What temperature should the A/C blow?

Question

The A/C output temperature on my 2012 Chrysler Town & Country feels diminished, especially in the rear seats' A/C vents. So, I had the refrigerant drained, measured, and topped off (dealership did the work, free of charge), but I'm still unconvinced the A/C is cooling to spec. I have not yet measured the output temperature, but I'd like to know what to expect in a "well-functioning" system.

From my research, it appears that a vent output temperature of 40°F to 45°F (4°C to 7°C) measured after driving for 10 minutes at 45+ mph would be reasonable on a 85°F (29.4°C) day with 70% relative humidity. Measurement would be taken from a console vent using a precise stick & dial thermometer.

Is this so? Are there tables that would provide better diagnostic guidance? Is there a particular thermometer known to be good at measuring vent outputs?

Answer 1

Set these test conditions:

• Engine set to 1800 rpm
• Recirulation air direction on
• Fan speed on low
• Windows up
• Temp set to max cold
• Air outlet to dash vents
• Vehicle in the shade
• Run for at least ten minutes, longer on a hot day.

It can take much longer on in hot weather or if the vehicle is parked in the sun.

Measure at least 3 inches into the center dash vent. Temperature should be 40 to 42 deg F. Higher fan speeds, resulting in high air volume, will result in higher vent temps. Rear cabin systems are normally 10 degrees warmer because they have a larger volume of space to cool.

The system is limited on low side temp by a thermo-switch that turns the system off at 34 degrees F. This is located in the evaporator. Given less than perfect heat transfer to the air about 40 degrees at the vents is the best it gets.

Answer 2

Best way to test the state of the A/C is to measure the "superheat" and the "Subcool" numbers. You'll need a manifold gauge set and a digital thermometer that you can attach to the refrigerant lines. Attach the digital thermometer's probe to the low pressure suction line (wrap it with electrical tape to hold the probe firmly against the line) on the metal part of the line within a few inches of the low side service port. Also connect your gauge set. Start the Vehicle and put the AC on MAX AC, full cold temp and max fan speed. Let the AC run for about 10 or 15 minutes to allow the pressures and temps to stabilize, then note the temperature reading on the low side pressure gauge for R134a refrigerant. (ie 30 PSI @ 35* on the gauge) This is the temp that the r134a begins to start boiling. Then measure the temperature of the line itself near the service port after it comes out of the evaporator using your digital thermometer. It should be higher than what is showing on on the pressure gauge. Subtract the pressure guage temp from the measured line temp. This is your superheat number. To get the Subcool number its the same concept but on the liquid line coming off the condenser, near the high side service port.Read the High side temp shown on the high side pressure guage... (ie 130* @ 200PSI) and subtract the actual measured line temp at the high service port from the temp shown on the pressure gauge (should be lower than the pressure gauge temp). This number is your "subcool" number. On r134a systems about you want about 15-20 degrees of subcool. if its less than that, the system needs more freon.. if its over that, it has too much freon. For your superheat numbers, around 15 degrees is about the best you're gonna get. less than that, its under charged, more than that, its overcharged. The purpose of superheat and subcool is to tell you about where in the system the freon is either changing from vapor to liquid (subcool) or from liquid to vapor (superheat), you want this change to occur in the center of the evaporator or condenser.. by changing the superheat and subcool numbers higher or lower (by adding or removing freon), you're moving the position of the where the evaporation/condensation point happens further up or down in the evaporator or condenser from top to bottom. The higher the numbers (up to a point), the more liquid freon is in the evaporator/condenser.