Refrigerant System Performance Check

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:

Normal A/C Refrigerant System System Pressure Temperature Relationship:

A/C Clutch Cycling Rates:

1. The system pressures are low (A/C compressor suction) and high (A/C compressor discharge).
2. An A/C clutch cycle is the time the A/C clutch is engaged plus the time it is disengaged (time on plus time off).
3. The A/C clutch cycle times are the lengths of time (in seconds) that the A/C clutch is ON and OFF.

The following procedure is recommended for achieving accurate diagnosis results in the least amount of time:
1. Run engine at 1500 rpm for 10 minutes.
2. Operate the A/C system on max A/C (recirculating air). Run A/C blower motor at max speed.
3. Stabilize the in-vehicle temperature at 70° F to 80 ° F (21° C to 27° C).
a. Connect a manifold gauge set to the system. Purge the air from the red and blue hoses by loosening the fittings at the gauge set. Open them only long enough for air to escape and then tighten the fittings.

NOTE: The test conditions, specified at the top of each chart, must be met to obtain accurate test results.

b. Start the engine and turn the A/C system ON.
c. 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 22 and 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 40 and 47 psi. Somewhere between 40 and 47 psi, the A/C cycling switch should close and the suction pressure should start to drop.

NOTE: 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.

d. Determine the A/C clutch cycle rate per minute (A/C clutch on time plus off time is a cycle).
e. Record the A/C clutch OFF time in seconds.
f. Record the A/C clutch ON time in seconds.
g. Record the center A/C register discharge temperature.
h. Determine and record the ambient temperatures.
i. Compare the test readings with the applicable previous charts.
4. Plot a vertical line for the recorded ambient temperature from the scale at the bottom of each chart to the top of each chart.
5. Plot a horizontal line for each of the other test readings from the scale at the LH side of the appropriate chart.

Refrigerant System Pressure And A/C Clutch Cycling Time Evaluation Chart:

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 ensure the concern has been corrected.

NOTE: In ambient temperatures above 27° C (80° F), the A/C clutch will not normally cycle off. This will depend on local conditions and engine/vehicle speed. Also, 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 A/C compressor operation. A rapid cycling A/C clutch is usually an indication that the system is low on refrigerant.
^ Refer to Insufficient or No A/C Cooling-A/C Evaporator Core Orifice Cycling A/C Clutch System Diagnosis chart. F: Insufficient or No A/C Cooling