In a Nutshell:
A good High Temperature Reading for 134a = twice the ambient temperature + 50.
For Example at 85° F, you would expect a high side pressure of 85x2=170+50 = 220 psig.
A good Low Temperature reading should be between 30-40 psig.
This means the temperature of the refrigerant entering the evaporator is between 35 and 40° F.
These may vary depending on ambient temperature.
Refer to the chart below for pressure readings.
Measuring Temperature & pressure revolves around all the basics:
Remember, all the examples given are approximate values only.
Air conditioning and refrigeration is ALL about HEAT and the intensity of it! Air Condition is just a means of moving heat from one place to another. The amount of heat concentrated within a certain volume with determine it's temperature. If you remove heat from that same certain volume, it's temperature will fall (become cooler). The Automotive Air conditioning system is a mechanical means of acomplishing this, using compression to reduce a certain volume to a smaller size. That smaller volume still contains the same amount of heat as before, but now the heat is more concentrated within that smaller volume, and therefore it's temperature is increased. The heat is letterally "squeezed" out of the refrigerant gas. Then the hot high pressure refrigerant gas (could be approximately 180F to 200F degrees)is moved to an area where it's heat can be transfered to the outside air (condenser), it's temperature is therefore reduced, and still being under high pressure, it's characteristics cause it to condense to a warm high pressure liquid (could be approximately 130F to 150F degrees). The high pressure liquid refrigerant then moves to the refrigerant control, which is a calibrated designed restriction, (orifice tube or TX valve) that divides the high pressure side from the low pressure side, acting like a small garden hose nozzel, it allows a small stream of refrigerant to spray into the low side. The liquid refrigerant, now being under low pressure boils and ABSORBS HEAT from in the evaporator (could be approximately 30F to 35F degrees). The low pressure refrigerant gas and mist flow from the evaporator out the evaporator outlet pipe (could be approximately 45F to 50F degrees)to the accumulator, which provides a place for the excess refrigerant mist to turn to gas, and also has a bag of dryer chemical in it, to trap any moisture that may get into the system. Then the low pressure refrigerant gas is sucked from the accumulator, through the suction line, back to the suction port of the compressor (could be approximately 55F to 65F degrees), where the process begins again. At each portion of the Air Conditioning circuit, the temperature and pressure will indicate how the refrigerant within it at that point is behaving, and has a direct bearing on the air conditioning performance.
As an example on the High Side, at the condenser, if the refrigerant is behaving properly, the refrigerant gas will enter at a high temperature, and the refrigerant liquid will exit at a lower temperature (because it has transferred heat to the outside air). The difference is the temperature drop across the condenser (could be approximately 30F to 40F degree drop at idle or 40F to 60F degree drop at 1500 RPM, and the high pressure gauge reading will corrospond to a value on the temp/press chart for the lowest temperature on the high side, meaning the high pressure reading on a R134a system should be 212 PSI if the refigerant is exiting the condenser at 135F degrees. If you don't have enough temperature drop across the condenser and high side pressure is too high, look at the possibility of an overcharge, air in the system, condenser, air flow and fans for a problem.
As an example on the Low Side, at the evaporatorer, if the refrigerant is behaving properly, the refrigerant liquid will enter and boil at a low temperature (could be approximately 35F degrees), and the refrigerant gas and mist will exit at the evaporator outlet pipe at a higher temperature (could be approximatly 45F to 50F degrees, because it has picked up heat from within the vehicle). Then the refrigerant gas and mist pick up more heat and are completely vaporized back unto a gas as they are sucked through the accumulator and through the suction line back to the compressor suction port. The temperature at the compressor suction port should be higher (approximately 55F to 65F degrees, because the refigerant gas/mist has picked up more heat and completely vaporized since leaving the evaporator). There should be approximately a 10F to 20F degree diffence between the evaporator outlet pipe and the line at the compressor suction port at idle, or 2F to 10F degree drop at 1500 RPM, and the low pressure gauge reading will corrospond to a value on the temp/press chart, meaning the high pressure reading on and R134a system should be 30 PSI if the refigerant is boiling in the evaporator at 35F degrees. If you don't have enough or any temperature drop from the evaporator outlet pipe to the line at the compressor suction port and the low pressure reading is high, look at the possibility of an overcharge. The correct charge will cause the line at the compressor suction port to be slightly warmer than the evaporator outlet, ensuring that liquid refrigerant will not be sucked back into the compressor. An overcharge will cause the system to not cool as well, pressures will be higher, and liquid refrigerant being sucked back into the compressor, shortening it's life. If you have very low pressure on the low side, the evaporator outlet pipe is much warmer than it should be, check for a low charge or restriction at the orifice tube or TX valve.
