1994 nissan altima ac problems

when i use my ac, it is cold when i have the rpm's up but when i come to a red light or in idle...the air is not as cold as it used to be. can anyone help me with this problem. thx

Check the cable adjustment to the heater control valve. If the heater control valve has never been replaced, take it out and see if it will completely block the coolant. If it leaks even a tiny bit it is like the heater is on at the same time as the A/C. The A/C is capable of cooling the hot water when the RPM is higher but the air would not be as cool as it used to be, and the A/C is less efficient at idle. Check both fans begin turning as soon as the A/C is switched on.

You might have to have the R134a (freon) levels checked as it sounds like it might be just a bit low.If you can check it yourself with quality guages that would be great....(check both high and low side pressure along with noting the ambient temp...you can go to ackits.com for specific ranges)if not take it to a reputable a/c mechanic and have it checked. Hopefully all you might need is a recharge of the R134a. Just went through an evap change on my daughters 94 Altima which was easy enough and the whole thing set me back about $200 by doing the whole replacement myself. Hope this helps you out.

Here is why I don't jump to a conclusion that this system is low on freon. Consider these facts. Nothing is written about the A/C compressor cycling quickly. If the heater control valve lets a little hot water flow through the heater core, determining the ambient temperature and trying to match pressure ranges on a chart is impossible. Ambient temperature at the evaporator will vary according to coolant temperature, coolant flow will vary with engine RPM, and the ranges on a chart can not allow for a slow leak or a leak that has progressed. Charts show ranges that are effected by several conditions, both internal and external. You probably noticed that there is one fan that blows air across the heater core and the evaporator. A/C systems become low on freon because they have a leak. When A/C systems are a bit low, the A/C compressor will cycle more frequently. Compressors cycle because the low pressure switch disconnects the compressor clutch as the low side pressure decreases which is related to the temperature of the evaporator. As the engine and A/C compressor turn faster the low side pressure decreases compared to what it runs at idle. Adjusting freon volume according to readings on the manifold gauges is a guess, or a guide. There aren't any freon levels to check? The most accurate method of setting the correct weight of freon in a system is to add the amount listed on the label on the car to an evacuated system. This system may have the exact freon charge now. Over 30 years ago and having only a basic understanding of A/C, I overcharged plenty of A/C systems that had a leaking HCV. I saw some of the most confusing off the chart readings on the gauges and only caused those A/C systems to become less efficient. Since that time I have diagnosed and repaired more than... maybe a lot more than 1000 A/C and refrigeration systems. My experience doesn't guaranty that I am right about this A/C problem. Don't start out adding freon first as a diagnosis like so many people have, that have yet to learn how to perform the add freon service correctly. Does the A/C clutch click in and release every few seconds? Conditions that are consistant with a leaking HCV are A/C takes a lot longer to cool down and the air won't be as cold. The A/C compressor may not cycle at all or very seldom. Checking the cable adjustment is easy. The heater control valve and cable adjustment are near the middle of the firewall conecting the heater hoses.