Car ac not cooling when weather is hot.

[JohnHere]

Ok JohnHere, I will do what you say, I think is a really good way to start with, and now it makes sense that the AC doesn't cool enogh, because in the AC Shop they only charge like 1 can of 12 oz.

Right...one 12-ounce can of refrigerant is roughly only half of what should be in there.

But now I have some question about what you say JohnHere: How do I know that the system has the correct amount of oil?, if it has an excesive amount, how do you get rid of that? and for curiosity, how can an ac system deliver cold air and being so down on refrigerant with only 12 oz?

It's probable that the new compressor came pre-charged with the recommended amount of oil, so the shop might not have put in any. Take a look at the label on the compressor to see what it says about the oil. If it says nothing, my guess (and it's just a guess) is that about 5 ounces is enough for a system like this.

There's no dipstick or gauge that tells us how much oil is currently in the system. So if the new compressor was "dry" from the factory, the oil would have had to be put in before assembling the system--either the entire amount into the compressor or distributed among the various components.

Unless there's a specific amount listed on the compressor label that was pre-installed, there's no way to gauge the amount of oil in the system. The only way to tell whether there's 5 ounces (or however much) in the system, you'd have to start from the beginning by emptying and flushing every component and then recharging with that amount of oil.

You might get some cooling for a while with a low charge of refrigerant. But eventually the evaporator will ice-up, blocking airflow. Insufficient refrigerant will also starve the compressor of oil and ruin it. So I wouldn't continue to run the system if you think it has only a partial charge. Are you sure that you have only one 12-ounce can of refrigerant in it?

And, do you think is neccesary to change the compressor oil and refrigerant once a year?, cause the people that installed the system told me that I have to do that service, or my compressor will not last.

The oil and refrigerant don't get "used up" in a sealed A/C system unless it has a leak or some kind of mechanical problem. So, no, you certainly don't need to change the oil and refrigerant every year.

And now I'm going to give you some readings. I was quickly testing my new AC gauges, without meeting the conditions for reading the pressure, just for learning how to do it. So the car were running for like 2 minutes before I connected the gauges.
Ambient temperature: 28°C
Relative humidity: 30%
I hope my digital thermometer is accurate.
One window rolled down and recirculation on.
At Idle:
Low: 28 psi
High: 198 psi
At 2000 rpm (I guess, my car doesn't have tachometer)
Low: 12
High: 203

At 28C or 84F, these pressures look pretty good. I ask again, are you sure that you have only one 12-ounce can of refrigerant in the system?

[JohnHere]

The compressor uses 6oz of PAG100 oil JohnHere, they drain out the oil that came out with the compressor, and added new oil.

Great. No need to worry about the amount of oil, then.

And I'm really sure that the system has only one can of refrigerant, I saw the hole process at the ac shop, and also I asked the man if he will only put one can, and he told me "yes, this is all that this system needs".

A 12-ounce charge seems inadequate to me. I'm not sure what's available in your country, but is it possible that the shop used a can that was larger than 12 ounces? The 12-ounce cans are common here in the USA, but 20-ounce small cans are also available.

I was using this chart to read the pressures, but I don't know if it is good, or if this readings are in idle or at 1800rpm.
But when I use this chart the low side always seems to be low and the high side is correct.
Do you think this reading are correct? Or there is another source for normal readings?

I'd guess that most pros don't use charts because they can be confusing, and they go by experience instead. For instance, looking at the chart you posted (for 85F ambient and 45-55 PSI on the LP side) would yield an evaporator temperature of about 50-58F, not nearly cold enough to provide a comfortable cabin and air temperatures at the center vent of 40-45F. You want an evaporator temperature of freezing or just above freezing, which would be about 28-30 PSI on the LP side, to achieve acceptable center-vent temps.

[JohnHere]

I did not read what the can label said, but I remember that was a small can like the ones I have of 12oz from chemours.
Do you think I should still evacuate the system and put 23oz to begin with?

It probably was a 12-ounce can. I wonder, though, whether the shop put in more than one can since you mentioned that it was cooling pretty well in a recent post, suggesting that it is correctly charged. And if it is cooling well now, I would leave it alone. If the refrigerant doesn't leak out, it should continue working since every component is new.

And talking about the chart, theres a source to learn how to read the pressures of a car ac system, or I should only focus in charging the correct amount for now?.

