Digital HVAC Issue

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
522
The digital HVAC controls in my Envoy are acting weird.

When I set the heat to 75f, it blows out scorching hot air, not 75f air. But if I set it to 60f (the coldest it will go), then it blows out ice cold air. Any temperature in between 90f and 60f, it simply does not adjust to. Its very annoying because we drove 3 hours last night to the cabin and I had to keep the windows cracked in the back because the truck would not blow anything but 90f air. :mad: And toggling the system on/off was too distracting.

Is there a temperature sensor that went bad? Or is there a way to recalibrate the blend door to coordinate with the requested temperature?

Anybody else have this issue?
 

n0kfb

Member
Dec 8, 2011
104
Does your truck have any other problems?

I had a problem with my ambient temperature sensor showing 28 degrees when the temperature was actually in the high 60's. The cabin was getting warm in the sun and the A/C refused to come on when the system believed the outside temperature was so cool.

-- Dan Meyer :coffee:
 

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
522
n0kfb said:
Does your truck have any other problems?

I had a problem with my ambient temperature sensor showing 28 degrees when the temperature was actually in the high 60's. The cabin was getting warm in the sun and the A/C refused to come on when the system believed the outside temperature was so cool.

-- Dan Meyer :coffee:

Not at all. I replaced the ambient temp sensor about 2 years ago because the exterior temp was wayyyy off. But its spot on right now and has been since I replaced it.

Im not having a problem getting the cool or hot air. Im having a problem getting the air to adjust temperature; When I set my zone to 75f, it will continue to blow 90f air. If I turn my zone to 60f, it will blow freezing cold air, like it should (its 16f out right now). When I set it to 75f, I want 75f air to come out of the vent, not 90f. Makes it difficult when you are on a road trip cruising at about 75mph, with the fan on the lowest setting, set to the floor, and even though the HVAC is set to 75f, its pushing out 90f heat. Had to crack the rear windows an inch or two to keep things regulated.

I just noticed this too. Its always worked as requested, if I asked for 75f air, it would adjust and push out 75f air. Now it seems I only get hot or ice cold. :sadcry:
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Pulling HVAC-B fuse (rear #36) for 30 seconds should reset the system. Reinsert the fuse, then when you turn the ignition on, do NOT touch any controls for 90 seconds as it recalibrates.

If that fails to work, tell us whats happening on the passenger size zone when you use its control to take charge of just that side.

There are sensors in the ductwork that the manual systems don't have. Those could have problems, and can be interrogated by a Tech II tool to home in on the exact one that's bad. Otherwise you could be in for a shotgun guessing session.

You're not covering or accidentally pushed in the sunload sensor, the little dome in the center of the dash up front. That still wouldn't explain your symptoms.
 

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
522
the roadie said:
Pulling HVAC-B fuse (rear #36) for 30 seconds should reset the system. Reinsert the fuse, then when you turn the ignition on, do NOT touch any controls for 90 seconds as it recalibrates.

If that fails to work, tell us whats happening on the passenger size zone when you use its control to take charge of just that side.

There are sensors in the ductwork that the manual systems don't have. Those could have problems, and can be interrogated by a Tech II tool to home in on the exact one that's bad. Otherwise you could be in for a shotgun guessing session.

You're not covering or accidentally pushed in the sunload sensor, the little dome in the center of the dash up front. That still wouldn't explain your symptoms.

My twilight sensor is fine, unaltered.

Im afraid to pull the #36 fuse because the truck has gone 2 years now without a power interruption and I do not want any of my actuators stripping themselves out during a "relearn" after a fuse pull or battery disconnect. My truck is a 2002 and Im on all of the original actuators - Ive read what happens if they re-learn after such an age. :frown:

I do, however, have a friend that owns a TECH II - I could ask him to check it out I suppose.

And its the same behavior on either zone - passenger or driver. I tried it on AUTO mode today and it seemed to not be as hot. I just want 75f heat, is that too much to ask for?!?! lol. :biggrin:
 

The_Roadie

Lifetime VIP Donor
Member
Nov 19, 2011
9,957
Portland, OR
Weak actuators, IMHO, deserve to be discovered and weeded out when you have a convenient time to swap them. Not wait for your battery to die in the winter time, force a recalibration, then discover you don't have a defrost function when you really need it. But that's just me. :wink:

Kill the weak. They slow you down and you can't eat them if they're too skinny. Oh, wait ... that was the old Roadie.

