AC MaintenanceThis post describes in laymen's language how an automobile A/C works. It also describes in detail the components of an A/C system and what to check when the A/C does not blow "cold air". The example is a VW Golf TDI A/C system
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Air Conditioning PrinciplesThis article did not post properly. Go to: www.vwtdirepair.com
Air Conditioning Principles
Air conditioning is the process in which air inside the passenger compartment is cooled, dried, and circulated. Heat is removed from inside the vehicle and then transferred to the outside air.
All air conditioners, whether it is an auto A/C, household refrigerator, or home HVAC, work on the same principles. A liquid refrigerant is changed to a gas and then back to a liquid. If a change of state of the refrigerant is to take place, heat transfer must take place. The rules that apply to refrigerant are:
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Refrigerant in a gaseous state collects, absorbs, and holds heat.
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Refrigerant in a liquid state releases that heat.
For your Golf, Beetle, or Jetta TDI's to blow "cold air", R-134A refrigerant must pass through and change state in three (3) components and one (1) receiver-dryer, and one (1) expansion valve that make up the closed A/C system:
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Compressor - A device that pressurizes the heated refrigerant and is driven by a belt mounted on the engine driveshaft.
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Condenser - Is a radiator for refrigerant that transfers the heat that was absorbed in the passenger compartment to the cooler air blowing through it as the vehicle is driven.
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Evaporator - Is a small radiator that is located under the dash in the passenger compartment. Liquid refrigerant entering the evaporator creates is a pressure loss. The liquid refrigerant absorbs heat from the air flowing across the evaporator. It then boils and changes state to a vapor before it enters the suction port of the compressor.
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Receiver-Dryer - Is a canister that stores the liquid refrigerant. It contains a desiccant that removes moisture from the system.
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Expansion Valve - Is a metering device that controls the amount of refrigerant to the evaporator.
The R-134A refrigerant travels and changes from a liquid to a vapor state. If any of the above devices were to fail, the A/C will not blow "cold air".
Below are some facts that you should know as you learn to handle refrigerant and work with the components that make up your Golf, Beetle, or Jetta TDI's A/C system:
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Fact 1 - There is no such thing as an A/C system that is 100% sealed and does not lose some refrigerant charge. An A/C system that loses 1/2 ounce of R-134A refrigerant per year will take over thirty (30) years to lose one (1) pound.
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Fact 2 - "Refrigerant does not wear out, it leaks out".
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Fact 3 - Auto A/C systems do not have a dip stick for you to check the refrigerant level. Things are far more complicated to successfully add refrigerant. You have to use common sense and be proficient in the use of the manifold gauge set to properly service your A/C system.
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Fact 4 - To add the proper amount of refrigerant to your A/C system required by VW, you would have to exhaust the present charge, and evacuate the system. Use a charging cylinder (Dial-A-Charge from www.robinair.com Robinair) to meter the proper charge into the A/C system.
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Fact 5 - A low-charge with a large leak is responsible for nine (9) out of ten (10) compressor failures. A compressor oil charge is approximately six (6) ounces. If a leak develops in the system, the oil and refrigerant will disappear into the atmosphere. This leaves the system with little or no cooling. The compressor will succumb to excessive temperatures, destroy the components, and seize from lack of lubrication.
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Fact 6 - A charging hose and R-134A refrigerant sold by retailers, known as a charging kit, is a poor choice to add refrigerant to your A/C system. More is sometimes not a good thing. The system will now have an improper refrigerant charge and will not blow "cold air".
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Fact 7 - All A/C systems work on the principle of the refrigerant changing states. The only difference is the servicing techniques. For example, the auto A/C uses service valves to evacuate and recharge the system on the low-side. On the other hand, the household refrigerator is charged on the high-side in a vacuum. It is known as a "sealed system" as there are no service valves. When a problem occurs, a domestic refrigeration A/C technician would braze saddle schrader valves on the suction and discharge lines to the compressor. The system can then be entered for service. Pressures are monitored with the standard manifold gauge set.
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Fact 8 - A can of R-134A refrigerant is not a 1 lb can. The contents of the can is only twelve (12) ounces. Subtract four (4) ounces of refrigerant that are in the three (3) hoses. That can is now eight (8) ounces of refrigerant. Keep this in mind when you add refrigerant to the Golf TDI A/C system.
