Have an '06 Trailblazer LT w/auto climate contols. Had an issue with it not switching to vents at all, would only stay on floor/defrost. I replaced one of the actuators yesterday, there are 2 under the driver's side dash. Now when you turn the vehicle on it will blow out of the top vents finally, but after a couple minutes it will switch to the floor only by itself. It's not in auto, it's in the top vents only mode. If I switch to floor defrost that works and when I go back to top vents only it's just the floor again. Is that other actuator bad? Or the main control unit in the dash? Everytime you shut the car off and then back on, it will do the same thing over and over. Disconnected battery, pulled the fuses under the rear seat (that does nothing at all, the air still works and the display is on, not sure what those fuses are for if they don't do anything) tried the calibration multiple times to no avail. This happened overnight, no dead battery or change to the vehicle. Any ideas? I'm thinking the main unit is malfunctioning? Appreciate any ideas/help. Thanks
HVAC issue
- Thread starter jahnny1
- Start date
There are three actuators on the drivers side. I will assume you did not replace the drivers side temp control actuator as temperature was not your issue. The next most likely guess would be the mode actuator which is the one in the middle of the three. The defrost actuator is the last one and that is the one closest to the firewall. I always recommend replacing the mode and defrost at the same time since they are the easiest to access and they sort of work together to some extent to direct the airflow.
I am uncertain which specific outlets you refer to. Top vents = defrost? Or top vents = Panel? (Panel are the ones that blow at you)
New info, unplugged the battery for about 5 minutes, started the vehicle, the HVAC came on, is in auto with A/C on. I've not touched it and it's been almost 5 minutes and all the top vents are working properly somehow. I turned the A/C off and turned the temp down to 60° and it's still blowing out of the top (panel) vents only. I'm referring to the panel vents being the issue. Yes, the middle one is the one I replaced. The firewall side on I saw shuts off the floor and makes it defrost only I noticed. The middle one controls which vents get air. I didn't do the temp as that one wasn't the issue. It's been almost 10 minutes and it's still only going thru the top vents.
Mooseman
Moderator
For future reference, we have this thread with diagrams and part numbers of the actuators.
a/c control issues 2005 TB (Front actuators/diagram)
a/c control issues 2005 TB (Front actuators/diagram)
I read that but it doesn't really sound like my exact issue thou. It's similar to the control issue, but I've not seen anyone else who has had it start up with the panel vents working and 90 seconds later it automatically switches to the feet mode. I don't have temp control issues and I didn't disconnect any power or make any changes that would have caused this. Just happened overnight all of a sudden. I ordered another actuator to replace the one closest to the firewall and the main control module. I've made several trips around town today, not touched the controls and it's stayed in the panel vent mode and adjusted the fan speed accordingly. When I get home I will try manually selecting the vent position and see what happens after that.
I switched the airflow to all the different locations, waiting for several minutes between them and they all worked. This was while the fan speed was still on auto. I lowered the fan speed and had it on the panel vents for several minutes and still worked. I went thru all the same steps with the fan speed on manual control changing thru all the directions and all seems to be working normally. I turned the vehicle off leaving the controls on the panel vents and manual fan speed control. I turned the vehicle off, restarted it and ran it for several minutes and the panel vents remained operational the whole time. So I'm not sure what the reason is for it working now, but I'll 🤞🙏 it stays working properly. Thanks for the input.
This sort of jumps out at me and might explain something. There are two fuses under the rear seat that supply all power to the HVAC control module. The only way I can see your system continuing to work after pulling fuses is that maybe the wrong fuses were pulled. Unless the HVAC module has all power removed the recalibration will not happen.
When you disconnected the battery that made sure that power was removed from the HVAC module. I think that since there is some power to the HVAC module even without the key inserted or turned on, the recalibration may happen as soon as you reconnected the battery even before you turned the key on. (not certain of this)
If this is true then the system only recalibrated when the battery was reconnected and not when you pulled fuses.
I pulled #39 & 44 I believe, 10A & 30A fuses for HVAC/HVAC1 and the system still worked. I'm sure the recalibration must be happening as soon as the battery is reconnected as I didn't hear or see anything upon turning the key on. I disconnected the battery 3X throughout this process and somehow the last one, after letting it sit disconnected for over 5 minutes seemed to do the job. I was only leaving it disconnected for about a minute before. I've seen several people say 15-30 seconds and others 30 minutes so it's all over the place for things. This is a GM so I know it's full of electronic glitches, I have multiple GMs but this is my work beater so I just want it to work fairly properly. Triple checked and i did pull the right fuses so strange how it has no effect, what are they for then?
