil (Outside the cabin)
How well refrigerant is circulated through the system
In troubleshooting, the first thing you'll want to do is to hook up a gauge set (You can 'rent' these from most local auto parts chains at no cost) to the High Pressure and Low Pressure ports of the system, to get an idea of how well the system is functioning and to perform some simple diagnostics.
Keep in mind that the reason the ports are called Low and High "sides" is directly related to which 'side' of the compressor they are located. Remember, the compressor's job is to create high pressure, so the 'high' side is after the refrigerent goes through the compressor.
There are two ports on the system:
The Low Pressure Port
Its location in the flow of the system is before the compressor. This port's connector is the smaller of the two ports. This is where you would add refrigerant (if required) and the hose color on the gauge set is generally Blue.
The High Pressure Port
Its location in the flow of the system is after the compressor. It has the larger of the two ports and the hose on the gauge set is generally Red.
Connect The Gauge Set
With the engine off, hang the gauges from the hood latch mechanism so that you'll minimize tangling of the hanging hoses. Attach the hoses to the high and low pressure ports (they are sized differently, so that you won't confuse them) and open the valves.
Read The Gauges
Even though the gauges have different scales, they should read the same pressure if the engine has been off. You should see around ~125psi on both gauges. If you are reading significantly lower than that, then the system probably needs more refrigerant. If the gauges read zero, the system has a leak and is not holding pressure at all.
Test The Air Conditioning
First, verify the hoses are clear of any moving parts in the engine compartment. Start the car with the A/C on full and roll down the windows. Make sure the compressor's clutch has engaged, let it run for about 5-10 minutes and take a reading. If the compressor's clutch won't engage, the system either has an electrical problem, the compressor's clutch has failed or the refrigerant pressure is too low to allow the system to switch on. A minimum pressure level is required to allow the pressure switch to activate the clutch. This is a safety feature that prevents the compressor from overheating if there is not enough refrigerant in the system to allow for adequate compressor lubrication.
Read the Gauges Again
With the A/C on and the car running, you should see about ~20-30 psi on the low side (Blue) gauge in 70/75-degree weather and about ~200-300psi on the high side (Red). In 100-degree weather, you could see up to ~45psi on the low side. A good rule-of-thumb is that the high side should be roughly 10 times the low side, if the system is operating properly.
If the gauges show lower overall pressures but the high side is still roughly 10 times the low side, then you are probably low on refrigerant, and may have a small leak.
If the gauges show pressure but are still equal while the compressor is engaged, then your compressor is not functioning properly. (make sure both gauge valves are open when you are taking the readings)
If the low side is very low and the high side is very high, then there is a blockage/clog in the system somwhere, likely the expansion valve.
It is normal for the compressor to 'cycle' on and off and you will see this reflected in your readings. Understanding when the compressor is on and when it is off will allow you to take proper readings. You'll be able to hear the change between modes.
Adding Refrigerant
If your troubleshooting has determined that you are low on refrigerant, then you'll need to add some to the system. A valve should come with the rental of the gauge set that allows connection to a standard can of 134a refrigerant.
Attach the R134a Can
Unscrew the handle on the valve all the way, then thread the fitting tightly onto the can of refrigerant. Attach the output of the valve to the Yellow Hose which is connected to the center of the gauge manifold. Close all the valves on the manifold. With the can upright, screw in the valve handle all the way so that the top of the can is punctured by the pin in the handle, then back off the valve a few turns to retract the pin, which allows refrigerant out of the hole. You should hear the hiss of the refrigerant entering the yellow hose.
Let it Flow!
First, verify the hoses are clear of any moving parts in the engine compartment. With the hoses on the gauge manifold connected to the ports, start the engine and turn the A/C on high. Open the Low Side (Blue) Gauge Valve to allow the refrigerant to flow from the can into the system while monitoring the gauge. Keep the can upright, and do not attempt to turn the can upside down during the filling process! It is normal for the can to become very chilled as the refrigerant expands out of the can. If the refrigerant does not flow, verify there is not an intermediate valve which has not been opened, as some gauge manifold sets have a separate, additional shut off valve for the yellow hose. Watch the gauge and when the pressure is within range, close the Low Side (Blue) Gauge Valve, then close the can valve.
Repair Considerations
If the system has been opened to outside air, you'll need to replace the drier/filter to ensure that any moisture inthe system is absorbed by the dessicant in the drier. Most auto parts chains won't warranty the compressor unless the drier/filter is replaced with the compressor. After performing any repairs that 'open' the system to air, you will need to evacuate the air from the system with a vacuum pump, verify that the system can 'hold' a vacuum for 20 minutes or so, and then re-charge the system with refrigerant. If you replace your compressor, remember to buy oil (different types for different cars and refigerant systems) to put into the system, so that your compressor doesn't overheat and fail. The oil is designed to co-exist with the refrigerant. You'll need to ask the parts guy to look up what kind of oil and how much to put in. If your old compressor 'cored out', you may need to flush the entire system, and you may need to completely replace the evaporator and condenser coils, becuase they are nearly impossible to 'flush' adequately.
Thanks to SecretAznMan4u for getting me started on this, and for his assistance in diagnosing my own A/C problem.
