What would cause A/C temp to fluctuate?

**Hoofhearted** · 05-21-2012 03:44 PM

So its 106 degrees today and my '11 has always done this. I'll be going 70 with cruise control set on flat ground with the air on recirculate and fan set to 5 of 7 speed and the AC will randomly blow warm air for 1-3 secs. Is there something wrong with it or am I expecting to much of the AC system? I've rented compact cars in the past that never did this.

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**EJ257** · 05-21-2012 03:56 PM

Where the hell do you live that it's 106°?!

**Hoofhearted** · 05-21-2012 03:58 PM

Originally Posted by EJ257

Where the hell do you live that it's 106°?!

Las Vegas, NV

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**rage-wrx** · 05-21-2012 04:12 PM

Originally Posted by Hoofhearted

Las Vegas, NV

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Move,lol.

I think mine does the same.It does when I use the heater.

I haven't used the AC yet this year.So I don't remember if its the same with the AC.

Must be the Climate control trying to maintain the set temperature.

**Hoofhearted** · 05-21-2012 04:27 PM

Originally Posted by rage-wrx

Must be the Climate control trying to maintain the set temperature.

I have it set to what ever the lowest temp is so I don't think that is the case.

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**SD_GR** · 05-21-2012 04:31 PM

It's normal if your pressures are correct and the thermostat is working I suppose. If you are concerned, have someone check those things specifically.

**DemonWRX** · 05-21-2012 07:06 PM

Mine does the same...both hot and cold

I would be willing to bet that it is the system momentarily running uncooled/unheated air through the system to check what temp the cabin air is at.

I suspect this as an unexpected side effect of the economy temp specific thermostat. If it were old-school, you would not have temp numbers on the dial, and would have to manually modulate the temp/fan speed. With the system having designated numbers (65/75/85), and auto fan speeds, it has to have some way of accurately reading cabin air temp. Unfortunately, a standard passive thermostat like your house has (one lead somewhere central with either a coil or a thermocouple) is not sufficient in a car as in most locations it would read radiant temp readings from the surrounding metal and interior parts. So you will need some sort of fan to force air past the thermostat that is isolated from metal parts or interior trim parts that are directly exposed to the sun. I know, in a perfect world, it would have a separate fan system that sucked sample temp air from the cabin and shot it outside silently, but that would add weight, so no thanks.

**Anub1s** · 05-21-2012 07:08 PM

This thread makes me glad I have the manual controls. Never have a hiccup with AC or heat delivery.

**Hoofhearted** · 05-21-2012 08:05 PM

Originally Posted by DemonWRX

Mine does the same...both hot and cold

I would be willing to bet that it is the system momentarily running uncooled/unheated air through the system to check what temp the cabin air is at.

I suspect this as an unexpected side effect of the economy temp specific thermostat. If it were old-school, you would not have temp numbers on the dial, and would have to manually modulate the temp/fan speed. With the system having designated numbers (65/75/85), and auto fan speeds, it has to have some way of accurately reading cabin air temp. Unfortunately, a standard passive thermostat like your house has (one lead somewhere central with either a coil or a thermocouple) is not sufficient in a car as in most locations it would read radiant temp readings from the surrounding metal and interior parts. So you will need some sort of fan to force air past the thermostat that is isolated from metal parts or interior trim parts that are directly exposed to the sun. I know, in a perfect world, it would have a separate fan system that sucked sample temp air from the cabin and shot it outside silently, but that would add weight, so no thanks.

That makes sense. I wish there was a way to dissable it like pulling a fuse or disconnecting a wire from the sensor. I never have it set to anything other then what ever the max cold is.

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**Hoofhearted** · 05-22-2012 03:59 PM

So its been happening so much that I feel like there is something wrong. So I made a appointment amd I feel like they are just going to give me the run around. Over the phone he said the system does cycle on and off while I'm driving but I don't understand why it would cycle off if its still hot in the cabin and really hot outside. I don't know how AC systems work so what I said might not make any sense to someone who knows about this kind of stuff.

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**DemonWRX** · 05-22-2012 04:31 PM

Did some digging, and I have two possible causes.....

One, if your cabin filter is real dirty it could be restricting the air flow, causing the AC to disengage so the evap does not freeze solid.

Two, take it to the dealer and have the freon level checked. If the freon is low, the pressure switch could be causing it to disengage/re-engage sporatically

**Hoofhearted** · 05-22-2012 05:08 PM

Thanks for checking on that. The car has 9k miles so maybe the cabin filter is dirty but I doubt it, ill check it. The dealer said they will check the levels and fill them up of they are low. Is this something that should be covered under warranty or should I expect to pay for it? Car was bought in Oct '11

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**DemonWRX** · 05-22-2012 05:12 PM

AC freon level should be under warranty I would think...it has not even been a year. Not to mention if it is low, I would have them check for leaks. The cabin filter will not be a warranty item.

For the filter, strongly recommend doing it your self....my dealer charges like $89 to do that (man, what some people are willing to fall for)