Look closely at your gauges. You'll see that the outer ring of numbers in PSI directly corresponds to an inner ring of numbers in degrees Fahrenheit, or sometimes, degrees Celsius. So, for instance, if you look at the LP gauge, a reading of 30 PSI would approximate the temperature of the evaporator, which is read on the inner ring--in this example, about 35F.

On the HP gauge, a rule of thumb that has proven to be fairly accurate says that the pressure should approximate a multiplier of 2.3 to 2.5 times the ambient temperature. Ambient temperature is the temperature actually measured directly in front of the vehicle's grille and not what the weather service reports. So for an ambient temperature of 85F, the pressure on the HP gauge should read about 196-213 PSI.

All of the above presumes that a given system is fully charged to specs with R-134a and contains the correct amount and type of oil.

But I think that now I have a bigger problem. I can't get my vacuum pump down to 30inhg, I only get lik 25 to 26. It's a chinese RS-1 1 stage pump of 3cfm 1/4hp. It says "ultimate vacuum 5Pa" but I don't know that that means. I have thighten all the hoses, but I cant pass 26, even with only the gauge connected to the pump. The AC gauge holded the vacuum all night long.
I was thinking of removing the intlet of the vacuum pump and like putting some teflon or something to help seal more, but I'm not sure if it will works.
It is normal that if you turn off the pump without closing the ac gauge you loose the vacuum to 15?

A vacuum pump should pull down to at least 29 inches of mercury (in/Hg) at sea level. Anything less won't achieve an effective evacuation. First, make sure that your gauges, especially the LP gauge, are zeroed. If your pump can't reach that level of performance, I would change the oil using one of the well-known professional brands. We don't know what type of oil was originally used, and it might be an inferior product. Use oil intended for vacuum pumps only...nothing else. If the pump still doesn't pull down to at least 29 in/Hg, then I would request a replacement. Don't use teflon tape on the inlet fitting because any loose shards of tape might be sucked in and clog something.

[tbirdtbird]

Agree about the vacuum level. 26 is not adequate, and can leave air and moisture behind, both enemies of good AC.
Once you get the vacuum pump issue fixed, vac down a good hour or more to remove the moisture which is still in the system at this time. Some vac 30 min, but this is not sufficient. I use a digital vacuum gauge and it takes at least an hour to get to 1000 microns, and ideal is 500 microns, which is hard to achieve. Simply put, the better the vacuum, the better the performance.

I think you said you are in central America so we may not be familiar with the brands of oil available there. Usually the cheapest is hardly ever the best. A sharp technician will change his pump oil quite frequently and believe me it makes a difference. Up here, Black Gold, Robinaire, and NuCalgon are good brands.

And forget those charts, we do not use them. They are misleading.

[JohnHere]

Also, can you name some good brand for oil? The pump come with an oil that say #100

I generally refrain from mentioning specific brands on our Forum. However, you may contact Tim, the site sponsor, who knows A/C products inside and out and can get you whatever you might need, and shipped quickly if it's in stock.

[JohnHere]

As discussed, if you can pull a decent vacuum on the system for a couple of hours, sure, go ahead and recharge it. The system will be completely empty of refrigerant after you evacuate it. As mentioned early on, start with 23 ounces and, if necessary, inch-up from there.

[swampy 6x6]

hi
1st vacuum pump change oil
test pump and check to see if it can reach 29 inHg
2nd check gauges are not leaking internally --replace sealing washers and tap o rings
---install new manifold seals
---install new hose rubbers

Vacuum system for 45 mins to 1 hr
Vacuum leak test 15mins

Charge with gas/liquid both high and low open , engine off ignition on , compressor will engage when pressure switch psi is reached
close taps
start car
charge low side only with only gas
-----------charge limitations -------------------
low side 20--40psi cycling up and down ,it will show TX VALVE WORKING
HIGH SIDE 180--250psi
ALWAYS ALWAYS check at idle and fast idle 1200-1500 engine rpm
sight glass in liquid line or dryer full very few bubbles that only appear when compressor first engages
vent temp cycling 5-10 deg cel
suction line from evaporator to compressor very very cold
t/x valve system measure sub cooling

Perfect low side 30psi Why to prevent freezing
Perfect High side is 150-180psi but commonly up to 250psi Why condenser size ,,BIG is always better

******* Is the heater tap on ,if in doubt clamp heater hose so its OFF
always check VENT TEMP on recirculation with only 1 or2 doors open , medium fan speed

The ambient temp vs PRESSURE CHART IS NOT a good reference . It was displayed earlier .

[swampy 6x6]

hi
The procedures on how to charge and Charging limitations guide has been mentioned