13 Suburban Rear HVAC Erratic Operation.

[George B]

I have been chatting with a few here about my rear HVAC operation but decided to start a thread. I am going to add the posts we have already and then proceed with some more diagnostics on my Tech2 as time permits. So far the unit operates as expected when controlled by either the front or rear control panel except for when placed in AUTO. It then leaks cold air out the upper vents for the rear when they are supposed to be off. I have not confirmed what comes out of the lower vents in the rear when this occurs. One additional note is that the temperature coming out of the rear vents is somewhat cooler than the fronts unless the heat is turned up to 80* or so. I have no history indicating any service of the related HVAC components except that I replaced the rear control panel some time ago. I cannot say if this issue is related to that or not.

 

[George B]

I have a question for the tech2 owners here. When using the special functions to command heat or cool in the aux heater I noted that when I command heat it gets colder and when I command cool it gets warmer. Is this a fluke with the tech2?

Both HVAC units operate as expected except that the rear tends to blow slightly cooler air and there is some air leaking in the vents on the roof when it blowing to the floor in auto/heat.

That would seem to me that the wrong actuators are installed. This is what ours did when I put the wrong one in on the driver's side.

The strange thing is the rear heater works as it should. When using the vehicle controls heat is heat and cool is cool just like it should be.

I have been in the back with the panels off and my tech2 in hand it worked as commanded with the tech2, there are only 2 actuators in the rear. the one nearest the rear blower motor controls hot/cold and the one further up at the Y in the vents controls air up/down, you can put in the wrong actuator it will fit, plug in and everything, it just will not work right (i know because I tried it lol). so assuming you have the correct actuators in there it could something else like a bad control module or faulty sensor in the vent, there are 2 thermistors in the same rear vent area where the actuators are, they should have 5volts power at the harness and the sensor should change resistance when heat is applied (like from a hair dryer or heat gun), it's unlikely one of them would be bad but if you are in there may as check them to rule it out.View attachment 265748 View attachment 265749

Thanks, I will have to play with it more I guess. It’s confusing because it works to a point and heats/cools when it should but not as warm as it should in my opinion. I will have to figure out which sensor is which back there. The tech 2 lists 4 of them.

I could swear I remember someone mentioning something working backwards with the tech2 clone in special functions.

I was back there troubleshooting mine because I was getting cold air in the back when I put it on heat, unless I had it set all the way up to full heat, if I turned it down 1 degree it would blow cold, the tech2 kept telling me there was a sensor feedback error, I searched high and low for that dam "sensor", even paid for a diagram which oddly enough came from a amazon website that showed a air sensor up in the headliner drivers side rear near the rear hatch which did not exist on my truck. I finally figured out that the main hvac control I had bought from rockauto had the wrong firmware and I had to have it reprogrammed to my vin, this was before I had ever tried to program with the tdsweb and the tech2 and didn't want to brick my bcm or something, once the hvac control was reprogrammed everything worked fine again.
one thing I learned from that is if you do a full scan with the tech2 it will try to find ANY option that may exist on a gm vehicle and if it does not find one of those options it will tell you no communication with that module. It does not know if you have it or not it just say's hey can't communicate, so this can be misleading if you are not sure if you have that module, sensor, etc or not.
getting back to those little sensors I pictured there is like 6-8 or even 10 of them in these trucks I am still not sure exactly it depends on if it is swb or lwb, they are all over most of them are in the front air vents. I found 2 in the rear vents one is pretty accessible when you have the rear hvac passenger side panel off the other is more forward and you have to take the passenger side mid pillar cover off to access it, if the tech2 say's there are 4 there could be 2 more in the vents more up in the headliner or down under the carpet.
previous experience tells me you may just have a lazy actuator if you are not getting warm enough air, especially if the tech2 says there is a "feedback error" I had that problem on the front passenger side it would not blow the same temperature as the drivers side, replaced the actuator and problem solved.

what you can do is just watch the actuator as you control the hvac without tech2 connected, manually adjust your heat/cold and watch the actuator move, if you set the hvac to full heat the door should be fully closed at the mark on the actuator, if it isn't then the actuator motor is probably funky.

