It’s still not blowing cold enough

By Richard Hawkins, MACS contributor

Author’s note: When writing last week’s article about customer’s expectations of A/C systems, I recalled a tech call and some testing I was involved with along about the same time.

I do not recall the specific application, but the tech call involved a car with a single expansion valve system that utilized an electronically controlled variable displacement compressor. Customer complaint: It’s still not blowing cold enough.

The call went something like this:

Me:  Hello.  This is Richard with A/C tech support.  Fred at the location that services you asked me to contact you about a vehicle you are having an A/C issue with.

Technician:  Yes. Thanks for calling. This customer came in with this car (he provided the vehicle information) and said it isn’t blowing cold enough and wants me to make it blow colder.  He just recently bought it and this is the first “hot spell” we’ve had since he’s had it.

He said that his 20-year old pick-up truck with over 150,000 miles on it will blow colder and says that indicates there is something wrong with the A/C system on this car.

(The information he provided indicated that the car was about 3-years old and had low mileage on it and didn’t appear to have had any work done on the A/C system.

Me:  What is the ambient temperature, humidity and vent temperatures you are getting?

Technician:  The ambient temperature is 88° F, humidity 60% and it is blowing 46° F from the vents with the system in recirculate and medium blower speed.

Me: What are your pressures?

Technician: 180PSI on the high side and 37PSI on the low side.  He keeps a temperature gauge in his vent and he said that his pickup will blow air that is right at 40° under the same conditions.

Me: That doesn’t surprise me. The system on the car is likely doing the best it is going to do. It utilizes an electronically controlled variable displacement compressor that isn’t going to allow the evaporator to get as cold as the one in the pick-up truck. 

The system in the truck has a fixed displacement compressor controlled by a clutch cycling switch that isn’t going to cycle the compressor off until the low side gets down to about 23 or 24 PSI. That is going to allow the exterior of the evaporator to get pretty close to 32° F.  When the system cycles off the cycling switch is going to allow the low side pressure to rise back up to 40 to 44 PSI before the compressor will turn back on. The temperature of the evaporator will rise and this is called evaporator re-heat. Then the temperature of the evaporator will be cooled back down again.

The evaporator is actually getting colder than it needs to be to cool the vehicle and the re-heating of the evaporator and cooling it back down wastes energy.  The system on the car with the electronically controlled compressor eliminates this reheating and keeps the evaporator at a more constant temperature.  It may not blow air quite as cold as the 20 year old pick-up but it will certainly cool the vehicle.

Technician:  That’s interesting. I sort of felt like there wasn’t anything wrong with the system on the car.

Me: We did a test on some similar vehicles not too long ago you may find interesting. It involved a 2013 Toyota Camry with about 6,000 miles on it that had an electronically controlled compressor and 1999 Astro van. The Astro van had over 200,000 miles on it and had a noisy compressor. These tests were conducted about 20 minutes apart in a room that was 82°F and had about 20% humidity. We obtained the following pressures and vent temperatures:

Camry: High side: 150PSI.  Low side: 37 PSI.  Vent temperature: 46° F

Astro Van: High side: 185 PSI. Low side: 26 PSI.  Vent temperature: 39° F

It would be a good idea to check blend door operation and maybe even recover all the refrigerant from the system and charge it back up to specifications to make sure it has a proper charge in it if the customer is willing to pay for that service. However I don’t think you are likely to find anything wrong with that system.  It is likely going to come down to trying to educate that customer. 

Technician: Sounds like I may have my work cut out for me with this guy.  Thanks for the information.

Sometimes it seems like everyone expects 40° F from their A/C but due to heat loads (in the case of last week’s article) and system design that just is not possible.

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