DEFEATING THE AIR CONDITIONINGBOGEYMAN.STREAMLINE YOUR PROCESS FOR NAVIGATING AND UNDERSTANDING THE LIFE CYCLE OF AIR CONDITIONING

TABLE OFCONTENTS

A/C Pressure and Temperature

Air Conditioning Components

How the Components of an A/C System Function

The Refrigerant Cycle

A/C Pressure Diagnostics

Diagnosing A/C Issues

Air Conditioning Temperature Readings

Common A/C Problem Areas

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DEFEATING THE AIR CONDITIONING BOGEYMANAutodata typically sees most use of the Service Air Conditioning module from early May through to late August with a peak in late June. However, with the changing climate threatening to deliver higher temperatures throughout the year, the length of time customers will be keeping their air conditioning on is likely to increase – and with it the need for service.

For some technicians servicing air conditioning can be daunting – without a strong knowledge of the basics of why AC works, it can sometimes feel like working in the dark. In this technical article we shine some light on the AC bogeyman – and hopefully show he’s not as scary as his reputation suggests!

REFRIGERANT CYCLE

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A/C systems generally have similar parts and all have two sides, representing low and high temperature and pressure. The above diagram shows some basic components and change in refrigerant pressure/temperature between the sides.

AC systems work by compressing a gaseous refrigerant, condensing it into a high-pressure liquid, and then throttle it to a low-pressure liquid to boil and remove heat from a heat sink. There are several different means of accomplishing each of these steps, but for the purposes of the below we are giving a general overview of the purpose of each component. We are also omitting for the purposes of this analysis control devices such as sensors and switches.

Compressor: This is typically a belt-driven pump. Its job is to pull refrigerant in and compress it into a high-pressure vapour. It may have an electromagnetic clutch to engage or disengage it, or it may have a spill valve to control the flow. On hybrid/EV vehicles this will usually be driven by an electric motor instead of a belt.

Condenser: This is a big finned heat exchange – usually mounted in front of the radiator. This is why the high-pressure vapour is positioned immediately behind the compressor. Air passes through the fin to cool and condense the high-pressure vapour and turns it into a high-pressure liquid. Air is either motivated by the cooling fans or by air ram as the vehicle is driving.

Receiver/Dryer: This is a reservoir for the high-pressure liquid. It stores and filters the refrigerant and also has a desiccant to remove water from the refrigerant. This unit is typically mounted in the condenser.

Thermal Expansion Valve or Fixed Orifice: This is where the refrigerant goes when it is compressed, filtered, and condensed. The thermal expansion valve is a small orifice that the high-pressure liquid is pushed through – turning it into a low-pressure liquid. It works much like a can of spray paint – high-pressure liquid inside the can sprays out as a low-pressure liquid. If you’ve ever spray-painted your hand you will feel the paint is colder than the can – this is due to the drop in pressure spreading the same amount of thermal energy over a much larger area.

AIR CONDITIONING COMPONENTS

A Fixed Orifice performs the same job as a TXV but does not vary in size at all; the manufacturer decides the best diameter for most situations. A Fixed Orifice can be anywhere along the line between the condenser and the evaporator – this should be specified in the service information.

Evaporator Core This is the heat sink inside the vehicle. After the low-pressure liquid is sprayed from the TXV or fixed orifice into the evaporator, it evaporates into a low-pressure vapor. Since the refrigerant is not under pressure, it boils into a gas. This further pressure decrease drives the cooling effect of the A/C system.“brrr!” in the A/C system.

Lines and Hoses These are the conduits that take refrigerant in its varying states to where it needs tobe; these should offer no resistance to flow at all, they’re simply tubes that should flow freely.

Courtesy of Honda

Thermal expansion valves (also called TXV or just Expansion Valves) vary the size of the opening based on the temperature of the evaporator – see the diagram on the left

The sensing bulb is filled with a gas that expands when warm, pressing down on the diaphragm in the valve. This pushes the valve open and allows more high-pressure liquid to spray into the evaporator as low-pressure liquid. As the evaporator cools, the sensing bulb contracts and the valve is allowed to close. In newer TXVs this is integrated into a single block, but the function remains the same.

