Auto not working for climate control

jonesumd · December 9, 2017

Hi folks,

I have another issue with my truck. Since day one of owning my truck, I leave my climate control to 70 with the Auto button on. This morning while driving to work on a colder day (around 30 degrees), I noticed very little warm air coming out of the vents. If it push one of the vent position buttons on, warm air comes out fine (for the specified vents). If I push the Auto button again, it goes back to barely any air coming out. While on Auto, if I crank the heat to Hi, I can hear the air kick in more in the front window defroster area but it's as if the air is circulating within the dash, not much coming out the defroster vents. The other vents still have almost no warm air coming out at Hi.

What could be the issue?

I really don't want to bring the truck down to the dealer again. I only have 3,646 miles on it and it's been to the dealer 5 times already for faulty AC components.

Bought new in April 2016. I've brought it down to the dealer so far for the following:

2016-09 @1,412 miles : AC stopped working - claim they couldn't find leak, re-add dye & refrigerant (over 50% low)
2016-10 @1,692 miles : AC stopped working - replaced AC expansion value
2017-04 @2,089 miles : AC stopped working - replaced AC condenser with another gen1 condenser (gen2 wasn't available yet)
2017-06 @2,525 miles : AC stopped working - replaced AC expansion value again & seals
2017-08 @2,826 miles : AC stopped working - replaced AC evaporator core

m3n00b · December 9, 2017

Man that sucks it's been to the dealer so much. You can change how high the fan blows on auto in the menus

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carnau · December 9, 2017

Ok so they should have never put the Auto function on a vehicle. I am going to try to explain it and give it to you from the book as well.

In automatic operation, the HVAC control module maintains the comfort level inside of the vehicle by controlling the A/C compressor clutch, the blower motor, the air temperature actuators, mode actuator and recirculation actuator.

To put the HVAC system in automatic mode, the following is required:

The auto switch must be activated.
The air temperature switch must not be in either the full hot or full cold position.

Once the desired temperature is reached, the blower motor, mode, recirculation and temperature actuators automatically adjust to maintain the temperature selected. The HVAC control module performs the following functions to maintain the desired air temperature:

Monitors the following sensors:
- Ambient air temperature sensor
- Lower left duct air temperature sensor
- Lower right duct air temperature sensor
- Upper left duct air temperature sensor
- Upper right duct air temperature sensor
- Ambient light/sunload sensor
Regulate the blower motor speed
Position the air temperature actuators
Position the mode door actuator
Position the recirculation actuator
Request A/C operation
Control of the A/C compressor

When the warmest position is selected in automatic operation the blower speed will increase gradually until the vehicle reaches normal operating temperature. When normal operating temperature is reached the blower stays on high speed and the air temperature actuators stays in the full heat position.

When the coldest position is selected in automatic operation the blower stays on high and the air temperature actuators stay in full cold position. The mode actuator remains in the panel position and the recirculation actuator will remain in the recirculation position.

Under cold ambient temperatures, the automatic HVAC system provides heat in the most efficient manner. The operator can select an extreme temperature setting but the system will not warm the vehicle any faster. Under warm ambient temperatures, the automatic HVAC system also provides air conditioning in the most efficient manner. Selecting an extreme cool temperature will not cool the vehicle any faster.

Now that is the book answer. Here it is in Layman's terms.

If you set the temperature at 70 degrees, it is going to monitor the duct temperature sensor and it is going to blow air out of the dash at 70 degrees (or so it thinks) but from the upper duct sensor you still have another 6 to 10 inches of travel before the air comes out the deflectors. So by the time the air gets to your hand outside of the deflectors it has already dropped UNLESS the vehicle has been running so long that the complete inside of the vehicle is 70 degrees. Which means the plastic duct work also has to hit 70 degrees to keep the moving air from cooling down.

Another thing it monitors is the cab temperature. It doesn't care how long it takes the cab to get to temperate. It just knows that if the temperate at the upper duct is 70 degrees it is eventually get the inside of the cab to 70 degrees. Which is true... tomorrow.

If you want to check and see if your vehicle is operating correctly take an automotive thermometer and stick it in the middle vent and see if the air coming out of the vent is within a few degrees of what you have it set on in the dash.

If you set it to automatic, eventually it will automatically get to that temperature. But it won't be as fast as you want it to be.

Also moving air cools faster in the winter and warms faster in the summer depending on ambient temperature. So the blower is going to try to automatically adjust to what it believes is the best setting.

Basically what I am saying is it isn't going to blow out 75 degree air at 70 degrees if the cabin temperature is -30 degrees... It's gonna blow 70 degree air at the upper temperature duct and 60 degrees at the deflector because of how much the air cooled down between the upper air duct and the deflector.

alrockaz · December 9, 2017

I think you can use carnau's post to see if your system is working correctly. Auto does take away control and might not operate the way you want but I haven't had any issues. My point? Your system on Auto may or may not be working properly.

The only thing I disagree with in that post is that the truck does get to the desired temp as fast as I want it. Here in Phoenix, when it's 110 out it blows cold air at full speed for quite a while to cool down the truck before it slows down. It doesn't get too cold here, but on the past few mornings it's been blowing hard to heat up the truck with the temps in the upper 40s to low 50s, and I'm also set at 70.

