Always Learning. CHAPTER 12 A/C System Inspection and Diagnosis.

Always Learning

CHAPTER 12

A/C System Inspection and Diagnosis

Automotive Heating and Air Conditioning, 6/eBy Thomas S. Birch

Copyright © 2012, 2010, 2006, 2001, 1997, 1991 Pearson EducationUpper Saddle River, NJ 07458 • All rights reserved.3

After studying Chapter 12, the reader will be able to:

1. Inspect an A/C system to determine whether it is operating correctly, and if not, make further tests to locate the cause of the problem.

2. Connect a manifold gauge set to an A/C system and check system pressures.

3. Determine whether system pressures are normal, and if abnormal, determine the cause of the fault.

4. Check a TXV and determine whether it is operating properly.

5. Locate the source of a refrigerant leak.

6. Complete the ASE tasks related to A/C system diagnosis.

OBJECTIVES



KEY TERMS

• Back-seat • Compound gauge • Delta T • Electronic leak detectors • Front-seat • Hand valves • Manifold gauge set • Mid-seat

• Rub-through • Service hose • Service unit • Shutoff valve • Ultrasonic leak detector • Vacuum pump • Valve core depressor



INTRODUCTION

• If the HVAC inspection has determined a fault in the A/C system, further checks can be made to determine the exact cause of the problem. • This further evaluation usually consists of a check of

system pressures and temperatures while also checking for unusual noises, vibrations, and smells.

• A/C SERVICE REGULATIONS



TECH TIP

• Remember that the purpose of the A/C system is to make the evaporator cold, slightly above 32°F (0°C). We determine its temperature by feeling the suction line where it reenters the engine compartment; it should be cold to touch. The suction line warms up as it goes to the compressor, but it still should be cool to touch. The discharge line leaving the compressor should be hot to very hot; use caution when you feel this line. Remember that the condenser must give up heat to the ambient air, so it must be hotter than ambient temperature.



A/C PRESSURE CHECKS

• SERVICE UNITS• A service unit contains:

• High- and low-side pressure gauges• A vacuum pump• A refrigerant supply and method controlling the

charge• A method of adding oil to a system

• Most modern service units also include:• A method of recovering refrigerant from a system• A method of recycling that refrigerant for reuse



A/C PRESSURE CHECKS

• GAUGE SET• ELECTRONIC GAUGE SETS• CHECKING GAUGE ACCURACY

• CONNECTING AND DISCONNECTING GAUGES TO A SYSTEM



TECH TIP

1. If the protective cap is missing, dirt will enter the port and valve core, being attracted and trapped by the oil film. When the system is serviced, this dirt will enter either the gauge set or the system. Dirt particles can damage the valves and the gauges in the gauge set, damage the compressor, or plug small orifices if they enter the system. An unprotected port should be cleaned before connecting a hose to it.

2. Many service fittings are permanently welded onto the line; some are threaded onto the line and are replaceable.



TECH TIP

• There are two major types of R-134a couplers: manual and automatic. Both types are quickconnect with a ring that must be slid upward to allow connecting and disconnecting. Manual couplers have a knob that is turned inward to depress the service valve. Do not overtighten this knob, as it can damage the valve.



Why Do Some Systems Have More Than Two Service Ports?

• Some systems that use an STV have a second low-side service port to allow the use of a third gauge. A third gauge is a single, compound gauge, similar to the low-side gauge, and is often attached directly to a service hose. The third gauge is usually attached to the third service port, which is downstream from the STV, closer to the compressor. Comparison of the low-side pressure with that of the third gauge allows the technician to determine whether the STV is open or closed.



What Is a Service Valve?

• Some early A/C systems (R-12) use service valves at the service ports. A service valve has a stem that can be rotated using a 1/4-inch square socket or box wrench. This valve is set to one of three positions: back-seat , mid-seat , or front-seat . During normal operation the valve is kept in the back-seat position: This closes the service port while the compressor port is left open. During service operations the valve is mid-seated: This opens both the compressor port and the service port. During some special operations the valve is front-seated: This closes the compressor port while the service port is left open. A service valve is fitted with protective caps for both the service port and the valve stem. While working with service valves, open-end wrenches, adjustable wrenches, and pliers should never be used on the valve stem. These tools will ruin the 1/4-inch square stem and prevent the use of the proper wrench.



