Fazsoliz: If the A/C system is not properly charged, it will not cool properly. Usually it is noticed at low speeds because the compressor is turning slower. If you bring the RPM up, you would probably notice it would start to cool a little better.
HOWEVER, there is a danger if the system has been over charged and you do not have enough air flowing over the condenser to keep the head pressures down. The hotter it is outside and the more overcharged the system, the higher the pressures will be in the system. They can reach dangerously high pressures in excess of 300 psi. This can cause hoses to blow, a/c evaporators and condensers to fail and compressors to fail.
If the refrigerant is contaminated or if the system had been opened and had not been properly evacuated before charging, it could have moisture in it as well as air.
The fan which is driven by the engine is attached by a viscus coupling (designed to offer more resistance as it gets hotter, thus making the fan turn closer the the engine's rpm's). If this part is not working properly, it will not make the fan turn fast enough at slow speeds to draw the air over the a/c condenser and radiator.
I have found in very hot climates, that installing an electric fan in place of the engine driven fan made a vast improvement on the performance of the a/c at idle and low speeds. I have an old Mercedes diesel wagon which I converted from R-12 to 134-A.
The a/c would freeze you out on the highway, but you would be better off rolling the windows down in town. I removed the engine driven fan and installed an electric fan. Now the a/c will freeze you out at low speeds if you bring the rpm up to 1100, but is cold enough even at 700 rpm.
You must also check for any type of debris caught up in the a/c condenser. The more trash accumulated in the fins, the more difficulty it will have in dissipating heat. The fan schroud also must be on the radiator, otherwise the fan will have little effect in pulling
air over the condenser.
Also, put the system on recirculate after a minute or so. This way you will be cooling the air in the cabin and slowly bringing the temperature if the cabin air down and not having to fight the need to constantly cool hot air coming in from the intake vent.
I hope that what I have offeref for suggestions will help you.
1. Check your in cabin air filters first. Replace them once a year around Thanksgiving for best AC performance.
2. Check and replace your fan clutch on the factory mechanical fan. If bad (the clutch wobbles on the shaft slightly when torqued back and forth), replace it with Hayden clutch fan--this model is cheaper than the Nissan part and works much better.
3. Delta Current Control makes the best e-fan solutions available in the world as of 2009. Their model 3516-A is a variable speed drop in replacement for factory mechanical fans on engines up to about 800 hp.
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