Jeep Cherokee (XJ): A/C performance

The air conditioning system is designed to provide the passenger compartment with low temperature and low humidity air. The evaporator, located in the heater-A/C housing on the dash panel below the instrument panel, is cooled to temperatures near the freezing point. As warm damp air passes through the cooled evaporator, the air transfers its heat to the refrigerant in the evaporator and the moisture in the air condenses on the evaporator fins. During periods of high heat and humidity, an air conditioning system will be more effective in the Recirculation Mode.

With the system in the Recirculation Mode, only air from the passenger compartment passes through the evaporator. As the passenger compartment air dehumidifies, the air conditioning system performance levels improve.

Humidity has an important bearing on the temperature of the air delivered to the interior of the vehicle.

It is important to understand the effect that humidity has on the performance of the air conditioning system. When humidity is high, the evaporator has to perform a double duty. It must lower the air temperature, and it must lower the temperature of the moisture in the air that condenses on the evaporator fins. Condensing the moisture in the air transfers heat energy into the evaporator fins and tubing.

This reduces the amount of heat the evaporator can absorb from the air. High humidity greatly reduces the ability of the evaporator to lower the temperature of the air.

However, evaporator capacity used to reduce the amount of moisture in the air is not wasted. Wringing some of the moisture out of the air entering the vehicle adds to the comfort of the passengers.

Although, an owner may expect too much from their air conditioning system on humid days. A performance test is the best way to determine whether the system is performing up to standard. This test also provides valuable clues as to the possible cause of trouble with the air conditioning system.

Review the Service Warnings and Precautions in the front of this group before performing this procedure.

The air temperature in the test room and in the vehicle must be a minimum of 21 C (70 F) for this test.

(1) Connect a tachometer and a manifold gauge set.

(2) Set the heater-A/C mode control switch knob in the Recirculation Mode position, the temperature control knob in the full cool position, and the blower motor switch knob in the highest speed position.

(3) Start the engine and hold the idle at 1,000 rpm with the compressor clutch engaged.

(4) The engine should be at operating temperature.

The doors and windows must be open.

(5) Insert a thermometer in the driver side center A/C (panel) outlet. Operate the engine for five minutes.

(6) The compressor clutch may cycle, depending upon the ambient temperature and humidity. If the clutch cycles, unplug the low pressure cycling clutch switch wire harness connector from the switch located on the accumulator (Fig. 6). Place a jumper wire across the terminals of the low pressure cycling clutch switch wire harness connector.

Fig. 6 Low Pressure Cycling Clutch Switch - Typical
Fig. 6 Low Pressure Cycling Clutch Switch - Typical

1 - LOW PRESSURE CYCLING CLUTCH SWITCH
2 - ACCUMULATOR

(7) With the compressor clutch engaged, record the discharge air temperature and the compressor discharge pressure.

(8) Compare the discharge air temperature to the Performance Temperature and Pressure chart. If the discharge air temperature is high, see Refrigerant System Leaks and Refrigerant System Charge in this group.

A/C performance

(9) Compare the compressor discharge pressure to the Performance Temperature and Pressure chart. If the compressor discharge pressure is high, see the Pressure Diagnosis chart.

