1. Service Fittings and Pressure Gauges

2. Service fittings
Service fittings are connectors in the A/C system that allow the system to be evacuated [vacuumed] and recharged with both refrigerant and oil.
The service fittings also permit gauges to be connected to the system for diagnosis.
All automotive A/C systems have at least two service fittings.

3. Service fittings for R-134a systems
A/C systems on cars built since 1993 have push on service fittings similar to air hose couplers.
The design of the fitting allows the A/C system to be connected to a gauge, vacuum pump, refrigerant tank or A/C service machine with almost no loss of refrigerant.
After the coupler is attached to the service fittings the knob on the coupler is rotated clockwise until tight.
This prevent accidental disengagement and ensures a tight seal between gauge assembly and A/C system.

4. Service fittings for R-134a systems
The high side and low side service fittings for R-134a are two different sizes. This ensures that the service hoses and gauges are not attached to the wrong side of the system. [Idiot proof]
In normal operation the service fittings are covered by a dust cap that is treaded onto the fitting’s internal thread. The dust caps are color coded [Red=high side Blue+ low side] and also have a letter H or L on the face of the cap.
The cap has an o-ring and is designed to prevent leakage if the valve in the fitting fails.

5. Service fittings for R-134a systems
The low side service fitting is located on the suction line and the high side fitting is on the discharge line at a point where they are easily accessible.
The valve inside the service fitting is non-serviceable so locating the fitting on the hose makes repair of a leaky fitting relatively cheap and easy.

6. Service fittings for R-12 vehicles
Prior to 1987 all A/C service fittings had an 7/16-20 externally threaded fitting on both the low and high side.
The same size connection was used for 30 lbs. refrigerant tanks, gauges and hoses.
Between 1987 and 1993 the high side service fitting was changed to 3/8-24 for most domestic systems to try to make them idiot proof.

7. Schrader valves
Both high and low fittings used a Schrader valve similar to the type found on a tire valve to seal refrigerant leaking out of the fitting.
The Schrader valve can usually be removed with the same tool used to remove Schrader vales in tires.
Tire valves should never be used in an A/C system as they are not designed to work with the refrigerants and refrigerant oil.

8. Service fittings for R-12 vehicles
The problem with the threaded fittings and Schrader valves on R-12 systems was that every time you connected and disconnect service hoses to the fittings an ounce or two of refrigerant was lost to the atmosphere.

9. Pressure gauges
A set of stand alone pressure gauges can be directly connected to the A/C system for diagnosis, evacuating and recharging.
Most shops will use the pressure gauges built into the A/C service machine.
Gauges can be mechanical or digital.
Digital gauges have more bells and whistles but mechanical gauges are more robust and inexpensive to replace if damaged.

10. Pressure gauges
High side and low side pressure can be displayed simultaneously on a digital pressure gauge systems.
Mechanical pressure gauges have a low side gauge that reads from 30 inches vacuum to 120 psi.
The high side gauge reads from 0 to 500 psi.

11. Measuring vacuum
Vacuum is pressure that is lower than the surrounding atmospheric pressure.
The most common unit of measure of vacuum is inches of mercury [ in. hg. ]
A perfect vacuum will suck mercury up a glass tube a vertical distance of 30 inches, at seal level.

12. Measuring pressure
The standard unit of measuring pressure in the USA is in PSI [pounds per square inch]
There are 3 common metric pressure units:
BAR
1 BAR is the same as standard atmospheric pressure [15 psi]
Used by most European manufactures
Kg/cm [Kilograms per square centimeter]
Commonly used by Asian manufacturers
Kp [Kilopascals]
Used mostly by domestic manufacturers

13. Gauge pressure vs. absolute pressure
Most pressure gauges are designed to read zero when exposed to normal atmospheric pressure.
Some gauges are designed to read zero when they are exposed to a near perfect vacuum and will read 15 psi or one BAR when exposed to atmospheric pressure. This type of gauge is called an absolute pressure gauge.
The scale of the gauge will be labeled PSI A .
Regular gauges that read zero at normal atmospheric pressure are labeled PSI G or just plain PSI.