AIR CONDITIONING COMPONENTS

HOW THE COMPONENTS OF AN A/C SYSTEM FUNCTION The whole A/C system is designed to take advantage of the pressure/temperature relationship of refrigerant. R-134a is the most commonly used refrigerant with R1234yf more frequently used in modern systems. Both however have a very similar pressure/temperature profile. The refrigerant cycle can be described as having two parts – the high and low side.

The high-pressure side is the path of the refrigerant after the compressor, and includes the compressor, condenser, dryer, lines and half of the TXV. The high-pressure side is also high temperature.

The low-pressure side is the other side of the TXV, the evaporator core, and the line going back into the compressor. The low-pressure side is where the cooling effect of the A/C system is felt.

The below chart shows the relationship between pressure and temperature for R-134a. At 20 degrees Celsius (68°F) the refrigerant is essentially 68 PSI. On a 20°C day, you will see around 68 PSI of static pressure in the system. Static pressure is when the refrigerant has equalised so both sides are the same pressure.

If the compressor is used to squeeze the gas to around 270 PSI, the temperature rises to around 66°C (152°F). The compressor is forcing more molecules of refrigerant into a smaller area – like any other substance, this produces heat as atoms collide and their kinetic energy is converted into thermal energy.

-20

-10 0 10 20

30

40

50

60

70

80

90

400

TEMPERATURE (°C)

PR

ES

SU

RE

(P

SIG

)

350

300

250

200

150

100

0

R134A TEMPERATURETO PRESSURE CURVE

PRESSURE

PSIG/Hg*

TEMP

(C°)

-22 -52,2

-20 -48,3

-18 -45

-16 -42,2

-14 -39,4

-12 -36,7

-10 -35

-8 -32,8

-6 -30,6

-4 -29,2

-2 -27,8

0 -26,1

1 -24,4

2 -23,3

3 -21,7

4 -20,6

5 -19,4

6 -17,8

7 -17,2

8 -16,1

9 -15

PRESSURE

PSIG/Hg*

TEMP

(C°)

10 -13,9

11 -13,3

12 -12,2

13 -11,1

14 -10,6

15 -9,4

16 -8,6

17 -7,8

18 -7,2

19 -6,1

20 -5,6

21 -4,7

22 -3,9

23 -3,3

24 -2,8

25 -1,7

26 -1,1

27 -0,6

28 0

29 0,6

30 1,4

PRESSURE

PSIG/Hg*

TEMP

(C°)

31 2,2

32 2,8

33 3,3

34 3,9

35 4,4

36 5,0

37 5,6

38 6,1

39 6,7

40 7,2

41 7,8

42 8,3

43 8,9

44 9,4

45 10,0

46 10,3

47 10,6

48 11,1

49 11,7

50 12,2

51 12,8

PRESSURE

PSIG/Hg*

TEMP

(C°)

52 13,3

53 14,2

54 14,4

55 13,9

56 15,0

57 15,6

58 15,8

59 16,1

60 16,7

61 17,2

62 17,5

63 17,8

64 18,3

65 18,9

66 19,2

67 19,4

68 20

69 20,3

70 20,6

71 21,1

75 22,8

PRESSURE

PSIG/Hg*

TEMP

(C°)

80 24,4

85 26,1

90 27,8

95 29,4

100 30,6

105 32,2

110 33,9

115 35,3

120 36,7

125 37,8

130 39,4

135 40,6

140 41,7

145 42,8

150 44,2

155 45,6

160 46,7

165 47,8

170 48,9

175 49,7

180 50,6

PRESSURE

PSIG/Hg*

TEMP

(C°)