OK, new Roadie talking. Coddle the weak systems in your vehicle, hoping upon hope that they won't let you down when you're driving through the Valley of the Shadow Of Death.:yes:
 

MichEnvoyGuy

Original poster
Member
Dec 3, 2011
522
the roadie said:
Weak actuators, IMHO, deserve to be discovered and weeded out when you have a convenient time to swap them. Not wait for your battery to die in the winter time, force a recalibration, then discover you don't have a defrost function when you really need it. But that's just me. :wink:

Kill the weak. They slow you down and you can't eat them if they're too skinny. Oh, wait ... that was the old Roadie.

OK, new Roadie talking. Coddle the weak systems in your vehicle, hoping upon hope that they won't let you down when you're driving through the Valley of the Shadow Of Death.:yes:


:rotfl: :rotfl:

Well, I'd rather not kill any actuators if I dont have to... especially because right now its 8f outside where Im at and Im nowhere close to a parts place (up in northern Michigan for the weekend). I can see actuators stripping out reallllllllly easy with those kinds of temperatures too :lipsrsealed:

I thought there was a button sequence you could press to have the head unit do a self check? Or is a fuse-pull or a battery disconnect the only way to get a recalibration?

Note: The last time my friend hooked up his TECH II to my truck, he did say something about some HVAC DTC's but asked me if I was having any problems with it and I wasnt (at the time). Ill have to ask him about that....
 

Denali n DOO

Member
May 22, 2012
5,596
I was just wondering if you had solved this issue? I too am having similar problems, since yesterday I believe. I have the digital dual automatic climate control. Yesterday I had the windshied replaced, probably not related but I saw Roadie mentioned that twilight dome thing on the dash. So when I got it back I noticed the external temp read 16 when in fact it was about 25-27 outside. I did the combined button push, mode+front def+rear def. It then started at 41 and kept counting down and stopped at 27. Perfect, so I thought. But now I get same as you did. I start at 15 and cool air comes out, I increase one digit at a time and the air temp stays the same all the way to 31, as soon as I hit 32 the heat comes blasting out. Not sure if I screwed it up by pressing those 3 buttons together. I wonder if I should pull that HVAC #36 fuse for a reset or may this cause more problems? let me know what your outcome was if you could. Anyone else have a suggestion?
 

BRomanJr

Member
Dec 9, 2011
371
Denali n DOO said:
I was just wondering if you had solved this issue? I too am having similar problems, since yesterday I believe. I have the digital dual automatic climate control. Yesterday I had the windshied replaced, probably not related but I saw Roadie mentioned that twilight dome thing on the dash. So when I got it back I noticed the external temp read 16 when in fact it was about 25-27 outside. I did the combined button push, mode+front def+rear def. It then started at 41 and kept counting down and stopped at 27. Perfect, so I thought. But now I get same as you did. I start at 15 and cool air comes out, I increase one digit at a time and the air temp stays the same all the way to 31, as soon as I hit 32 the heat comes blasting out. Not sure if I screwed it up by pressing those 3 buttons together. I wonder if I should pull that HVAC #36 fuse for a reset or may this cause more problems? let me know what your outcome was if you could. Anyone else have a suggestion?

The Auto HVAC system has a few more sensors as Roadie explained. It also prefers if you set and forget it, because it can respond somewhat slowly to control panel changes.
I have now and have had GM's Auto HVAC systems in many vehicles including my Envoy and the sensor most overlooked is the INSIDE Ambient sensor. This sensor draws interior air across a thermistor and informs the control panel. The control panel usually buffers this signal and combines it with outside air temp to decide what to do.

In the Trailblazer/Envoy/variants line, the interior sensor is to the left of the drivers head in the B pillar (with a small round grille) This sensor contains a tiny fan (not to be confused with the Blower fan) to draw the interior air across the thermistor for accuracy. A few things can cause inaccurate readings. Most likely is dust accumulation, also the tiny fan motor can slow and/or stop from "inhaling" dirt and dust.

When this tiny fan motor and/or the thermistor become dirty, it ends up giving readings closer to outside temps. (outside sheet metal is not far away). This usually causes too much Hot air on cold days and too much Cold air on hot days. I am not aware of a code for the failure of the tiny fan motor.

Occasionally I blow the dust and dirt out of the sensor while installed, but have had times on other vehicles (2000 Blazer LT) where I had to remove the sensor and clean it while checking for proper tiny fan operation.

FYI:
Sometimes I point my leftmost Driver dash vent up and over towards (not directly at it) the sensor to "help" it get an airflow.

In response to the Dash sensors, there is a Left and a Right Sunload sensor on the Automatic HVAC (one for each zone) and the Ambient Light sensor for the Auto Lights shares the same central Dash housing, but is not part of the HVAC system. Sunload sensors can go bad and should set a code.
 

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