I realize Auto may not be everyone's cup of tea, or it may not perform to everyone's expectations, but it works well for me. I suspect yours may not be working properly especially given the issues that you've had.

tbarn · December 9, 2017

With all due respect to Carnau, he is wrong. GM auto climate control systems will blow as hot or cold air that is available depending on engine temp and ac system function. As an example. Temp is set to 70 degrees and it's 10 degrees outside. Truck cold started. As engine temp approaches full temp, the hvac system set on auto will blow full hot until the cabin temp approaches 70 degrees. Then it will back down the temp door and fan to attempt to attain and hold 70 degrees. As it is driven, it will raise and lower the output temp and fan if it has to to maintain a 70 degree cabin temp although they should be subtle changes. I love my auto climate control. Some times I disagree with it in which case I just kick it up or down a couple of degrees. The only time it's not in auto is for defrost.

Largely Unknown · December 9, 2017

Thanks, very helpful to all.

I love the auto system, but at the same time have been trying to understand how best to use it.

I have one question about the settings menu, where you select the fan speed...does that mean the speed to which is settles down to, after the truck's interior reaches the desired temperature? It seems to run continuously at that selected speed for the rest of the drive. The "high" setting gets a bit noisy for conversation, so I backed it down to "medium". I'm hoping that when it's hot or cold, the fan will automatically adjust up or down as needed.

As a minor side issue, I also have the auto defrost setting unchecked (off) in the settings menu, yet I noticed this week it's coming on (rear defrost switch lighted) when its cool outside.

Mdsjohna · December 9, 2017

When it’s on auto and is calling for warm you are going to get air from the floor vents mostly. A small amount will come out of the defroster vents and a bit out of the outermost dash vents. Are you expecting lots of flow out of the dash or defrost vents? If so it isn’t going to happen unless you actually select those.

When it’s calling for cool is when the dash vents should blow.

I believe the fan speed setting is just a max setting. So if you set it to low it will never blow harder then low when set to auto. I leave that set at max. On second thought I think you might have different fan settings then I have in my 14.

I like the auto but my last truck actually showed you what vent position and fan speed it was at.

The following screen shot isn’t a silverado but I think you can expect it to be kind of similar.

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Donstar · December 9, 2017

14 hours ago, jonesumd said:

I really don't want to bring the truck down to the dealer again. I only have 3,646 miles on it and it's been to the dealer 5 times already for faulty AC components.

It appears that you are getting some good information. The point I would like to add is don't give up going to your dealer if you are still having problems. Service departments seemingly adhere to prescribed steps to remedy a problem. I suspect they have further steps they can take to make yours operate as intended!

newdude · December 9, 2017

I use the Auto in my Colorado. I think where your issue is lies in the temperature in which you are requesting. AUTO can and will set the system best to heat/cool the entire cabin. If you want warmer air, try starting the temperature at 80 or 84. The system will pump warm/hot air in to get the cabin to 80 degrees (or whatever you set it to) and then regulate fan speed to maintain that temperature. At 70, its going to start "blending" cooler air to drop the temps inside the cabin to 70.

I guess what I'm trying to get to is that the AUTO system almost works like your house. You set your temp, and the system does its thing to get to and maintain that temp. Instead of 70, try 72-76 range.

Grumpy Bear · December 9, 2017

My climate control works like a dream in both my truck and my Buick. My wife's Buick and her Terrain are never to her satisfaction but work perfectly when I drive them. The difference is patients of the person and reasonable expectations.

If it's 30 degrees outside and the truck has sat overnight the water/oil are also at 30 degrees. Set your thermostat where you wish. It will not turn on until there is enough heat in the system to actually accomplish it set goal. Cranking the fan up faster actually delays the time it takes to reach temperature. Turning up the thermostat does zip when there is no fire in the furnace.

Some cars like Honda Civics have such large cooling systems, in a relative sense, it could idle in the driveway until the tank ran empty and never get hot enough to open the thermostat on a cold day. My V6 truck is much the same. Load creates heat. Just get it moving and in a few miles BINGO! I have to say heated seats make this more bearable.

carnau · December 9, 2017

16 hours ago, tbarn said:

With all due respect to Carnau, he is wrong. GM auto climate control systems will blow as hot or cold air that is available depending on engine temp and ac system function. As an example. Temp is set to 70 degrees and it's 10 degrees outside. Truck cold started. As engine temp approaches full temp, the hvac system set on auto will blow full hot until the cabin temp approaches 70 degrees. Then it will back down the temp door and fan to attempt to attain and hold 70 degrees. As it is driven, it will raise and lower the output temp and fan if it has to to maintain a 70 degree cabin temp although they should be subtle changes. I love my auto climate control. Some times I disagree with it in which case I just kick it up or down a couple of degrees. The only time it's not in auto is for defrost.

I am not above being wrong trust me. But unless something has changed this is the same way the systems have worked since I started working on Auto AC units back in the early 90's. (I actually worked on an automatic ac unit in a 1973 Cadillac Elderado with less that 5000 original miles on it the other day which was awesome. I will post a video of it.) It will raise and lower the fan speeds based on inside air temperature and duct temperature and the only monitored blend door temperature sensor is the upper duct temperature sensor.

Regardless the Automatic system is a terrible design because what people think should be automatic isn't the way automatic actually works.

Again, it's just my opinion about how automatic is terrible, But facts on how the system actually works unless the new system has changed. I am going to do more research on it though to make sure I am not giving you faulty information.