185 51,7

190 52,8

195 53,9

200 54,7

205 55,6

210 56,7

215 57,5

220 58,3

225 59,4

230 60

235 61,1

240 61,7

245 62,8

250 63,3

255 64,4

260 65

265 66,1

270 66,7

275 67,2

280 68,3

285 68,9

PRESSURE

PSIG/Hg*

TEMP

(C°)

290 69,4

295 70,6

300 71,1

305 71,7

310 72,5

315 73,3

320 73,9

325 74,4

330 75,6

335 76,1

340 76,7

345 77,2

350 77,8

355 78,3

360 78,9

365 79,4

370 80,0

375 80,6

380 81,1

-20

-10 0 10 20

30

40

50

60

70

80

90

400

TEMPERATURE (°C)

PR

ES

SU

RE

(P

SIG

)

350

300

250

200

150

100

0

R134A TEMPERATURETO PRESSURE CURVE

PRESSURE PSIG/Hg*

TEMP (C°)

-22 -52,2

-20 -48,3

-18 -45

-16 -42,2

-14 -39,4

-12 -36,7

-10 -35

-8 -32,8

-6 -30,6

-4 -29,2

-2 -27,8

0 -26,1

1 -24,4

2 -23,3

3 -21,7

4 -20,6

5 -19,4

6 -17,8

7 -17,2

8 -16,1

9 -15

PRESSURE PSIG/Hg*

TEMP (C°)

10 -13,9

11 -13,3

12 -12,2

13 -11,1

14 -10,6

15 -9,4

16 -8,6

17 -7,8

18 -7,2

19 -6,1

20 -5,6

21 -4,7

22 -3,9

23 -3,3

24 -2,8

25 -1,7

26 -1,1

27 -0,6

28 0

29 0,6

30 1,4

PRESSURE PSIG/Hg*

TEMP (C°)

31 2,2

32 2,8

33 3,3

34 3,9

35 4,4

36 5,0

37 5,6

38 6,1

39 6,7

40 7,2

41 7,8

42 8,3

43 8,9

44 9,4

45 10,0

46 10,3

47 10,6

48 11,1

49 11,7

50 12,2

51 12,8

PRESSURE PSIG/Hg*

TEMP (C°)

52 13,3

53 14,2

54 14,4

55 13,9

56 15,0

57 15,6

58 15,8

59 16,1

60 16,7

61 17,2

62 17,5

63 17,8

64 18,3

65 18,9

66 19,2

67 19,4

68 20

69 20,3

70 20,6

71 21,1

75 22,8

PRESSURE PSIG/Hg*

TEMP (C°)

80 24,4

85 26,1

90 27,8

95 29,4

100 30,6

105 32,2

110 33,9

115 35,3

120 36,7

125 37,8

130 39,4

135 40,6

140 41,7

145 42,8

150 44,2

155 45,6

160 46,7

165 47,8

170 48,9

175 49,7

180 50,6

PRESSURE PSIG/Hg*

TEMP (C°)

185 51,7

190 52,8

195 53,9

200 54,7

205 55,6

210 56,7

215 57,5

220 58,3

225 59,4

230 60

235 61,1

240 61,7

245 62,8

250 63,3

255 64,4

260 65

265 66,1

270 66,7

275 67,2

280 68,3

285 68,9

PRESSURE PSIG/Hg*

TEMP (C°)

290 69,4

295 70,6

300 71,1

305 71,7

310 72,5

315 73,3

320 73,9

325 74,4

330 75,6

335 76,1

340 76,7

345 77,2

350 77,8

355 78,3

360 78,9

365 79,4

370 80,0

375 80,6

380 81,1

HOW THE COMPONENTS OF AN A/C SYSTEM FUNCTION

AIR CONDITIONING PRESSURE DIAGNOSTICS Static pressure: This is always the first step in diagnosing an A/C cooling complaint. Check the ambient air temp, refer to the pressure/temp chart, and check the pressure. If the pressure is low, you have a low refrigerant charge; if the pressure is high you may have non-condensable gas such as nitrogen or oxygen in the system. In both cases you should evacuate and recharge the system. Refer to the service manual for refrigerant type and capacity.