TECH TIP

• As the R-12-style service hose is being connected to a Schrader valve port, refrigerant often escapes because the valve depressor usually makes contact with the valve stem before the seal ring contacts the seat. This leak can be reduced by using the following procedure: Thread the knurled nut on the port one or two turns, grip the hose and push it quickly and firmly against the port, and tighten the nut to hold the hose in place. The knurled nut should be tightened only finger-tight. The seal and seat can be damaged if pliers are used. If the seal leaks while finger-tight, the sealing ring in the hose end should be replaced.



PERFORMANCE TEST

• A performance test is used to determine whether the A/C system is operating properly and to indicate what is wrong if the system is not operating normally. • Inspection checklists are TS 5 and TS 6 in the

NATEF Correlated Task Sheets.



PERFORMANCE TEST

• SYSTEM TEMPERATURES• THERMOMETERS

• MEASURING RELATIVE HUMIDITY• INTERPRETING SIGHT GLASSES• INTERPRETING PERFORMANCE TEST

READINGS• ABNORMAL CONDITIONS• VERIFYING REFRIGERANT CHARGE LEVEL• ON-VEHICLE TXV TESTS• ELECTRONIC DIAGNOSTIC TOOL



TECH TIP

• An infrared thermometer with a flexible probe can be used to measure the temperature of an item that is difficult to get to.



TECH TIP

• A direct-reading, electronic humidity tester is available that provides fast, easy humidity measurements. Technicians can quickly measure in-vehicle humidity, which should be about 30% to 50% RH in vehicles with A/C systems that are operating properly.



A/C SYSTEM REFRIGERANT LEAK TESTS

• ELECTRONIC LEAK DETECTOR CALIBRATION• EVAPORATOR INSPECTION• VACUUM PUMP LEAK TESTS



TECH TIP

1. Check the high side for leaks right after shutting the engine and system off while the pressure is the highest.

2. To increase system pressure for leak checking, heat the accumulator or receiver–drier. A heat gun can be used or you can wrap the unit with a small heating pad.

3. Some technicians consider the Schrader valve and service port cap as expendable items. They replace these parts when making a major service to a system. A faulty Schrader valve can be removed by unscrewing it just like a tire valve; in some cases, a tire valve service tool can be used. A special service tool is used in very tight locations. A new A/C Schrader valve should be installed; tire valves will not work properly.



TECH TIP

1. Some R-134a service ports are threaded into the metal line. A damaged service port can often be replaced by unscrewing the port from the line and installing a new one.

2. A leaking high-pressure relief valve can be replaced using a new valve and gasket.



SUMMARY

1. Technicians use system pressures and temperatures to determine if a system is working properly and what is causing the problem if the system is not working properly.

2. The pressure in different sections of a system is linked to the temperature of that section.

3. Service equipment such as manifold gauge sets and service units must be dedicated to a single refrigerant type.

4. Technicians check their pressure gauges periodically to ensure that they are working properly.



SUMMARY

5. Performance tests follow a set procedure.

6. High-side pressure is related to ambient temperature.

7. Pressures that are higher or lower than normal indicate the nature of the system’s problem.

8. The best way to ensure that a system has the proper refrigerant charge is recover the refrigerant and recharge it to the specified amount.

9. Several types of leak detectors can be used to locate refrigerant leaks.



REVIEW QUESTIONS

1. An A/C gauge set contains a(n) ______________ ______________ and a(n) ______________ ______________ pressure gauge, at least two hand valves, and ______________ ______________ .

2. An A/C service unit will include ______________ ______________ , a(n) ______________ ______________ , and a(n) ______________ ______________ .

3. An A/C service unit can also include equipment to ______________ and ______________ refrigerant.

4. A low-side, R-12 service port is the same as a 1/4-inch ______________ ______________ .

5. A high-side, R-134a service port is ______________ than the low-side port, and both ports use a(n) ______________ ______________ style coupler.

6. The low-side gauge and hose are colored ______________ , and the high-side gauge and hose are colored ______________ .

7. When the hoses are connected to an R-12 system on a 70°F day, both gauges should read about ______________ ______________ .

