If the low side gauge shows a drop again, the

valve is not stuck. Clean the surfaces of the evaporator outlet and the capillary coil or bulb. Make sure the coil or bulb is securely fastened to the evaporator outlet and covered with insula- tion material. Operate the system and check per- formance.

Repair Procedure:

If the test did not result in proper operation of the expansion valve, the valve is defective and must be replaced. Recover all refrigerant from the system and replace the expansion valve and the receiver-drier. Evacuate and recharge the system with refrigerant, then check AC opera- tion and performance.

0 100 200 300 400 500 HIGH TEMPERATURE LOW TEMPERATURE 150 120 90 60 30 0 10 20 30

System High Pressure Side Restriction

LOW SIDE LOW HIGH SIDE NORMAL-TO-HIGH

Tip: Air from vents in the cab is only slightly cool. Look for sweat or frost on high side hoses and tubing, and frost appearing right after the point of restriction. The hose or line may be cool to the touch near the restriction.

Cause: There could be a kink in a line, or other restric- tion in the high side of the system.

Figure 8-11

Gauge reading, system high pressure side restriction.

Repair Procedure:

After you locate the defective component con- taining the restriction, recover all of the refrig- erant. Replace the defective component and the receiver-drier. Evacuate and recharge the sys- tem with refrigerant, then check AC operation and performance. 0 100 200 300 400 500 HIGH TEMPERATURE LOW TEMPERATURE 150 120 90 60 30 0 10 20 30 Compressor Malfunction

LOW SIDE HIGH HIGH SIDE LOW

Tip: The compressor may be noisy when it operates.

Cause: Defective reed valves or other compressor com- ponents. If the compressor is not noisy, there may be a worn or loose compressor clutch drive belt.

Repair Procedure:

If you find the belt worn or loose, replace or tighten it and recheck system performance and gauge readings. To inspect and service the com- pressor, you must isolate (front seat the stem type compressor service valves) and recover re- frigerant, or fully recover R-12 from systems containing Schrader valves. Remove the com- pressor cylinder head and check the appearance of the reed valve plate assembly. If defective, replace the valve plate and install with new gas- kets, or replace the compressor assembly.

Figure 8-12

Gauge reading, compressor malfunction.

Chapter 8 – Troubleshooting & Service Procedures

If you find particles of desiccant in the compres- sor, remove and replace it and the receiver-drier. Before doing so, back flush other system compo- nents (except the expansion valve) using a flush- ing kit. If there are stem type valves and you isolate the compressor, the rest of the system must be purged of refrigerant before you can disconnect and flush system components (Chap-

ter 9 describes the flushing procedure). After

flushing, reassemble the components. Always check the oil level in the compressor, even if you install a new or rebuilt unit. Tighten all connec- tions and evacuate the system. Recharge the air conditioner with refrigerant and check system operation and performance.

Note: Rotary compressors have a limited oil reser- voir. Extra oil must be added for all truck installations 0 100 200 300 400 500 HIGH TEMPERATURE LOW TEMPERATURE 150 120 90 60 30 0 10 20 30

Condenser Malfunction or System Overcharge

LOW SIDE HIGH HIGH SIDE HIGH

Tip: The air from vents in the cab may be warm. In R- 12 systems there can be bubbles in the sight glass. The high pressure hoses and lines will be very hot. Don’t forget to check the engine cooling system components—fan and drive belt, fan clutch operation, and the radiator shutter.

Figure 8-13

Gauge reading, condenser malfunction or system overcharge.

Cause: The condenser is not functioning correctly or there may be an overcharge of refrigerant inside the system. Another possibility is lack of (ram) air flow through the condenser fins during test- ing. Engine cooling system component malfunc- tion can cause high pressure by blocking air flow (radiator shutter) or not providing air flow (fan clutch) in sufficient quantity.

Repair Procedure:

Inspect the condenser for dirt, bugs or other de- bris and clean if necessary. Be sure the con- denser is securely mounted and there is ad- equate clearance (about 1-1/2 inches) between it and the radiator. Check the radiator pressure cap and cooling system, including the fan, fan clutch, drive belts and radiator shutter assem- bly. Replace any defective parts and then re- check AC system operation, gauge readings and performance.

If the problem continues, the system may be over- charged (have too much refrigerant inside). Recover the system slowly until low and high pressure gauges read below normal, and bubbles appear in the sight glass. Then add refrigerant (charge the system) until pressures are normal and the bubbles disappear. Add another quarter to half pound of refrigerant and recheck AC sys- tem operation, gauge readings and performance. If the high gauge readings do not change, you should recover all of the refrigerant and flush (it may be partially plugged) or replace the con- denser. Also replace the receiver-drier or accu- mulator. Then connect the components and evacuate the system. Recharge the air condi- tioner with refrigerant and check system opera- tion and performance.

Chapter 8 – Troubleshooting & Service Procedures 0 100 200 300 400 500 HIGH TEMPERATURE LOW TEMPERATURE 150 120 90 60 30 0 10 20 30

Thermostatic Switch Malfunction

LOW SIDE NORMAL HIGH SIDE NORMAL

Tip: The low side gauge needle may fluctuate in a very narrow range compared to a normal range. The compressor clutch may be cycling on and off more frequently than it should.

The low side gauge needle may fluctuate in an above normal range as the clutch cycles. This may be an indication that the thermostat is set too high (someone may have attempted to adjust the factory setting). A new thermostat may have been installed incorrectly (capillary tube not in- serted between the evaporator fins in the proper position).

Cause: The thermostatic switch is not functioning prop- erly or at all.

Repair Procedure:

Replace the thermostatic switch. When you re- move the old thermostat, replace it with one of the same type. (They operate in a factory preset temperature range.) Take care in removing and handling the thermostat and thin capillary tube attached to it. Don’t kink or break the tube. Position the new thermostat capillary tube at or close to the same location and seating depth be- tween the evaporator coil fins as the old one. Connect the electrical leads.

Figure 8-14

Gauge reading, thermostatic switch malfunction.

Note: See the Thermostat section in Chapter 10. Fan clutch, radiator shutter, condenser, compres- sor, and the newer air and water valve control systems are covered in Chapter 10.