Normal operation: The high side is usually at least double the static pressure, plus 100 PSI in humid conditions. So at 68 PSI, the rule-of-thumb pressure range for the high side would be 140-240PSI. The low side is usually 30-40 PSI unless ambient temperature is very hot – in which case it will be higher unless the evaporator can maintain a colder temperature. Off idle on a TSV-style vehicle, you will see the pressure drop as low as 10PSI. A Fixed Orifice will usually hold the low side steady between 30-40PSI regardless of RPM.

High side too high, low side normal: If the high side approaches 350-400PSI, then the condenser is unable to rid itself of enough heat to turn the high-pressure gas into a liquid. Make sure the fins are not obstructed. Make sure the cooling fans come on and blow towards the engine. Fans that blow backwards will push head from the radiator and engine into the condenser rather than cooling the condenser down.

Low side to low, high side normal: If the low side is too low then the compressor is pulling on the suction side but there is not enough refrigerant flow to feed it. This suggests either an obstruction in the evaporator or suction line, or the TXV has failed. Before replacing anything, put the system into a deep vacuum for several hours to boil off the water in the system. Ice can form in the TXV and develop this type of complaint when there is moisture in the system.

Low and high side low: If when the compressor engages, the low side drops but the high side does not rise – or the high side also drops – there is an obstruction between the compressor and the high side pressure port. Refrigerant is backing up along the line but not reaching the gauge.Low side high, high side low: If the high side and low side do not change from static pressure when the compressor is engaged, the compressor is not pumping and has failed.

Pressures normal, not cooling: Assuming the blend doors are working normally, the system may be insulated from the inside. Something is keeping the cold low side refrigerant from making the evaporator fins cold. This can be caused by stop-leak, or by an excessive amount of refrigerant oil.

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DIAGNOSING A/C ISSUES. When you have an idea of what’s going on with your pressures, it’s time to start hands-on diagnosis.

This typically involves temperature-sensing equipment – either a temp gun, probe, or thermal camera. If you are using a temp gun, note that measuring extruded aluminium lines can give false readings – cover the line in masking tape and measure the temperature of the tape for better accuracy.

AIR CONDITIONING TEMPERATURE READINGS

If you suspect an obstruction anywhere in the system, you will need to check the temperature of the line and components. See the below guide for temperatures you should expect to see on a normal system at around 20°C (68°F):Discharge line from compressor: This is the hottest part of the system – it should be 15-30 degrees hotter than the liquid line coming from the condenser. At 20°C you should see anywhere from 54-79°C depending on humidity.

Along the condenser: You should see a gradual drop in temperature from the discharge line to the liquid line. This will typically be a drop of around 15-30 degrees. If you see a drop of more than 50 degrees, the condenser may be plugged.

Liquid line to expansion valve: This is the ‘warm’ line – this should be around 15-30 degrees cooper than the discharge line. At 20°C, you should see 40-63°C. There should be even temperature all the way to the expansion valve.

Expansion valve: The liquid line going in should be at 40-63°C at 20°C ambient air temperature. The suction line coming out will be 0-6°C almost regardless of the ambient air temperature if the evaporator is working correctly.

COMMON A/C PROBLEM AREAS

Check the temperature drop across the condenser – a 50-degree drop is a problem.

Check the temperature all along the lines; you should not have any drop at all in the middle of a line, unless that’s where the Fixed Orifice is located.

If the temperature of the lines is lower than suggested by the pressure on your gauge, this suggests stop-leak or excessive refrigerant oil in the system.

Autodata’s service air conditioning module contains connection diagrams, refrigerant quantities and types, as well as oil types and viscosities. With Autodata Diagnostic & Repair, get system layouts, component information, and in-depth interactive wiring diagrams for AC systems.

To find out more or to try Autodata today, visit www.autodata-group.com

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