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?

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    You should expect to see anywhere from mid 40s to high 30s on a working ac system. The best way to tell is with a manifold gauge set and watch high side pressure.
    – Ben
    Jun 17, 2016 at 13:48
  • Note that when measuring the A/C effectiveness, a digital thermometer in the vent is ideal. Have all the windows open, and the ventilation on fresh air, not recirc, so that you can measure exactly what's happening. My recommendation would be to take it to a car A/C specialist, and have them check it out. There are a lot of variables in car A/C (high side pressure, low side pressure, humidity..) - it's a specialist area once it gets beyond 'stick a pound can of R134A in and see what happens' (which is what I think your dealer did)
    – PeteCon
    Jun 17, 2016 at 14:57
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    @bishop: Feel free to try both methods. For me, I want to know what the actual AC is doing, and not be confused by any conditions in the cabin - although it's the conditions in the cabin that dictate comfort, of course. I have a simple digital cooking thermometer that I use; I have a laser type, but you need to aim it at exactly the same point every time to get a comparable reading.
    – PeteCon
    Jun 17, 2016 at 16:10
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    Is the air in the back warmer than expected, or is there a lack of air? I have a similar vehicle and the duct for the rear had popped apart so there was almost no airflow in the back.
    – JPhi1618
    Jun 17, 2016 at 17:20
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    @Jphi1618 There is air flow, but substantially warmer (by feel) than the dash flow.
    – bishop
    Jun 17, 2016 at 17:24

2 Answers 2


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.

  • Thanks, that matches my expectations given what I know of the A/C system architecture. Also, should "Eco mode" be disabled? (I believe that cuts or limits the A/C function when idling.)
    – bishop
    Jun 17, 2016 at 15:34
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    @bishop Eco mode is, for most cars, recirculation mode. If it has other specific effects on your system I am not aware of what they might be. Jun 17, 2016 at 16:33
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    Recirculation mode takes air from inside the car, and cools it again. This is more efficient than taking hot outside air and cooling that. You get better cooling performance, and just a bit better fuel economy depending on conditions. You definitely want recirculation mode. (that's the one with the louder blower motor noise... louder because blower is right next to the recirculation air inlet.) Recirculation mode also takes humidity out of the equation. That really affects performance.
    – zipzit
    Jun 17, 2016 at 21:11
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    At least on Audi cars, the ECO mode on climate control systems disables the AC completely and just uses the outside air to cool the cabin. Might be different on Chrysler. Jun 21, 2016 at 7:58
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    @bishop, old thread, I know. I just wanted to point out to everyone else here that the "Eco Mode" the OP is referring to on a Chrysler/Dodge van is for fuel economy, not directly related to the HVAC system. This forum doesn't suggest it changes how the A/C operates, but doesn't rule it out either since no one reaches a conclusion on what it does other than change the timing and shift points. Jul 11, 2017 at 20:14

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.

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