A/C system not working even after recharge!

[jdeville]

yesterday i went and bought some refrigerant for my car’s A/C system. today i put it in and i followed all the instructions correctly. but when i got in the car to check if it was working, i was still getting warm air (its about 90 degrees out today). i know i do not have a leak in the system, because at first when i checked the pressure, it was way over charged. almost at 150 psi on the gauge the can came with. i took my car to my uncles shop and had him empty the system so i could recharge without creating a leak. charged it today, and still nothing.

any ideas what could be causing this? would really love to have A/C in this weather!

Topic

[EricTheCarGuy]

You first need to find out if the AC system can work at all, there could be some other issue like a faulty compressor, an electrical problem, or contamination in the system that’s not allowing it to work, you really can’t just put refrigerant in and expect it to work you need to make sure the entire system can work by checking it over first. You also need to put a vacuum on the system before charging to verify that there aren’t any leaks and remove the moisture.
http://www.youtube.com/watch?v=lglPJuBXVeE

[jdeville]

there was quite a but of moisture in the system from what my uncle said. i thought maybe he vacuumed it out, but maybe he didnt. could that be just from the system not operating for a long period of time, or a bad part in the system?

thanks for the help thus far 😀

[college man]

If the system was over charged. The high pressure switch
may have opened and not reset. also compressor damage
may have occurred from the over charge.

[jdeville]

how would i go about resetting the high pressure switch??

[college man]

look for a little button on it. You would need to push the button.
if there is no button. then either it did not reset or its stuck open.
I would check it with an ohm meter.

[jdeville]

any idea where the switch is located on a 1998 honda civic??

[college man]

whats the year make model of the car?

[Rick]

I’m about to write a “book”, but sometimes the answer to a question is, “Well, let’s start at the beginning…”

You really have to consider that the A/C is a couple different “systems”. You have a system that circulates refrigerant and a system that controls it. There is also variation from year to year and manufacturer to manufacturer on the basic system types and specific controls. The old school system would have a high pressure cut-off and a low pressure cut-off switch as well as an evaporator temp sensor. These were wired in series with the A/C select switch on the dash (or an A/C relay controlled by the dash switch) and the compressor clutch. So if the pressure reached a high cut-off point (usually because of lack of cooling load), the compressor would cycle off and then on again when the pressure got lower. Most systems also have a relief valve on the back of the compressor which will blow if pressures get extreme. The low pressure cut-off served to save the compressor if the refrigerant leaked out since the compressor depends on lubrication circulating with the refrigerant. To diagnose this type of control system, you would just trace out the circuit and follow it until you find the problem (like a failed pressure sensor either stuck on or stuck off).

Now most of what I’ve had to deal with is controlled by the ECU, so that pressing the button on the dash just gives the computer your “A/C request”. The computer logic then determines when to run the A/C compressor based sometimes on engine load, but also on the evaporator temperature and a high-side pressure transducer (and even based on cabin temperature sensors in an automatic temperature control system). The pressure transducer is not an on-off switch, but a sensor that gives a voltage signal to the computer depending on the pressure. This allows the computer to sense and react to the actual pressure and evaporator temperature as it determines whether to activate the compressor. As you may imagine, diagnosing that type of system best starts with a scan tool that can report real-time PIDs for the A/C request switch status, high-side pressure, evap temperature and clutch on/off control status. You then compare what you are seeing to what is happening with the system to see if anything is wrong and then pinpoint the problem using your voltmeter, etc. Diagnosing this type of system is a little more complicated without the scan tool, but as was suggested above, for example, you can check the pressure sensor reading to see if it changes as you read pressure changes on your gage or if it’s just open or a perfect short. It would be even better if you can find factory information on what readings to expect.

So that’s the control side of things. On the refrigerant circulation side of things, the basic idea is that you have a refrigerant (r-134a) that is made to change from gas to liquid in order to remove heat from the cabin and release it outside the car. These systems are generally described as having two sides, the high side and the low side which are named for the amount of pressure you’d expect on each side of the system while in operation. Starting with the low side, the refrigerant is at a low pressure and is in the form of a gas. From there it is pumped into the compressor where it is compressed into a high temperature, high pressure gas (and the high side of the system begins here). This gas flows into the condenser, which is essentially a radiator in front of your car’s regular radiator, where heat is transferred from the gas to the atmosphere causing the high pressure, high temperature gas to cool (though it would still be hot to the touch) and change into a liquid form or “condense”. So the line that comes out of the condenser is usually referred to as the “liquid” line.

From here, there are two types of systems categorized according to their “metering” system.

