First let me acknowledge the cautions that go with this topic. Folks much smarter than me warn us not to just add R134, to use a professional. That notwithstanding:

Today it's in the 90's and humid. On days like this I can't help but wonder if the ac is at peak efficiency. Is there any info about what the conditioned air temp at the vent should be given a specific outside temperature?

Lots of variables I know [fan speed, recirculation on or off, humidity level]. We'll never be precise, but I wonder if there's any guidelines out there.

FWIW I only have access to a cheap low pressure guage. I'm looking for a way to see if I'm at the : "this is about the best you are going to get" point or not without disturbing the refrigerant.

Thanks for your thoughts.

John

 

The best automotive A/C system I have ever measured was producing temperatures around 46 to 48 degrees F at the center vents with the system set on maximum cool on afternoons in the high 80s to low 90s F here in central North Carolina. This was on my 1999 Dodge Ram when it was maybe 3 years old. It will be 20 years old this coming September, still has its original compressor, has never required a refrigerant recharge during my ownership (since late December 2000), and continues to produce very cold air today. I have never checked the temperatures on my wife's 2006 XK8 simply because it continues to be quite sufficient for her purposes....

 

Thanks Jon89. I'll measure it tomorrow and post the numbers.

John

 

My quick take is that a normal A/C system will leak a small fraction of its refrigerant every year. As long as you monitor the loss of cooling (i.e. no complete loss of refrigerant). you can "top-off" the system every few years with a cheap can/gauge from the parts store by bringing the fluid level into whatever zone on the gauge is considered filled. I usually go for the "normal" refrigerant, nothing with added promises of sealant, extra oil, high mileage, etc. as I figure only the refrigerant needs topped off. Professional services become required for anything beyond that.

Separately, the ventilation system itself can develop leaks and give you warmish air even if the A/C system is working just fine. I seem to have max cooling with recirc turned on (and locked, press and hold) and vents on Face. Air coming out is then _glacial_.

 

It's 101 here today, and I have the vents on my coupe shut nearly down just to keep from freezing my fingers off, so yes, these systems can do a fine job keeping your car cool.

With your simple gauge, test the low side pressure with the engine running and compare that to the temperature corrected pressure charts. Service as needed.

 

Automotive AC systems, as a rule of thumb, usually produce cold air that is 30-40 degrees F cooler that ambient. That means if the outside air temp is 90 your output at the vent will be around 60-70. Now at MAX AC the system closes off the outside supply and uses recirculated air and as the temps go down the temp at the vent goes down as well. When the temp of the evaporator nears 42-45 F some systems will start to restrict the flow of coolant to keep the matrix from freezing up. Other systems use a sensor that controls the temp by allowing warm/hot air in from the heater core through the "blend door". If your vehicle has a adjuster that is a hot/cold control, it probably uses the first type. If it has a settable temp control , it probably uses the latter. Air flow also plays a big part, as the more and faster the air flows the quicker it cools off the cabin. My XK8 is not a large cabin yet the AC is marginally OK. The air flow and speed just isn't enough on very warm days but it is comfortable. I also have a Mercury Grand Marquis and its AC set at 68 and MAX AC can make ice cubes at the vents! It has a very fast fan and plenty of air flow.

 

If you haven't done it already I recommend cleaning and resealing the ducts and flaps in the dash.

Mine were in pretty bad shape. I suspect it was due to prior owner always running a/c with top down.

 

The coldest I've met was a "learning experience": came to hot car, started engine, a/c on max. Shut doors and waited a while. Opened door, nice cold car

BUT A/C ICED UP SO COULD NOT TURN IT OFF!!

Had to turn engine off and wait for a/c to deice. Yes - a slider control for a/c. This was long ago. Can't recall the car.

 

One thing that is not so obvious, and I got this from my professional Air Con man, is that the air con fluid also contains a lubricant. If you let your air con pressure drop the parts do not get lubricated and we all know what happens when you do not lubricate moving parts.

 

Yes, typically PAG oil, and it's why at low pressure normally the a/c will not switch on (it's why there are both hi & lo pressure detecting switches/sensors).

What I think of as "proper" A/C recharging machines remove the gas (& some oil), weigh it, hold a vacuum to both check for leaks and boil off any moisture, then recharge with the correct weight of gas and add oil to the correct total.

 

Yesterday I put a thermometer in the center vent. Roof up, 78 degrees outside.

35mph, ac set to 66 degrees,temp showed 46 degrees from vent . Turned recirculation on, temp dropped to 42 degrees. Turned temp down to lo cool, watched temp go down to 41 degrees. With recirc on, temp slowly dropped until I had to stop at 40.3 degrees.

if I decide to read low pressure and add r134, I'll post updates.

John

 

I worked on Semi's / over the road trucks, which have large cabs, and pretty lousy construction compared to most cars. Lots of leaks and drafts. We could most usually achieve a 30 degree drop from the outside airtemp, on very Hot 100' degree F and high humidity days, this was about the best you would get. So we relied on that as a bench mark if the system pressure's were where they should be, and we got a 30 degree drop, all was good. Most would achieve about 35 without much trouble and with the system on Max / Recirculate, most would do close to 40 Degree drop. Which was considered excellent. So the numbers you listed are very good, and I'd say the system is functioning properly. I'd probably leave it alone. Cause if it ain't broke, don't fix it!