Gruber
Senior Member
- First Name
- Mark
- Joined
- Jan 27, 2018
- Threads
- 2
- Messages
- 2,309
- Reaction score
- 1,521
- Location
- TN
- Vehicle(s)
- 2018 Honda Civic Sport Touring; 2009 Honda CR-V EX-L
Definitely don't set the fan speed to high to check the vent temperature.Over the years these are some basics or guidelines I’ve read about & followed on how to check your vehicle AC temperature.
Check the center dash outlet with an AC thermometer with the AC set on recycle, temperature on maximum cold, blower speed on max, & eng rpm 1,000 to 1,500. Not sure about dual zone systems, maybe check each center vent. Sometimes it’s colder when the fan speed is on low & IIRC one manual said to do that. I’m presuming it’s because the air is moving slower through the evaporator & having more time to get the heat removed,
Outlet vent temp is typically 40 degrees below the ambient temp. Some manuals say it should not get colder than 38-40 degrees, some say not colder than 32. If too cold the evaporator will freeze up with moisture & block air flow. I’ve had this happen on a very humid day. I turned off the AC for about 10 minutes to let the evaporator thaw & then it worked OK.
When working on my old cars AC, sometimes I’ve driven around with the thermometer in the vent. If it gets below 38-40 degrees I’m satisfied. If it starts climbing to 50 degrees when stopped at a light I know there's a problem. However if it’s 90 outside you may never see 40 at the vent.
I don't think you can count on 40 F temperature drop by any properly working AC system, auto or home. This is certainly not typical and not as it should be. If you do see 40 F cooling, it might be in reference to the temperature outside, while you are recirculating, so it's not what you are supposed to be measuring. You should in this case measure the difference from the temperature inside the car. Or it is actually cooling too much and at risk of freezing over the evaporator, which should never happen. Generally, after it gets steady after some 15 minutes, the temperature difference between inlet and outlet should not be much more than 20 F and if it's 15 F it's still fine.
While always cooling by up to 20 F, the ability of the system to maintain a comfortable temperature on very hot and humid days depends on the size of the system, which practically means more pounds of refrigerant. Outside temperature might be 130 F and the temperature inside the car set to recirculation could be a cool 65 F. That doesn't mean that the delta T is 65 F. In this case the temperature of the air out of the vents might be 45 F, only 20 F difference from the inlet air.
AC systems tended to be bigger in older good cars and in luxury cars at any time, so they would provide adequate cooling even on extremely hot and humid days, but the temperature difference between inlet and outlet air in a properly working AC in steady state should still never significantly exceed 20 F.
Some Honda civics in the XX century had about twice or more refrigerant than today. And in the early 90's it was still R12, which has more cooling capacity per pound than R134a. There are many ways of saving energy when less than maximum cooling is needed, but no breakthrough was invented since 40 years ago to make 15 ounces of refrigerant have the same cooling power as 30+ ounces.
https://www.techchoiceparts.com/refrigerant-and-oil-capacities/honda
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