Cold Air Comes and Goes

The AC on my '95 Altima blows cold then hot; or hot then cold. Can’t figure out a pattern. Had the air mixer in the dash replaced but that didn’t fix the problem. Nothing showed up on an AC system diagnostic. Any ideas?

Do you live in a very humid area? AC “freeze up” can cause this problem. The AC will freeze and stop working until the heat in the engine bay defrosts the system. The AC will then start operating until it freezes up again and the whole cycle repeats. Try turning the AC temperature control to a little warmer setting. Most people leave the AC temperature control as low as possible which tends to make the problem worse.

If this is the problem, and that’s a big if… I don’t think that turning the temperature warmer is likely to help. The heater core is in the air circuit after the evaporator, not before. What would help greatly (again, only if this is the problem) is to set the air control to recycle instead of fresh. When it’s on the fresh setting, humidity from outside is constantly drawn in with the fresh air. When on the recycle setting, the system has much less humidity to deal with. The other thing that might help is to help the fan on a high setting.

Let me clarify my statement. You need to turn the “AC” to a warmer setting…not the heater. Leaving the AC on the coldest setting can cause the AC to freeze up especially in very humid conditions.

Missleman, clearly you don’t know how the AC works on this particular car (or most cars for that matter). The AC temperature control works by running the AC all the time, and then as you select a warmer setting, it starts to heat some of the air using the heater core. This is not an unusual way for this to work. Way back when, some after-market AC systems that had a temperature control would vary the on/off time of the compressor to reduce the temperature, but the factory system in this Altima does not work that way. Look it up.

Tardis…you clearly have an agenda here so spill the beans. I am not an AC expert but this method has always worked in the past for me. From what I understand from friends who are AC experts, this method works because it cycles the compressor more frequently. I doubt that you are an expert either because you want me to believe that the compressor never cycles. I don’t buy it. If you want to make an enemy just continue in your idiotic fashion. You must have paid for your “Top 250” rating because your remarks cast serious doubt on it being a legitimate title.

Some Honda’s cycle the AC compressor when you up the temperature control. Virtually no other cars cycle the compressor to vary temperature output. (Most do cycle the compressor if the pressure goes too low. Some will cycle it if the evap temperature goes too low.) Read up on it. Output temperature control is accomplished by varying the amount of heat that is added from the heater core. Most cars do this with an air-temperature blend door that varies how much air passes through the heater core. Others control the temperature with a water valve that varies how much hot water flows through the heater core. I would love to have you site a service manual saying that the 95 Altima controls temperature by cycling the AC compressor. You won’t be able to.
(My “agenda” is that I don’t like it when people give incorrect advise that comes from experience [or not] with cars that are not like the OP’s car.)

I can accept that. Like I said…I’m not an AC expert. I see incorrect advice here all the time but I don’t want to get into moderating the website. I think you should stick to advice and let the website flow with right and wrong answers as it has done for years. Your “opinion” is no more right or wrong than anyone elses. Let the majority of like-minded answers speak for themselves. Personal attacks have no place here.

I’m sorry if you see any of this as a personal attack or moderation. It wasn’t and it isn’t. I think that it’s perfectly reasonable when someone knows something for an actual fact to mention that or to correct someone who doesn’t know and has made an incorrect guess.
I would think that the OP’s would appreciate that. I make guesses too, but I state that they are guesses and I back off when someone else says they are wrong. However, when I am stating a known fact, I don’t usually back down from someone who is guessing.
I learn from being corrected. Don’t you?

Thanks for all the input so far. I do live in a humid area, and under normal circumstances, recirculating the air worked great – but now when the air turns hot, it is really hot! I will try moving the cold indicator down a bit just to see if that is a factor. I don’t think so, because before the air mixer was replaced I had to keep it below the coldest point because it would create a terrible banging sound in the dash. Don’t have that problem anymore but the temperature problem hasn’t changed. Also, I’ve had the car for 12 years (and lived in the same place) and it has only happened the last two summers. (Global warming, perhaps?!..but that’s another discussion!)

I had a similar problem with my Civic. The air conditioning would come and go, with no particular pattern. The shop had the hardest time diagnosing the problem. They left the car idling for more than an hour, but nothing but cold air came out. I told them to keep it, and to let the A/C technician drive it home for a few nights until the problem repeated itself. Eventually, they discovered it was a short in the wiring harness on the compressor, and a new compressor solved the problem. The fact that I had a motorcycle to get around on in the meantime really helped when I said, “I don’t care how long you have to keep it, I want this problem solved.”

AC “freeze up” can cause this problem.

missleman, if this was the problem, I believe there would be a pattern. When the car is on the move at highway speeds, the A/C would work fine. Then, immediately when he exits the highway and stops at a red light, it would freeze up. This kind of failure is usually the result of failure of the condenser fan.

Okay, so let’s try to understand the problem better.
Is it that (a) the cooling stops and the air starts coming out at or slightly higher than the outside air temperature?
Or (b) the air starts coming out heated (much higher than the outside temperature)?

Have you checked to see if the AC compressor clutch is engaged when this happens?

Every time I took the car to the mechanic, the same thing would happen - it would just blow cold air. The other day I took it in while it was running hot and asked the mechanic to come out and check it. By the time he got to it, the cold air was back. So frustrating. I appreciate your information regarding the wiring harness on the compressor. I’ll mention it next time I take it in.

It seems to blow hotter-than-outside air – sometimes hard to tell.
Good question about the AC compressor clutch – I’ll have to ask my mechanic about it.

If it’s blowing out heated air and your compressor is still engaged, then you might have a problem with the blend air door or the control electronics. Don’t count out the blend air door motor assembly just because it was replaced in the past and it isn’t making the hard-stop (clicking) noise this time.

The blend air door was replaced last week, so my hope is that issue is ruled out – but I hear what you are saying. Just talked with someone at another shop and they mentioned the electronics as well.