A good way to check all the line temps is with an infrared thermometer. It's the best thing since sliced bread, and I would'nt be caught dead without mine!
Here's some info and guidelines for using it:
Using the Infrared Thermometer:
If you have an infrared Thermometer, you can easily measure the temps of the lines and get the charge just right. It works best on Dark Surfaces, so before starting, neatly spray paint the surfaces you intend to measure with some Flat Black Spray Paint and let dry. Wet surfaces should be wiped dry before measuring. Hold the thermometer close to and pointed at the object to wish to measure temperature. Since there is so much hot metal in the engine compartment and hot air blowing in the engine compartment, these will affect the thermometer to make reading inaccurate. To minimize these effects, take your readings quickly to keep the thermometer from getting hot, allow time for it to cool between each use, blow cool air on it between each use. Use the thermometer only when the thermometer is cool and stable. You must also use a heat shield around the component you are measuring, to keep stray heat away from the thermometer. Using a piece of light colored cardboard is a good heat shield against engine heat, exhaust manifold heat, and radiator heat. Place the heat shield in place quickly and measure the temperature quickly, then remove the thermometer and heat shield quickly, so that the heat shield itself does not become hot. Allow the heat shield to cool between uses.
These are APPROXIMATE TEMPERATURE numbers and will vary according to application, make and model, but are still good guidelines:
FOR AUTO RADIATORS:
At IDLE - Inlet Tank temp should range from 155 to 185 degrees, depending on thermostat and cooling system condition.
Outlet Tank temp should range from 130 to 170 degrees.
IMPORTANT-the DIFFERENCE between the inlet tank temp and outlet tank temp is the most important factor to determine how well the radiator is doing it’s job, which is TEMPERATURE DIFFERENTIAL ACROSS THE CORE.
With the A/C OFF, the temp diff should range from 20 to 35 degrees.
With the A/C ON, the temp diff should range from 10 to 25 degrees.
If there are problems with a too low temp diff and overheating, check temps across the core for evenness, check air flow through radiator, check fan & fan clutch and check coolant flow through core.
FOR AUTO AIR CONDITIONING R134a SYSTEMS:
At IDLE - Condenser inlet temp 170 to 180, Condenser Outlet temp 135 to 150 (usually a 30 to 40 degree drop in temperature across the Condenser), Evaporator Outlet temp 45 to 55, Compressor Suction temp 60 to 70 (usually a 15 to 20 degree drop in temperature from the evaporator outlet to compressor suction port). With the engine IDLING, you want to adjust the charge so the line at the Compressor suction port is about 10 to 20 degrees WARMER than the Evaporator Outlet pipe, and with the engine running at 1500 to 1800 rpm, you want the line at the Compressor suction port to be about 2 to 10 degrees WARMER than the Evaporator Outlet pipe.
At 1500 RPM - Condenser inlet temp 190 to 200, Condenser Outlet temp 135 to 155 (usually a 40 to 60 degree drop in temperature across the Condenser), Evaporator Outlet temp 45 to 55, Compressor Suction temp 55 to 60 (usually a 7 to 12 degree drop in temperature from the evaporator outlet to compressor suction port).
At 90 degrees ambient, for R134a systems, look for about 200- 220 Pressures on the high side and about 25-30 Pressures on the low side at idle, and at 1,500 RPM look for about 220- 230 Pressures on the high side and about 20-27 Pressures on the low side.
The Compressor should cycle off at around 20 PSI.
Note: Always keep a copy of the Temperature/Pressure Chart handy.
R134a
Temperature Pressure
(°F) (Psig)
-60.0 21.6*
-55.0 20.1*
-50.0 18.5*
-45.0 16.7*
-40.0 14.6*
-35.0 12.3*
-30.0 9.7*
-25.0 6.7*
-20.0 3.5*
-15.0 0.1
-10.0 2.0
-5.0 4.2
0 6.5
5.0 9.2
10.0 12.0
15.0 15.1
20.0 18.5
25.0 22.2
30.0 26.1
35.0 30.4
40.0 35.1
45.0 40.1
50.0 45.5
55.0 51.2
60.0 57.4
65.0 64.1
70.0 71.1
75.0 78.7
80.0 86.7
85.0 95.3
90.0 104.3
95.0 114.0
100.0 124.2
105.0 135.0
110.0 146.4
115.0 158.4
120.0 171.2
125.0 184.6
130.0 198.7
135.0 213.6
140.0 229.2
145.0 245.5
150.0 262.9
155.0 281.1
160.0 300.0
* - (in Hg) Vacuum