Pressure Diagnosis

Condition

Possible Causes

Correction

Rapid compressor clutch cycling (ten or more cycles per minute).
  1. Low refrigerant system charge.
  1.  See Refrigerant System Leaks in this group. Test the refrigerant system for leaks. Repair, evacuate and charge the refrigerant system, if required.
Equal pressures, but the compressor clutch does not engage.
  1. No refrigerant in the refrigerant system.
  2.  Faulty fuse.
  3.  Faulty compressor clutch coil.
  4.  Faulty compressor clutch relay.
  5.  Improperly installed or faulty low pressure cycling clutch switch.
  6.  Faulty high pressure cut-off switch.
  7. Faulty Powertrain Control Module (PCM).
  1.  See Refrigerant System Leaks in this group. Test the refrigerant system for leaks. Repair, evacuate and charge the refrigerant system, if required.
  2.  Check the fuses in the Power Distribution Center and the junction block. Repair the shorted circuit or component and replace the fuses, if required.
  3.  See Compressor Clutch Coil in this group. Test the compressor clutch coil and replace, if required.
  4.  See Compressor Clutch Relay in this group. Test the compressor clutch relay and relay circuits. Repair the circuits or replace the relay, if required.
  5.  See Low Pressure Cycling Clutch Switch in this group. Test the low pressure cycling clutch switch and tighten or replace, if required.
  6.  See High Pressure Cut-Off Switch in this group. Test the high pressure cut-off switch and replace, if required.
  7. Refer to the proper Diagnostic Procedures manual for testing of the PCM. Test the PCM and replace, if required.
Normal pressures, but A/C Performance Test air temperatures at center panel outlet are too high.
  1. Excessive refrigerant oil in system.
  2.  Blend-Air door inoperative or sealing improperly.
  3.  Blend-Air door motor faulty or inoperative.
  1. See Refrigerant Oil Level in this group. Recover the refrigerant from the refrigerant system and inspect the refrigerant oil content. Restore the refrigerant oil to the proper level, if required.
  2.  See Blend-Air Door under Heater-A/C Housing Door in this group. Inspect the blend-air door for proper operation and sealing and correct, if required.
  3. Perform Blend-Air door motor diagnosis, replace if faulty.
The low side pressure is normal or slightly low, and the high side pressure is too low.
  1. Low refrigerant system charge.
  2. Refrigerant flow through the accumulator is restricted.
  3. Refrigerant flow through the evaporator coil is restricted.
  4.  Faulty compressor.
  1. See Refrigerant System Leaks in this group. Test the refrigerant system for leaks. Repair, evacuate and charge the refrigerant system, if required.
  2.  See Accumulator in this group. Replace the restricted accumulator, if required.
  3.  See Evaporator Coil in this group. Replace the restricted evaporator coil, if required.
  4.  See Compressor in this group. Replace the compressor, if required.
The low side pressure is normal or slightly high, and the high side pressure is too high.
  1. Condenser air flow restricted.
  2.  Inoperative cooling fan.
  3.  Refrigerant system overcharged.
  4.  Air in the refrigerant system.
  5.  Engine overheating
  1. Check the condenser for damaged fins, foreign objects obstructing air flow through the condenser fins, and missing or improperly installed air seals. Refer to Group 7 - Cooling System for more information on air seals. Clean, repair, or replace components as required.
  2.  Refer to Group 7 - Cooling System for more information. Test the cooling fan and replace, if required.
  3.  See Refrigerant System Charge in this group. Recover the refrigerant from the refrigerant system. Charge the refrigerant system to the proper level, if required.
  4.  See Refrigerant System Leaks in this group. Test the refrigerant system for leaks. Repair, evacuate and charge the refrigerant system, if required.
  5.  Refer to Group 7 - Cooling System for more information. Test the cooling system and repair, if required.
The low side pressure is too high, and the high side pressure is too low.
  1. Accessory drive belt slipping.
  2.  Fixed orifice tube not installed.
  3.  Faulty compressor
  1. Refer to Group 7 - Cooling System for more information. Inspect the accessory drive belt condition and tension. Tighten or replace the accessory drive belt, if required.
  2.  See Fixed Orifice Tube in this group. Install the missing fixed orifice tube, if required.
  3.  See Compressor in this group. Replace the compressor, if required.
The low side pressure is too low, and the high side pressure is too high.
  1. Restricted refrigerant flow through the refrigerant lines.
  2.  Restricted refrigerant flow through the fixed orifice tube.
  3.  Restricted refrigerant flow through the condenser.
  1. See Liquid Line and Suction and Discharge Line in this group. Inspect the refrigerant lines for kinks, tight bends or improper routing. Correct the routing or replace the refrigerant line, if required.
  2.  See Fixed Orifice Tube in this group. Replace the restricted fixed orifice tube, if required.
  3.  See Condenser in this group. Replace the restricted condenser, if required.

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