 

[Doubeleive]

I have been chatting with a few here about my rear HVAC operation but decided to start a thread. I am going to add the posts we have already and then proceed with some more diagnostics on my Tech2 as time permits. So far the unit operates as expected when controlled by either the front or rear control panel except for when placed in AUTO. It then leaks cold air out the upper vents for the rear when they are supposed to be off. I have not confirmed what comes out of the lower vents in the rear when this occurs. One additional note is that the temperature coming out of the rear vents is somewhat cooler than the fronts unless the heat is turned up to 80* or so. I have no history indicating any service of the related HVAC components except that I replaced the rear control panel some time ago. I cannot say if this issue is related to that or not.

I would suspect the rear ambient temp sensor above the passenger rear seat in the headliner, it's same part as the front drivers side one. I have not heard of the rear hvac controller needing to be programmed to a vin but it's possible also I suppose

 

[George B]

I would suspect the rear ambient temp sensor above the passenger rear seat in the headliner, it's same part as the front drivers side one. I have not heard of the rear hvac controller needing to be programmed to a vin but it's possible also I suppose

I only see one of those little fan type sensors above the drivers seat. Nothing in the back.

 

[swathdiver]

I only see one of those little fan type sensors above the drivers seat. Nothing in the back.

I have two of them, one in back on the driver's side. The Denalis have 2 over the driver after 2009. plus the one in the back.

Anyway, I used the Tech-2 tonight to command the rear temperature hot and cold and it did what I asked it to do. Did the same for the driver's side temperature control.

 

[George B]

I have two of them, one in back on the driver's side. The Denalis have 2 over the driver after 2009.

Anyway, I used the Tech-2 tonight to command the rear temperature hot and cold and it did what I asked it to do. Did the same for the driver's side temperature control.

Thanks for checking. I don’t know where the rear ambient sensor would be on my 13. I don’t see one. I am also miffed that while the front and rear controls both cause the rear heat to get hot or cold as commanded my tech2 results in backwards operation. Strange...

 

[swathdiver]

Thanks for checking. I don’t know where the rear ambient sensor would be on my 13. I don’t see one. I am also miffed that while the front and rear controls both cause the rear heat to get hot or cold as commanded my tech2 results in backwards operation. Strange...

Yes it is. If the cabin is still operating at the temperatures you want it, I wouldn't worry about it too much yet. Are there any codes for the actuators? Are they usually within 5 numbers between commanded and actual? Were any of them changed that you know of?

 

[George B]

Yes it is. If the cabin is still operating at the temperatures you want it, I wouldn't worry about it too much yet. Are there any codes for the actuators? Are they usually within 5 numbers between commanded and actual? Were any of them changed that you know of?

I only have one code for an actuator after running a calibration and it is for the recirculate door position sensor open circuit. I watched the counts and it moves like it should but stops counting for a bit and then restarts. I can open and close that door without issue. One thing I question is that I didn't notice the rear changing positions during the calibration. I don't know if there is another calibration for the rear only or not.
It was loaded with codes due to a dead rear control panel I cleared all those and only got the one back.

My main concern is the rear temperature control and the leaky mode door when operating in AUTO.

I wanted to mess with it more today but our other vehicle was more needy.

 

[swathdiver]

I only have one code for an actuator after running a calibration and it is for the recirculate door position sensor open circuit. I watched the counts and it moves like it should but stops counting for a bit and then restarts. I can open and close that door without issue. One thing I question is that I didn't notice the rear changing positions during the calibration. I don't know if there is another calibration for the rear only or not.
It was loaded with codes due to a dead rear control panel I cleared all those and only got the one back.

My main concern is the rear temperature control and the leaky mode door when operating in AUTO.

I wanted to mess with it more today but our other vehicle was more needy.

Yeah, I half expect to get better gas mileage from the AC having to work less once all the actuators are replaced! LOL At one time or another, every one of mine has thrown a code at least once. Still have the Mode and Recirc sitting on the shelf along with the passenger side one. Then might as well get the rears done too. But first the new bumper cover and headlights and control arms and tires. Money money money! But it sure is nice not having a car payment for this big thing, just repairs and maintenance on my schedule, not the banks'!