Performance Test Diagnosis

Performance Test Diagnoses
This A/C system is different from other Nissan A/C systems. The typical Nissan A/C system uses a thermal expansion valve to provide a restriction which causes a pressure change and also controls refrigerant flow through the evaporator.

This system uses a Fixed Orifice Tube (FOT) to cause a pressure change, by restricting the flow of refrigerant to the evaporator. The flow of refrigerant is controlled by engaging and disengaging the compressor clutch based on low side pressure. Therefore, it is critical to consider clutch engagement and disengagement times for proper diagnosis.

The refrigerant flow is regulated by a fixed orifice tube for the front evaporator and a thermal expansion valve for the rear evaporator.

The best way to diagnose a condition in the refrigerant system is to note the system pressures (shown by the manifold gauges) and the clutch cycle rate and times. Then, compare the findings to the charts.
- The system pressures are low (compressor suction) and high (compressor discharge).
- A clutch cycle is the time the clutch is engaged plus the time it is disengaged (time on plus time off).
- Clutch cycle times are the lengths of time (in seconds) that the clutch is ON and OFF

Important - Test conditions
The following test conditions must be established to obtain accurate clutch cycle rate and cycle time readings:
- Run engine at 1,500 rpm for 10 minutes.
- Operate A/C system on max A/C (recirculating air).
- Run blower at max speed.
- Stabilize in vehicle temperature at 21°C to 27°C (70°F to 80°F).

The following procedure is recommended for achieving accurate diagnosis results in the least amount of time.
1. Connect a manifold gauge set to the system. The test conditions specified must be met to obtain accurate test results.
2. Start the engine and turn ON A/C system.
3. As soon as the system is stabilized, record the high- and low-pressures as shown by the manifold gauges. The low side should cycle between approximately 168.9 kPa (1.723 kg/cm2, 24.5 psi) and 276 to 324 kPa (2.81 to 3.30 kg/cm2, 40 to 47 psi). (As low pressure drops, high pressure should rise.) When the clutch disengages, the low side should rise and the high side should drop.
4. Determine the clutch cycle rate per minute (clutch on time plus oft time is a cycle).
5. Record clutch OFF time in seconds.
6. Record clutch ON time in seconds.
7. Record center register discharge temperature.
8. Determine and record ambient temperature.
9. Compare test readings with applicable chart.
- Plot a vertical line for recorded ambient temperature from scale at bottom of each chart to top of each chart.
- Plot a horizontal line for each of the other test readings from scale at LH side of appropriate chart.
At the bottom of the chart, additional cause components are listed for poor compressor operation or a damaged compressor condition.

The diagnosis charts provide the most direct and sure way to determine the cause of any concern in a poorly performing refrigerant system. However, if the vehicle is equipped with rear A/C, poor performance of the front or rear system may not cause abnormal pressures or clutch cycle time. In this case, poor performance may indicate a restriction of refrigerant flow to that system, or an evaporator problem. After servicing and correcting a refrigerant system concern, take additional pressure readings and observe the clutch cycle rate while meeting the conditional requirements to ensure the concern has been corrected.