The first type is an orifice tube system. In this system the liquid will flow through a device called an “orifice tube” which is basically a plastic piece with a filter screen and a tiny hole that sprays (atomizes) the liquid into the evaporator (which is a small radiator inside the cabin). The evaporator has low pressure internally and is where the low side of the system begins. Cabin air is passed over the evaporator transferring heat to the refrigerant cooling the air and causing the refrigerant to “evaporate” changing it into a gas. If you remember from basic physics, when a material changes from a liquid to a gas it must absorb heat to do so, hence the cooling effect. Now since the orifice is a fixed size, it does not change the amount of refrigerant metered into the evaporator depending on cooling needs. As a result, if too much refrigerant is sprayed into the evaporator because of low cooling demand, some of that refrigerant will not evaporate, but will exit the evaporator as a cold liquid along with the gas. As you may be aware, also from basic physics, a liquid does not compress. If you were to allow this excess liquid to flow into the compressor, the compressor would be destroyed as it tried to compress it. For this reason, the next part in the system is a little “can” known as the “receiver/drier”. The primary function of this part is to “receive” the excess refrigerant which falls to the bottom of the can and collects there allowing only gas to travel to the compressor. As the name implies, the can also contains a bag of desiccant to absorb any moisture from the refrigerant. At this point, the cycle starts over again.

The second type of system uses a thermal expansion valve (TXV) rather than an orifice to meter the liquid refrigerant into the evaporator. The TXV will have a “sensor” that mechanically measures the temperature of the evaporator and then mechanically changes the size of the orifice so that it sprays the proper amount of refrigerant into the evaporator for the cooling demand. As a result of this, no liquid will flow out of the evaporator so a receiver/drier is not required. What happens to the excess liquid you may ask? In this type of system, the liquid flows from the condenser into a small “can” called the “accumulator/drier” where it “accumulates” until needed by the TXV.

That should give you some basic background to be able to understand how the system should work and hopefully figure out why it is not working– either because the control system is not turning it on, or because of a problem with the refrigerant flow (clogged orifice, faulty TXV, etc.)

So, lets get onto at least one basic point about A/C pressure and charge. The little recharge kits with a guage on them and a “green” zone are extremely misleading in that they suggest that filling an A/C system is like filling a tire. You cannot judge the amount of charge in an A/C system this way because there is a fundamental difference between an A/C system and a tire (never thought I’d have to say that!). A tire is completely filled with a gas (air or nitrogen), an A/C system is filled with a material in both a liquid and gas state. Because a tire is filled only with a gas, it is possible for all practical purposes to measure the amount of air in the tire based solely on pressure (though it is good to consider temperature, it is usually not critical). Because an A/C system has liquid as well as gas in it, you have to consider the “vapor pressure” within the system. Vapor pressure is the pressure at which the liquid will evaporate at any given temperature– the vapor pressure of r-134a happens to be very close to the Farenheit temperature of the r-134a. So if you have a closed system (turned off) with a proper refrigerant charge of 21oz with 16oz of liquid refrigerant and 5oz of gaseous refrigerant at 70F, then the pressure would be about 70psi. This relationship stays the same if there is any liquid refrigerant at all in the system. So the same system, if it had 1oz of liquid in it and 3oz of gas even though it is completely under-charged would read at a pressure of about 70psi. Likewise, if the system was seriously overcharged with 20oz of liquid and 8oz of gas, the pressure would read 70psi. Basically, in any of these examples, if the pressure increased by adding refrigerant, more of the refrigerant would become liquid equalizing the pressure and if the pressure decreased by removing refrigerant, more liquid would evaporate equalizing the pressure at the “vapor pressure” for that temperature. Pressure can only be used to determine charge by using a method that takes temperature into account with the system in operation. Static pressure tells you nothing (unless it is well below ambient temperature, indicating an empty system.)

Why should you care? An over-filled or underfilled system will be inefficient. An extremely over-filled system will allow liquid to overflow from the receiver/drier and flow into the compressor causing it to be destroyed.

So, you ask, how do you properly determine whether a system is overcharged or undercharged? The best way is a method called “superheat”, but we don’t usually do that calculation in the automotive world. As a practical matter, the best way to do it is to start with an empty system (at vacuum) and fill it with the specified weight of refrigerant. Here it is important to remember that the A/C system does not “use up” refrigerant. If the system is undercharged, it means that there is a leak in it. If the system is overcharged, it means that someone who didn’t know what they were doing over-charged it in an attempt to fix it (ie by following the instructions on those stupid cans). As a practical matter, I should note that for most systems an oz or two over or under will not make much of a difference.

Chrysler has a nice thing in their factory service manual– they take the work out of the “superheat” method by including a chart that shows the proper relationship between liquid line temperature and high-side pressure. So you attach a temperature probe to the liquid line and compare it to the high-side pressure and judge the charge by whether or not these fall on, above or below the curve.

[Author]

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