Refrigerant System Performance Check

Normal Pressure/Temperature Relationships:

Normal A/C Cycling Pressure Switch Rates And Times:

Evaluation Chart:

Test equipment must be connected to the refrigerant system in order to make system tests. If a charging station is used, follow the instructions of the station manufacturer.

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

The following test conditions must be established to obtain accurate pressure readings:
- Run engine at 1500 rpm for 10 minutes.

NOTE: When ambient temperatures exceed 38°C (100°F), engine should be run at normal idle speed only. For tests in ambient temperatures between 38°C-43°C (100°F-110°F), system performance pressures will be the same as those for ambient temperatures shown on the chart in the 32°C-38°C (90°F-100°F) range. Center A/C register discharge temperatures will also be similar to those shown in the chart in the 32°C-38°C (90°F-100°F) range.

- Operate A/C system on MAX A/C (recirculating air).
- Run blower motor at maximum 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 Rotunda R-134a Manifold Gauge Set 176-R032A or equivalent, to the system.
- Run engine at 1500 rpm for 10 minutes.
- Operate A/C system on MAX A/C (recirculating air)
- Run blower motor at maximum speed
- Stabilize in-vehicle temperature at 21°C to 27°C (70°F to 60°F).
2. As soon as the system is stabilized record the high and low pressures as shown by the manifold gauges. Normally the suction pressure should decrease to a range between 152-193 kPa (22-28 psi) and the A/C cycling switch should open. When the A/C cycling switch opens, the suction pressure should start to rise to a range between 276-324 kPa (40-47 psi). Somewhere between 274-324 kPa (40-47 psi), the A/C cycling switch should close and the suction pressure should start to drop.

The discharge (high) pressure should operate the reverse of the suction pressure. When the suction pressure is dropping. the discharge pressure should increase. When the suction pressure is increasing, the discharge pressure should decrease.

3. Determine the A/C clutch cycle rate per minute (A/C clutch on time plus off time is a cycle).

NOTE: When the ambient temperature is above 26°C (80°F), the A/C clutch may not cycle.

4. Record A/C clutch off-time in seconds.
5. Record A/C clutch on-time in seconds.
6. Record center A/C register discharge temperature.
7. Determine and record ambient temperatures.
8. Compare test readings with previous applicable charts.
- 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.

If the point where the two lines cross on each of the charts falls within the dark band, the system is operating normally. If the lines cross outside the dark band on one or more of the charts, there is a concern and the specific cause must be determined.

At the bottom of the Evaluation Chart, additional cause components are listed for poor A/C compressor operation or a damaged condition.

The diagnosis charts provide the most direct and sure way to determine the cause of any concern in a poorly performing refrigerant system.

After servicing and correcting a refrigerant system concern, take additional pressure readings and observe the A/C clutch cycle rate while meeting the conditional requirements to make sure the concern has been corrected.

In ambient temperatures above 27°C (81°F), the A/C clutch will not normally cycle off. This will depend on local conditions and engine rpm/vehicle speed. A/C clutch cycling will normally not occur when the engine is operating at curb idle speed.

If the system contains no refrigerant or is extremely low on refrigerant, the A/C clutch will not engage for compressor operation. A rapid cycling A/C clutch is usually an indication that the system is low on refrigerant.