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I didn't even see the #36 one till now, weird how #44 is in there when it didn't come with manual control's. Makes more sense why the unit stayed on as HVAC B was still hot. What's still puzzling is that after the several battery disconnects it didn't work until I left it alone this last time. You'd have thought that fuse #39 would have killed the display on the main module or something? What would that fuse be for if it is removed yet nothing changes? Thanks for taking the time to go thru that and show this pics.
Fuse #39 only has power when the key is in the RUN position. So it likely is a signal to the module that it's time to get to work and might also supply some power to some circuits driven by the HVAC module.
#36 is always hot so that's what keeps the module alive.
Pretty much every module in these trucks still has power when the key is out and the truck is parked.
Must be for memory retention I would assume. I pulled those 2 fuses while the vehicle was running and didn't immediately see any changes and noticed everything was still powered up with them removed. It'd be interesting to know what stops working when they are removed thou.
It's a lot more than that. These modules are fully capable of communicating with each other as well as responding to scantool requests etc. If they were not then things like remote access with a keyfob would be impossible. I was not certain if the HVAC module acted like most others so I just went out to ask the truck. I knew the PCM goes completely dormant until key to Accessory but wasn't sure of others.
Here is who answered me without ever even opening a door.
0x81 Amplifier, 0x80 Radio, 0x1A TCCM, 0x40 BCM, 0x60 Cluster, 0x97 Onstar, 0x98 HVAC, 0xA0 Driver Door Module, 0xA1 Passenger Door Module, 0xA2 Liftgate Module, 0xA7 Rear Seat Audio.
All alive and ready to do as told.
That's nuts, especially for 15yrs old. I can understand why some of those (OnStar/door modules) would need to be active but rear seat audio takes the cake for that list. I feel much safer now knowing that the cluster can talk to the liftgate module while the vehicle is sleeping and vise versa with all the rest. Why does the HVAC need to be alive thou? Does the manual HVAC stay awake like the digital one I wonder?
Dr.Fiero
Member
A few more observations:
(1) The condition of an Older, Worn or End-Of-Life SUV Battery may be critical to the proper operation of ALL Modules, perhaps more so for those that are being kept "awake" post Key-Off situations.
(2) Check on the Voltage-Amperage Draw ALL of those "awake" sub-modules draw from the Battery.
(3) Your SUV runs a CAN-Bus version (High & Low Communications) of the GM Networks, so it is even more demanding on having available Battery Power. (See the attached Diagram).
(4) Your SUV would benefit from having a Refreshed Calibration performed on the PCM from either a Tech 2 or other Bi-Directional capable Scanner via an ACDelco TIS2WEB update. These may include updates versus any GM TSBs mentioning odd and sundry HVAC issues getting fixed along the way.
(5) You cannot overlook the possibility of having a Failing HVAC Automatic Control Module.
(1) The condition of an Older, Worn or End-Of-Life SUV Battery may be critical to the proper operation of ALL Modules, perhaps more so for those that are being kept "awake" post Key-Off situations.
(2) Check on the Voltage-Amperage Draw ALL of those "awake" sub-modules draw from the Battery.
(3) Your SUV runs a CAN-Bus version (High & Low Communications) of the GM Networks, so it is even more demanding on having available Battery Power. (See the attached Diagram).
(4) Your SUV would benefit from having a Refreshed Calibration performed on the PCM from either a Tech 2 or other Bi-Directional capable Scanner via an ACDelco TIS2WEB update. These may include updates versus any GM TSBs mentioning odd and sundry HVAC issues getting fixed along the way.
(5) You cannot overlook the possibility of having a Failing HVAC Automatic Control Module.
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Thanks for all that info. It has a 6 month old AGM battery so that should not be an issue, it worked amazing at -30°+ so I'm confident it's not that. It may have been a glitch and I have replacement parts coming if needed to swap out. I will look into that update as well if it starts acting up again. I greatly appreciate all the info/advice here, you guys are awesome!
His profile indicates a 2006 4.2 LL8. There should be no ISO 15765-4 Canbus there. Only the Class 2 serial data bus SAE J1850-VPW.