A basic A/C system is a pretty simple mechanism; there aren't a lot of moving parts to worry about. These are the basic pieces in order of operation:
Low Pressure Port (Smaller Connector)
Compressor - Compresses refrigerant into near-liquid state
High Pressure Port (Larger Connector)
Expansion Valve - releases/sprays compressed refrigerant into Evaporator Coil
Evaporator Coil - allows heat contained in cabin air to be absorbed by refrigerant
Condenser Coil - Acts as a radiator for cooling the refrigerant
Filter/Drier - Traps any H20 or debris and keeps it from entering the compressor
The variables include (but are not limited to):
How well the compressor is working (a moving part)
How much refrigerant is in the system loop
How well the Expansion Valve is working (on a BMW, a moving part)
Outside Air temperature
How well the air is circulated past the Evaporator Coil (Inside the cabin),
How well the air is circulated past the Condensor Coil (Outside the cabin)
How well refrigerant is circulated through the system
In troubleshooting, the first thing you'll want to do is to hook up a gauge set (You can 'rent' these from most local auto parts chains at no cost) to the High Pressure and Low Pressure ports of the system, to get an idea of how well the system is functioning and to perform some simple diagnostics.
Keep in mind that the reason the ports are called Low and High "sides" is directly related to which 'side' of the compressor they are located. Remember, the compressor's job is to create high pressure, so the 'high' side is after the refrigerent goes through the compressor.
There are two ports on the system:
The Low Pressure Port
Its location in the flow of the system is before the compressor. This port's connector is the smaller of the two ports. This is where you would add refrigerant (if required) and the hose color on the gauge set is generally Blue.
The High Pressure Port
Its location in the flow of the system is after the compressor. It has the larger of the two ports and the hose on the gauge set is generally Red.
Connect The Gauge Set
With the engine off, hang the gauges from the hood latch mechanism so that you'll minimize tangling of the hanging hoses. Attach the hoses to the high and low pressure ports (they are sized differently, so that you won't confuse them) and open the valves.
Read The Gauges
Even though the gauges have different scales, they should read the same pressure if the engine has been off. You should see around ~125psi on both gauges. If you are reading significantly lower than that, then the system probably needs more refrigerant. If the gauges read zero, the system has a leak and is not holding pressure at all.
Test The Air Conditioning
First, verify the hoses are clear of any moving parts in the engine compartment. Start the car with the A/C on full and roll down the windows. Make sure the compressor's clutch has engaged, let it run for about 5-10 minutes and take a reading. If the compressor's clutch won't engage, the system either has an electrical problem, the compressor's clutch has failed or the refrigerant pressure is too low to allow the system to switch on. A minimum pressure level is required to allow the pressure switch to activate the clutch. This is a safety feature that prevents the compressor from overheating if there is not enough refrigerant in the system to allow for adequate compressor lubrication.
Read the Gauges Again
With the A/C on and the car running, you should see about ~20-30 psi on the low side (Blue) gauge in 70/75-degree weather and about ~200-300psi on the high side (Red). In 100-degree weather, you could see up to ~45psi on the low side. A good rule-of-thumb is that the high side should be roughly 10 times the low side, if the system is operating properly.
If the gauges show lower overall pressures but the high side is still roughly 10 times the low side, then you are probably low on refrigerant, and may have a small leak.
If the gauges show pressure but are still equal while the compressor is engaged, then your compressor is not functioning properly. (make sure both gauge valves are open when you are taking the readings)
If the low side is very low and the high side is very high, then there is a blockage/clog in the system somwhere, likely the expansion valve.
It is normal for the compressor to 'cycle' on and off and you will see this reflected in your readings. Understanding when the compressor is on and when it is off will allow you to take proper readings. You'll be able to hear the change between modes.
Adding Refrigerant
If your troubleshooting has determined that you are low on refrigerant, then you'll need to add some to the system. A valve should come with the rental of the gauge set that allows connection to a standard can of 134a refrigerant.
Attach the R134a Can
Unscrew the handle on the valve all the way, then thread the fitting tightly onto the can of refrigerant. Attach the output of the valve to the Yellow Hose which is connected to the center of the gauge manifold. Close all the valves on the manifold. With the can upright, screw in the valve handle all the way so that the top of the can is punctured by the pin in the handle, then back off the valve a few turns to retract the pin, which allows refrigerant out of the hole. You should hear the hiss of the refrigerant entering the yellow hose.
Let it Flow!
First, verify the hoses are clear of any moving parts in the engine compartment. With the hoses on the gauge manifold connected to the ports, start the engine and turn the A/C on high. Open the Low Side (Blue) Gauge Valve to allow the refrigerant to flow from the can into the system while monitoring the gauge. Keep the can upright, and do not attempt to turn the can upside down during the filling process! It is normal for the can to become very chilled as the refrigerant expands out of the can. If the refrigerant does not flow, verify there is not an intermediate valve which has not been opened, as some gauge manifold sets have a separate, additional shut off valve for the yellow hose. Watch the gauge and when the pressure is within range, close the Low Side (Blue) Gauge Valve, then close the can valve.
Repair Considerations
If the system has been opened to outside air, you'll need to replace the drier/filter to ensure that any moisture inthe system is absorbed by the dessicant in the drier. Most auto parts chains won't warranty the compressor unless the drier/filter is replaced with the compressor. After performing any repairs that 'open' the system to air, you will need to evacuate the air from the system with a vacuum pump, verify that the system can 'hold' a vacuum for 20 minutes or so, and then re-charge the system with refrigerant. If you replace your compressor, remember to buy oil (different types for different cars and refigerant systems) to put into the system, so that your compressor doesn't overheat and fail. The oil is designed to co-exist with the refrigerant. You'll need to ask the parts guy to look up what kind of oil and how much to put in. If your old compressor 'cored out', you may need to flush the entire system, and you may need to completely replace the evaporator and condenser coils, becuase they are nearly impossible to 'flush' adequately.
Thanks to SecretAznMan4u for getting me started on this, and for his assistance in diagnosing my own A/C problem.