2001 a/c intermittent highway freeze up problem

[reever]

My Maxima a/c problem is described below:.......what is the problem do you think? 2001 Nissan Maxima a/c was empty this spring so my mechanic filled it with a can or two of R134 and added some sort of stop leak stuff. HE measured the R134 fluid content so it was the right amount put in....... Now my a/c works in city driving all the time but after driving for about 20 minutes on the highway with a/c on at high..... or low , the smooth fan noise is interupted and makes a muffled missing /clicking like sound and the air flow reduces to 5% of normal or less. If I stop and open the hood the frost is caked on the tube very thickly. If I stop the car or turn off the a/c during the highway drive for 5 minutes , the a/c comes back on again when I hit the button, but returns to the above bad performance within 5 minutes or less. We have adjusted downward the amount of R134 fluid in the system several times , still within the recommended levels of the gauge, but with no improvements in operation . We have leanded out a few things in the system. Any ideas as to what the problem is? Thanks, Bob

[loystock]

IMO, stop leak stuff is just a patch and could aggravate situation long-term. Your mechanic should have fixed the leak first before doing anything. And the way it was done, looks like your mechanic is not a certified A/C technician.

The A/C system should have been evacuated first before refrigerant is added. And there was no mention of oil. A fully evacuated system needs at least 6.1 oz of A/C Oil Type-S and 1.4 pounds of R-134a refrigerant. Using a manifold gauge, he should have measure the low-pressure and high-pressure side of the compressor and a thermometer for the discharge temp at the center vent, if they are in spec. It's possible that he overcharged your system resulting in higher pressure at highway speed, causing the high pressure switch in the drier to open, disabling the compressor, off-an-on. It's also possible that your compressor is failing due to lack of oil.

[reever]

Thanks so much for your reply. Very helpful and quick.

Do you think any of the following ideas would be a cure also?

- Your ac symptoms seem to point to the "low pressure control switch". This switch controls the compressors on & off needs. As the temp of the cold coil drops, the coil can freeze if the compressor keeps running thus freezing the condensation, blocking the air flow. The switch controls the compressor by sensing pressure. As the temp drops the pressure drops. At a pre set pressure the switch cuts off the power to the compressor, as the pressure/temp rise the switch cuts on the compressor, on & off....... -Replace your thermostat located at the evaporator case. It is called a thermo sensor only a dealer item.

[loystock]

The problem you are having is probably due to improper servicing done to your AC system. Since you system has been empty for a while, that means the lines have been exposed to water vapor (ambient air) and contaminants. The dryer (aka liquid tank, receiver, etc.)) has a dessicant which extracts moisture from the system and is probably saturated with water. The fact that the your mechanic simply recharged your system without evacuating it means that water and contaminants are still present in your system. Your AC is showing symptoms of excessive refrigerant, water presence and or probable contamination.

Water freezes once the evaporator discharge temperature goes down, clogging the low-pressure line. Contaminants, if present, may cause erratic expansion valve operation, resulting in frost in the lines. The same thing happens when you have too much refrigerant in your system, Anytime the system has been exposed to ambient condition for extended period, replacement of the dryer is a must. The compressor in your car has variable displacement. The compressor stroke varies, depending on set temperature, ambient condition and other parameters. If the stroke is always at maximum, then you will have very cold output from the evaporator.

So the best thing you can do is to take your car to shop with certified AC technician. -Have your system evacuated properly and fix any leak if still present.-Replace the dryer-Inspect/clean/replace the expansion valve of possible contaminants.-Evacuate the system again for at least 30 minutes (the technician should know the actual time required)-Add the required amount of AC oil and refrigerant.-Test the AC system - doors closed, windows open, AC in MAX COOL, MODE at pax vent, and air intake at RECIRCULATE With ambient temp @ 86F and humidity at 50-70%, the discharge temperature at the AC central vent should be 56-68F, Low Pressure at 31-37 PSI and High Pressure at 149-185 PSI. Please note that the temperature and pressure will vary as a function of humidity and ambient air temperature, etc..

The actions above should fix your AC system. If not, there is a built-in AC diagnostic which you or your technician can use to further isolate the problem, The detailed procedure is given in the FSM (Factory Service Manual) which is available for download from this site. It's covered in HA (Heating and Airconditioning) chapter.

Had your system been serviced properly, you could have simply gone directly into the built-in diagnostic. But it is impractical to do diagnostic if you know that the system may have too much refrigerant, no AC oil, presence of water and/or contaminants.

[reever]

Thanks Loystick for your time, comments and interest. This was really and fullsome and thought out response. Much appreciated. I get the impression that this type of problem could cost me much as good and so so mechanics take it on..................trying different things. So getting good info before I seek the fix was important .

Your info is great and Nissan specific which is wonderful. As I mentioned, the a/c works great in the city but freezes up on the highway somehow until I turn it off for a few minutes at which time it works again for only a few minutes before the airflow stops again and the tube under the hood is caked with frost.

Thanks again.

Bob

[reever]

Sorry, my last reply should have said Loystock, not Loystick!