You can quickly check the health of the Golf, Beetle, or Jetta TDI's A/C system by driving the vehicle with the A/C "ON" and the blower set to "High" until the system pressures stabilize. With the engine running (A/C "ON") feel the lines on the low-side and high-side of the system. You should see these results:
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The low-side should feel cool to the touch.
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The high-side should feel warm to the touch or "hot".
If the high-side is not warm or "hot" and the low-side is not "cool, there is a problem in the A/C system.
Before temperature tests can be made, the following is a check list for testing the A/C system.
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Set the A/C "ON-OFF" switch to "ON".
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Set the temperature control to maximum cooling in blue area.
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Set the blower on "High" or number four (4).
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Temperature inside passenger compartment should be stable and not getting cooler.
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Engine speed must be a minimum of 1500 rpm's.
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Compressor clutch must be engaged.
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All windows are in the "up" position.
The Golf, Beetle, and Jetta TDI have small capacity A/C systems and a loss of cooling will be more noticeable than a larger capacity auto A/C system. If you suspect that your A/C system has lost some of its cooling capacity, a temperature check at the vent registers inside the passenger compartment should be made. The discharge air from the vents should range from 38 degrees F to 42 degrees F.
Assuming that the temperature measured at the vent registers are out-of-range (38 degrees F to 42 degrees F) the manifold gauge set should be connected to the low and high-side service valves. With the engine "OFF" (static reading) the low-side gauge and high-side gauge should show equal readings of approximately 80 to 120psi if there is adequate refrigerant in the system. If, for example, the readings were 50psi on both gauges, this would indicate that there is refrigerant in the system but there is a low-charge. If the reading on the low-side gauge was near "0" would indicate that the system is "empty" (no charge).
To get a further look inside the A/C system, start your engine. Turn the A/C "ON" with blower on "High" and run the engine for fifteen (15) minutes. A fully charged system would show a pressure reading of approximately 35psi on the low-side gauge and pressure of 225 to 240psi on the high-side. This reading would be on a 90-degree day.
For example, if the pressures were 20psi over 150psi would be a strong indicator that the system has a low-charge. The 150psi on the high-side is the tell take here as the system is doing very little "work".
A low-charge would be an indicator that there is a leak in the system. Some symptoms to look for this condition include:
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Short cycling of the compressor clutch.
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Clutch will not engage.
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Little or no cooling.
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Oily residue on hoses, and components such as the compressor.
Going a little further, assume that the pressures were 70psi over 300psi. This would be an indicator that the A/C system would be over-charged. An over-charged system is a bad thing as the compressor will fail pre-maturely. The reed valves will warp and the compressor is rendered useless.
Here is a real world example. A compressor was installed on the Project Car TDI recently and the VW specification calls for twenty-four (24) ounces of refrigerant. After adding one (1) 12-ounce can, the A/C system began to cool. The system pressures were holding steady at 25psi over 180psi. After adding the second can, the A/C that was blowing "cold air" is now blowing "hot air". Suddenly, the system pressures skyrocketed to 80psi over 280psi. Those readings were off the chart as the A/C system now had all the indications of being over-charged.
Now, some refrigerant has to be bled from the system and recycled. The fifteen (15) count method can be used for this purpose. With engine running and "A/C "ON-OFF" switch set to "ON", blower on "High", open the low-side hand valve (blue) one (1) turn on manifold gauge set and count to fifteen (15). Close low-side hand valve and monitor the pressure readings on the gauges for five (5) minutes. Continue expelling refrigerant using this method until the pressures drop to a range of approximately 35psi over 225psi on a 90-degree day.
Note
If the engine has been running over thirty (30) minutes with the A/C "ON" on a 90-degree day, spray the condenser with water to lower the high-side pressure.
By an estimate, the total charge in the system is about eighteen (18) ounces. (only guessing - no Dial-A-Charge). The factory specification calls for twenty-four (24) ounces of refrigerant R-134A. The discharge air at the vent registers was 39 degrees.
Vibrations, wear-and-tear, rough roads, and high miles: All of these can be an invite for a refrigerant leak to develop in the A/C system.
When a leak is suspected, due to poor cooling, a visual inspection of the vehicles A/C system should be made. The place to start is at the grille area working your way back to the firewall. Check for traces of oil around the banjo fittings connected to the compressor (Suction and Discharge ports) and receiver-dryer. These fittings use 0-rings to seal the components for a leak-free connection.