8. In an R-134a system with normal operation, the low-side pressure should be about ______________ psi, and the suction line should be ______________ .

9. In an R-134a system with normal operation, the discharge line should be ______________ , and the high-side pressure will ______________ as ambient temperature increases.



REVIEW QUESTIONS

10. Bubbles are expected to appear in the sight glass during ______________ ______________ .

11. High relative humidity can cause ______________ high-side ______________.12. Low pressure on the low and high sides is commonly caused by

______________ ______________ .13. A faulty compressor can cause ______________ low-side pressure and

______________ high-side pressure.14. The refrigerant charge level can be verified by checking the ______________

and ______________ of the refrigerant at the condenser outlet.15. The most popular refrigerant leak detectors used today are ______________

______________ and ______________ ______________ .16. A system should have a pressure of at least ______________

______________ when checking for a leak.17. The leak detector probe should be moved along about ______________

______________ from the location being checked at a maximum rate of ______________ inch(es) per second.

18. A fluorescent trace will show up better if you wear ______________ ______________ .



CHAPTER QUIZ

1. Technician A says that you should begin A/C problem diagnosis with a thorough HVAC system inspection. Technician B says that from the inspection it can be determined what type of system the vehicle has. Who is correct?

a. A only

b. B only

c. Both A and B

d. Neither A nor B



CHAPTER QUIZ

2. Technician A says that oily residue on a fitting or hose can indicate a refrigerant leak. Technician B says that this residue will not affect a leak detector. Who is correct?

a. A only

b. B only

c. Both A and B

d. Neither A nor B



CHAPTER QUIZ

3. When a system is operating correctly, the lines and hoses on the high side should be cool to the touch.

a. True

b. False



CHAPTER QUIZ

4. Technician A says that pressure checks are one method of diagnosing A/C problems. Technician B says that trouble trees are used only for mechanical problems. Who is correct?

a. A only

b. B only

c. Both A and B

d. Neither A nor B



CHAPTER QUIZ

5. While discussing a manifold gauge set, Student A says that the high-side service hose is blue. Technician B says that the low-side pressure gauge goes up to 750 psi. Who is correct?

a. A only

b. B only

c. Both A and B

d. Neither A nor B



CHAPTER QUIZ

6. Technician A says that the manifold gauge set for R-12 can be used on all systems. Technician B says that R-134a systems use service ports with metric flare threads. Who is correct?

a. A only

b. B only

c. Both A and B

d. Neither A nor B



CHAPTER QUIZ

7. On a system that uses service valves, the valve is kept in the ______________ seated position for normal operation.

a. front-

b. back-

c. Mid

d. None of these



CHAPTER QUIZ

8. An R-12 system is shut off, and the ambient temperature is 70°F. The evaporator pressure should be about ______________ .

a. 50 psi

b. 70 psi

c. 83 psi

d. 100 psi



CHAPTER QUIZ

9. In most R-12 systems that use an STV, the normal low-side pressure should be about ______________ .

a. 15 to 20 psi

b. 15 to 30 psi

c. 28 to 30 psi

d. over 35 psi



CHAPTER QUIZ

10.High-side pressure is affected by ______________ .

a. ambient temperature

b. fan clutch operation

c. refrigerant charge level

d. All of these



CHAPTER QUIZ

11.A faulty A/C system has a high low-side pressure and a high high-side pressure. Technician A says that the system is probably starved. Technician B says that the system has a bad thermostatic switch. Who is correct?

a. A only

b. B only

c. Both A and B

d. Neither A nor B



CHAPTER QUIZ

12.______________ can be used to locate refrigerant leaks.

a. Soap and water

b. Flame-type testers

c. Electronic testers

d. Any of these



CHAPTER QUIZ

13.The engine should be running while the technician checks for leaks.

a. True

b. False



CHAPTER QUIZ

14.Technician A says that the system should be operated after injecting fluorescent dye into it. Technician B says that wearing yellow-tinted goggles can help you find the dye traces, and this also protects your eyes. Who is correct?

a. A only

b. B only

c. Both A and B

d. Neither A nor B

Always Learning. CHAPTER 12 A/C System Inspection and Diagnosis.

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