a/c problems

[edwin_jedi]

my ac turns on whenever I put the fan or heater on. Can you advice me or has anyone experience this. Thank you.

[michelkenny]

If you have the defrost on, the AC automatically turns on. It prevents fogging on the windows.

[rwanttaja]

my ac turns on whenever I put the fan or heater on. Can you advice me or has anyone experience this. Thank you.

It will definitely turn on if the heater control is in the Defrost or Defrost/Heat positions (turned all the way to the right or to the second position from the right). Using the AC dehumidifies the air and clears the fog from the windshield and side windows. You can manually turn off the AC, if you wish. However, the next time you start the car, the AC will be on again and you'll have to turn it off.

I've seen one or two reports of the AC turning on by itself with the heater switch in other positions. This weekend, I actually found my AC on with the switch in one of these other permissions. Not sure if I'd punched the AC button accidentally, or whether there is some sort of "Ghost AC" bug.

Why does Nissan set up the system so that the AC turns on with the Defrost setting? Lets look at the physics involved.

We're all familiar with the concept of humidity, the amount of water vapor in the air. The amount of water the air can carry is proportional to its temperature...warm air can carry more moisture, cold air less. Rather than indicate the amount of humidity by a weight or density value, we list it as "relative" humidity...a percentage of the maximum amount the air at the current temperature can carry. At 100% relative humidity, the air is carrying all the water it can. Try to introduce more water, and it remains as visible water (e.g., fog).

A good example is walking outside on a cool day. Your body expells moisture with every breath. The moisture remains evaporated in your body-temperature breath, but when the breath hits the cool outside air, it suddenly contains more moisture than the air can support...and you get bursts of fog as you exhale.

Imagine what happens in your Versa, then. Even when in the inside of the car is fairly warm, the glass can be pretty cool. After all, it's just a quarter-inch or so from the cold outside weather. As you breathe, the relative humidity of the air inside the car increases. When that moisture-carrying air passes by the cold glass, the air cools and suddenly can't support the same level of moisture. The water condenses out of the air, and gloms on to the closest physical item...the windshield. So your windshield fogs up.

Back in the 70s, GM fought this by making sure you couldn't turn off the car's ventillation system. There was no "off" position for the fan...it ran all the time. There were exhaust vents in the back of the cabin, so fresh air entered from in front and the moist air left in back. Didn't stop the windshield from fogging, but did delay it a lot. Especially compared to a Beetle with its legendary sealed interior and less-than-capable defoggers! Back in North Dakota, I used to have to install plastic screens on the inside of the windshield to try to minimize *frost* forming, much less fog.

Well, then...how do you get rid of or prevent windshield fogging?

There are two basic ways. First, you can ensure that the air that touches the glass doesn't contain enough moisture to condense. Second, you can warm the glass so that even when the glass cools it, it can still retain the water without condensation.

Most cars don't produce much warm air when first started, so the second approach usually doesn't work for a couple of minutes. The first approach is far more reliable. Hence, when you select one of the two Defrost positions on the Versa control, that's why the air conditioner turns on.

However, this does hurt fuel mileage. If you want to maximize economy, you might try the other defogging approach.

I had an opportunity this weekend, with a drive from Portland Oregon to Seattle on a cold, rainy morning. My windshield was fogging up nearly as soon as I started the car, and I hit the freeway with the AC doing the defogging.

After the car had warmed up, I turned the AC off and left the Heat Control in the defrost setting. After a minute or two, the windshield and the driver's side window (I was alone) were starting to fog.

I turned the heat control to full hot, cranked the fan to position 3, and cracked one of the rear windows open slightly to give an exit path for the moist air. Both the windshield and side window cleared up in a minute or two. I closed the rear window, and the windshield stayed clear. I backed the fan back to position 1, with no change. Finally, I backed the temperature back a bit, and all the windows stayed clear.

So...my recommendation is to let the Versa use the AC for defogging for the first couple of miles. Then try to keep the windows clear with heat and/or fan settings. Don't think you'll have to crack open a rear window, but keep it in mind if the windshield fog stays stubborn.

Ron
Modified by rwanttaja at 2:00 PM 11/27/2006

[marleyfan]

Ron,

I've been using the start with A/C on then use heat method as you suggested and find that it works fine. I turn the A/C off as soon as I can since my mileage is bad enough as it is.

[edwin_jedi]

My next question is, Nissan made it like this for the versa specifically? (standard?) or by mistake/glitch? I have friends and family members that have nissan's but they never encounter this. From the other forums it seems like that this problem never occurs to most versa owners. Thank you for all your replies.

[Ever Victorious]

This is normal for most modern cars. Every car I have owned manufactured since 1997 has had this.

[versabundus]

The reason you're aware of it is that the light comes on. On most cars the light doesn't come on in this mode even though the A/C is operating. The funny thing is that the manual states that the A/C will operate in the defrost mode but that the light will NOT operate. There does seem to be a glitch.

[proxim2020]

My Dodge Ram was designed the exact same way. The compressor will run when the defrost is turned on, but you will not get a light to let you know. The A/C control is disabled when defrost is on. Even though you can poke the button to turn the light on and off, that's all you're doing. The compressor runs on it's own schedule.

[brownbrix]

yea man i freak out just like you to but after i read the forums and the manuel then i was like feww though i had a bad v.