HVAC Air Inlet Door / Actuator Modes - Chevy Cobalt Forum / Cobalt Reviews / Cobalt SS / Cobalt Parts
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post #1 of 3 (permalink) Old 08-06-2018, 10:02 PM Thread Starter
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HVAC Air Inlet Door / Actuator Modes

I recently looked into the well known issue of the HVAC air inlet door and actuator which is located behind the passenger cabin air filter on Cobalts/G5s. I was wondering if anyone in the know can confirm - regardless of the setting of the HVAC recirculation mode being on or off when the ignition is ON, does the air inlet door ALWAYS CLOSE when the ignition is shut off, i.e. in engine off or accessory or completely off key position? I wanted to understand whether this door is opened and closed each time I turn off the engine and/or ignition completely. If it does always close, by which point does the air inlet door close (e.g. as soon as engine is off, or accessory position, or ignition completely turn off)?

Since I rarely if ever use HVAC recirculation mode, it would mean that the air inlet door would always be in the open position while the vehicle is in operation, but if the system logic is such that the air inlet door actuator always CLOSES the air inlet door when the ignition key is not in the engine run position, that would mean that the actuator is constantly being operated frequently.

Also, is it true that there are three HVAC actuator modules on these vehicles? I understand there's supposed to be 2 actuators for HVAC mode control and HVAC temperature control (AC-Delco appears to call this actuator a heater blend door) both of which are easily accessible to the right of the driver's foot well on the side of the centre console, once the knee bolster panel is removed. But the third actuator for the HVAC air inlet door is located in a difficult to access position inside the right end of the dash.
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post #2 of 3 (permalink) Old 08-11-2018, 09:11 PM
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I believe the logic is...
The BCM Closes the door at key on.
This is a simple motor assembly with no position feed back. At the time one turns the power off, no power is applied to that motor, sos the BCM does not know its position. Also you can confirm this by activating the motor by pressing the recirc button while the key is on and noting itís illumination. Then, turn power off, then back on. Upon power on, you will note the recirc button is NOT illuminated and the BCM applies power to close the door.

The rest of your statement appears true. IE @ total of 3 blend door motors. You can watch that recirc blend door and confirm itís operation by removing the recirc filter. Just before installing your new filter, cycle the switch and power and watch the doors operation.

BC
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post #3 of 3 (permalink) Old 08-13-2018, 04:24 PM Thread Starter
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Quote:
Originally Posted by 1966tbird View Post
I believe the logic is... The BCM Closes the door at key on. This is a simple motor assembly with no position feed back. At the time one turns the power off, no power is applied to that motor, sos the BCM does not know its position. Also you can confirm this by activating the motor by pressing the recirc button while the key is on and noting it’s illumination. Then, turn power off, then back on. Upon power on, you will note the recirc button is NOT illuminated and the BCM applies power to close the door.

The rest of your statement appears true. IE @ total of 3 blend door motors. You can watch that recirc blend door and confirm it’s operation by removing the recirc filter. Just before installing your new filter, cycle the switch and power and watch the doors operation.
Thanks for your input. I've since investigated this further by temporarily removing the passenger cabin air filter and aimed a flash light at the door and then monitor its behaviour under different modes. What I found is that GM designed the logic of the air inlet door to be the most sensible and produces the least wear and operation on that inlet door's actuator.

What I did confirm is that whenever the HVAC is put into RECIRCULATION mode, it is only on a per-ignition basis. This means that this mode is automatically cancelled upon the next ignition cycle. So if the user engages recirculation mode, the air inlet door will close and remain closed even after the ignition is shut off. However, since upon the next ignition, recirculation mode is cleared, the air inlet door will be set to the open position immediately upon ignition, due to the fact that the current HVAC mode on the next ignition will have recirculation mode defeated. So this means if the users turns on recirculation mode again during the next ignition cycle, the door will end up exercising twice - opening and then closing again.

However, I was pleased to see that if recirculation mode is off when ignition mode is on, then the air inlet door remains open at all times. Since I almost never use recirculation mode, this means that actually the air inlet door actuator on my particular vehicle is rarely ever used so that's a good thing to know because it's the most difficult to replace actuator if it fails (gear cracking / clicking noises, etc). I'll make sure I don't use recirculation mode from now on. Usually if I have air cond on when the car is hot such as parked under the sun, I just open all windows and drive it for about half or a minute and then close all windows and start air cond at that time, so no need to use MAX A/C mode by turning on recirculation mode. The air inlet door does NOT close even if air cond and/or air flow mode knob is set to defog range (i.e. any position of the air flow knob at centre position or further clockwise beyond centre position). The only time air inlet door closes is when recirculation mode is on. I had heard rumours that there would be more modes that the air inlet door would close but that was found to be untrue.

I was able check the other two actuators on the left side of the centre console just to the right of the driver's foot well. No need to even remove any panels whatsoever, just needed a flash light to spot those two actuators move when I adjusted the air flow mode knob and the air temperature knob and they are slowly rotating to their target positions normally. So they will be very easy to service if there is a failure since they are so accessible.

I recall that the air inlet door actuator is a 3-pin electrical connector, so I think that's just one ground terminal and two positive terminals for running the actuator motor in both directions. As you suggested, that actuator has no positional feedback, so I'm thinking that it is probably a dumb actuator where the BCM only energizes the motor in the desired direction for a fixed duration and assumes the door has reached its intended position and then power is cut off to the actuator, unless it is also monitoring current and then do a smart shut off when the BCM senses the motor has reached a stopped position while current is applied.

Last edited by gmdriver; 08-13-2018 at 04:27 PM.
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