14. Low side pressure gauge
The low side gauge normally reads both pressure and vacuum.
The area in green is vacuum.
The gauge should only read vacuum when the systems is being evacuated with a vacuum pump during servicing.

15. High side pressure gauge
The high side gauge has a range of zero to 500 psi.
Some gauges will have the dial shaded in red at pressures above 350 psi to indicate that the pressure is abnormal and potentially dangerous.

16. Normal rest pressure
Low side
High side
When the gauges are attached to the service fittings and the engine is shut off and cooled down, the pressure readings on both gauges should be about the same as the ambient temperature.

17. Rest pressure – hot engine
If the engine has been recently been running, heat from the radiator will warm the refrigerant in the condenser – raising the rest pressure 15 to 20 psi on both sides.

18. Engine running - A/C turned on
Both low and high side pressure will effected by ambient air temperature.
The low side pressure normally ranges between the mid 20s to 40 psi.
High side pressure ranges between 120 and 225 psi.

19. Pressure when cabin temperature is high
A vehicle parked in the sun in summertime may have a cabin temperature of 115 degrees or more.
When the A/C system is started the heat from the cabin air will boil off most of the refrigerant as soon as it enters the evaporator.
Evaporator pressure will be relatively high – in the mid 40 psi range while cabin temperature is high.

20. Normal cabin temperature
Once the cabin air temperature cools down to a comfortable level the low side pressure should stabilize around 27 psi.

21. Correction for relative humidity
More condensation forms on the surface of the evaporator when the relative humidity is high.
The additional amount of liquid water coating the fins of the evaporator restrict the amount of air passing through the evaporator – this raises the evaporator pressure since there will be less heat absorbed.
Some manufacture’s A/C pressure charts have humidity compensation columns to correct for the effects of high relative humidity.

22. Measuring relative humidity
Relative humidity is measured in percentage – where at 100% relative humidity the air is completely saturated with water vapor and any additional water will form foe or rain droplets.
Relative humidity is measured with a hygrometer.
Electronic hygrometers are the most common type.

23. Wet bulb / dry bulb hygrometer
In the past wet bulb/dry bulb hygrometer were used in servicing A/C systems.
Two thermometers mounted side by side – one with the bulb surrounded by a cotton wick that was soaked in water
The difference in temperature between the wet bulb and dry bulb was used to determine the relative humidity from a chart printed on the back of the hygrometer.

24. Humidity - comfort range
A relative humidity level between 50 and 60% is ideal for most people.
Besides cooling the cabin air the A/C system removes a significant amount of water vapor from the air – bringing the humidity into the comfort range

25. Defrost / defog and relative humidity
Whenever the HVAC mode is set to defrost/defog mode the A/C compressor should be turned on.
Cooling the air prior to heating it will allow any humidity in the air to condense and drip out the evaporator drain.
Fog or frost on the inside of the windshield will be removed faster if the air coming out the defrost vents is both hot and dry.
Most domestic cars automatically turn on the A/C compressor whenever the mode selector is in defrost mode.

26. High side pressure – winter operation
When the A/C is turned on the high side pressure is dependant on the ambient temperature and volume of air passing through the condenser.
On a cold winter day the high side pressure might not reach 100 psi

27. High side pressure – summer operation
In summer driving when the outside air temperature is in the 90s the condenser will not work as effectively as it did when the temperatures were in the 40s.
High side pressure can go as high as 225 psi in summer driving.

28. Compressor cycling
CCOT systems turn the compressor on and off when evaporator pressure or temperature exceed specified values.
When the compressor kicks in on a CCOT system the pressure in the low side slowly drops from around 34 psi down to the point where it shuts off at about 24 psi.
When the compressor is shut off the pressure slowly rises back up to 34 psi where it turns back on again.

29. Compressor cycling
In hot summer weather the compressor may run more or less continuously.
In cold weather operation the compressor may cycle on for a few seconds then turn off for a minute or more.
A compressor that runs for few seconds then shuts down for a 10 to 15 seconds is a sign that the system is severely undercharged.