Note
Care should be exercised when tightening the bolts of a banjo fitting. These bolts require a 6mm hex bit for removal and reinstalling of components. 10x8mm 0-rings are used for a seal. Do not torque above 15-ftlb or damage to 0-ring will result
After a visual check, the system is next checked for leaks with a UV leak detector kit.
The following check list is helpful when checking for leaks in an A/C system:
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Check for abrasions and defects in all hoses of A/C system.
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Check for leaks at the service valves with UV light and UV glasses.
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Check the compressor shaft seal with the UV light and UV glasses.
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Check the banjo fittings of suction and discharge hoses to compressor.
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Check the banjo fittings to the inlet and outlet of receiver-dryer.
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Remove cover to expansion valve that is located on passenger side next to firewall. Check connections for leaks with UV Light and UV glasses.
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Check tubes to condenser for oily deposits with the UV Light and UV glasses.
The evaporator is not accessible from outside the vehicle. It is located in the dash and checking for leaks is difficult. Check first by (1) Adding 2 ounces of dye to the A/C system on the low-side with manifold gauge set, (2) Place a white paper towel under evaporator drain tube (under car), and (3) Check water and dye collected on towel from evaporator with UV Light and UV glasses. If there is evidence of a yellow-green color on the towel with the UV glasses, there is a freon leak in the evaporator.
Refrigerant oil travels with the R-134A refrigerant in an A/C system. A large leak will cause oil and refrigerant to leak from the system into the atmosphere. This leaves the compressor vulnerable for failure due to lack of lubrication. Figures M1-1, M1-2, M1-3, and M1-4 show photos of a Sanden compressor that has been disassembled. From viewing the destruction in these photos, it is evident that the temperature build-up inside the compressor was more than the compressor could handle.
Figure M1-1 - Reed Plate Damage
Figure M1-2 - Broken Reed Valves
Figure M1-3 - Piston Hammer on Dome of Piston
Figure M1-4 - Loose Needle Bearings
An often overlooked cause of "poor cooling" is debris that clogs the condenser. Bugs, plastic bags, and grass from city and highway driving will prevent air from passing through preventing cooling of the refrigerant in the condenser. All debris should be removed and the condenser cleaned with condenser cleaner. Bent fins on the should be straightened with a fin comb.
The ALH TDI engine uses two (2) radiator cooling fans that are controlled by a thermostat switch. Rising refrigerant pressures will activate the cooling fans. A visual inspection of the fans should be made. A fan or fans that are turning slowly and not at full speed will result in poor cooling. If the fan did not come "ON", a fuse could be at fault. Replace the fuse or fan.
Caution
Never place your fingers in or around the cooling fans when working in the fan area. They can come on without warning with or without the key in the ignition. For added safety, disconnect the electrical system.
Proper air flow can be checked with a temperature or non-contact thermometer. Temperature testing and results for proper air flow at the condenser can be made in this manner:
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Place non-contact thermometer to inlet of condenser and record reading.
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Place non-contact thermometer to outlet of condenser and record reading.
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A twenty (20 to forty (40) degree drop from the inlet to the outlet should be seen.
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Temperature difference less than twenty (20) degrees indicates poor air flow.
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Temperature difference more than forty (40) degrees indicates a restriction in the condenser.
A restriction is a real world condition that is a "blockage" that usually occurs at the condenser. Some form of debris has lodged in the component and restricted the flow of the refrigerant. The condenser tubes passages on the Golf, Beetle, and Jetta TDI's are very small (less than 6mm). The most effective way to diagnose a restriction is with the manifold gauge set. The high-side gauge would move into the DANGER zone over 300psi.
A blockage in an A/C system is a bad thing. It causes a pressure build-up behind the restriction and damage to the components in its path behind it.
An effective way to locate a restriction is to make temperature checks throughout the A/C system. Large drops in temperature that are found should be repaired or components replaced.
An example of a restricted condenser is: (1) Inlet temperature measures 175 degrees (300psi) and (2) the outlet temperature measures only 100 degrees (110psi) would be an indicator that refrigerant is slowly leaking past the restriction. This means that the refrigerant is moving slowly or stalled in the condenser. The result is it has more time to "cool" in the condenser. However, the 300psi build-up of pressure in the condenser will eventually take out a component upstream, usually a seal or internal damage in the compressor.
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| rwheels - Aug 12, 2009 9:19 am | |||
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