Auto Air Conditioning & Heater Repair: a/c problem, thermo sensor, nissan maxima

Question
QUESTION: My auto 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 fine 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 usual smooth fan starts to  make a muffled missing /clicking like sound and the air flow reduces to 5% of normalor 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 drivefor 5 kinutes , 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 .

Any ideas as to what the problem is?

Thanks,  Bob



ANSWER: HI Bob

Replace your thermostat located at the evaporator case.  It is called a thermo sensor only a dealer item.

---------- FOLLOW-UP ----------

QUESTION: What do you think about the following as other possible cures for my problem?

Thanks so much for your first reply.

(1)   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.......     


(2)  IMO, stop leak stop 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 and disabling the compressor, off-an-on. It's also possible that your compressor is failing due to lack of oil.  

Answer
If we are talking about the 01 Maxima.  This vehicle DOES NOT have a cycling (low pressure control switch).  This system has an expansion valve therefore it must have a temperature sensor called a thermostat.  The stop leak section is correct it MAY cause more problem in the future mainly compressor failures.  But. evacuating and even vacuum is no reason to add 6.1 oz of oil to system this Will damage your system faster.  From your first information the only thing that can cause that is your thermostat at the evaporator.