30. Sight glass
Glass lens
A sight glass is used to determine if the A/C system has the correct amount of refrigerant charge.
Sight glasses are common in older vehicles that use a receiver drier and expansion valve.
A sight glass is a window into the pickup tube in the center of the receiver/drier.
The problem with the sight glass is that a system with a complete refrigerant charge looks almost the same as a system that is completely empty.

31. Sight glass – low refrigerant charge
Glass lens
If the receiver drier is almost empty vapor bubbles will be seen in the sight glass as the bottom of the pickup tube is above the liquid level in the receiver/drier.
Pickup tube
Liquid level

32. Determining if the system is completely charged
The only way you can accurately determine if the A/C system is completely charged is to recover the refrigerant and measure the weight of recovered refrigerant and refrigeration oil.
In a system with a sight glass if there is a froth of bubbles visible in the sight glass when the compressor is running the system is only partially charged.

33. Ambient temperature 85 deg. F
Undercharge
Ambient temperature 85 deg. F
Lower than normal pressures on both sides indicate an undercharge condition on CCOT and expansion valve systems.
VDOT system will have a normal low side pressure and a low high side pressure.

34. Severe undercharge – engine off
Ambient temperature 85 deg. F
If there is a leak in the system there may be a little bit of residual pressure.
When the pressure in the system is significantly less than the ambient temperature there is no liquid refrigerant remaining in the system.

35. Overcharge
If both sides have above normal pressure the system may be overcharged.
Overcharging decreases the system performance.
The compressor can be destroyed and the hoses, condenser and evaporator can rupture due to excess pressure.

36. Low side restriction
Restrictions in the low side… orifice tube, expansion valve or evaporator will result in lower high side and low side pressures.
When inspecting the system look for a sudden drop in temperature or ice around the lines and valves.

37. Compressor – defective reed valves
Rapid fluctuation of either high or low side pressure can indicate a leaking or broken reed valve.
If the reed valve is damaged vapor will flow back and forth in the lines instead of a normal one way flow.

38. High side restriction
A restriction in the discharge line, condenser or liquid line will cause above normal pressure in the high side and slightly higher than normal pressure in the low side.
Look for a large temperature drop or ice build up around high side components.

39. Defective electric fan
An inoperative electric fan can cause the high side pressure to rise to dangerous levels in less than a minutes A/C operation.
The electric fan should be running whenever the compressor is running.

40. Non-condensable gasses
Engine off and cooled down to ambient temperature
Ambient temperature 70 deg. F
Air trapped in the A/C system will not condense into a liquid when cooled.
Any air trapped in the system will raise the system pressure and prevent the pressure regulating controls from working correctly.

41. Vacuum and charging hose
Gauge and hose set
A gauge and hose set is often used to perform A/C diagnosis and service.
An A/C service machine is needed to recover and recycle refrigerant but if the A/C system has lost all of it’s refrigerant charge the gauge and hose set can be used in conjunction with a vacuum pump to evacuate and recharge the system after repairs have been made.
Low side service hose
High side service hose
Vacuum and charging hose

42. Vacuum and charging hose
Hand valves
The gauges are always connected to their respective hoses.
When the red [high side] hand valve is opened the red hose is connected to the yellow hose.
If the yellow hose is connected to a vacuum pump the high side will be evacuated.
High side hand valve
Vacuum and charging hose

43. Hand valves
When the blue[low side] hand valve is opened the blue hose is connected to the yellow [center] hose.
Low side hand valve

44. Hand valves
Both hand valves can be opened at the same time. When the system is being evacuated both valves are opened so that air and moisture in both the high and low sides of the system is removed
Low side hand valve
High side hand valve

45. Gauges and hose - vacuum pump connection
Low side service fitting
High side service fitting
Vacuum pump
When evacuating the system the vacuum pump is connected to the center of the gauge manifold.

46. Charging using gauges and hose set
Low side service fitting
High side service fitting
30 lbs. refrigerant container
The system can be charged by connecting a tank of refrigerant to the center hose.

47. Charging is done through the low side only
When charging an A/C system the high side hand valve must be closed whenever the engine is running.
Opening the high side valve with the engine running can cause the refrigerant container to explode..!
High side hand valve