Rear HVAC randomly turns on after engine is off

[tkdgirl]

Hoping to get some insight/help.

Getting my winter beater 04 Tahoe LT ready for the season. Last year,.my rear HVAC would randomly turn on once the car was off. Ended up draining my battery multiple times. Took to my mechanic and he said there's something going on with the aftermarket stereo that was installed. So I pulled the fuse and haven't had it happen again.

Today,.I took apart the dash to inspect the wiring for the stereo and any loose connections. When I did this, the rear HVAC came on, key out and engine off. No radio fuse either.

Any idea what's going on? I'd really like to use my stereo this winter. Here's a timeline.

1. New ATOTO A6 stereo professionally installed Dec 2018 and rear HVAC actuators replaced.
2. New stepper motors installed April 2019
* HVAC issue started summer 2019*
3. Checked the cluster connection and it's ok
4. Replaced the RP5-GM11 harness thinking it's the problem. Didn't fix it.
5. Replaced the HVAC fuses. Didn't fix it.

I have no idea how to use a voltage meter (but have one somewhere)

 

[Fless]

Are you setting the rear (overhead) HVAC control OFF (not AUTO) before you shut off the truck?

 

[tkdgirl]

Are you setting the rear (overhead) HVAC control OFF (not AUTO) before you shut off the truck?

Yes.

 

[tkdgirl]

To add, when I replace the radio fuse, it sparks.

 

[retiredsparky]

How old is your battery?
Is the vehicle only driven sporadically during some of the year?
Do you use a "smart" maintenance charger on the battery?

The reason I ask, is that a battery that sits for long periods of time builds up sulfation in the cells. This can damage the battery and cause strange symptoms in vehicles, even if the battery tests ok on a standard battery tester. Vehicles need good quality power from the alternator and battery to be able to function logically as they are designed. This is only a shot in the dark wild guess. Take it for what it's worth.

 

[tkdgirl]

How old is your battery?
Is the vehicle only driven sporadically during some of the year?
Do you use a "smart" maintenance charger on the battery?

The reason I ask, is that a battery that sits for long periods of time builds up sulfation in the cells. This can damage the battery and cause strange symptoms in vehicles, even if the battery tests ok on a standard battery tester. Vehicles need good quality power from the alternator and battery to be able to function logically as they are designed. This is only a shot in the dark wild guess. Take it for what it's worth.

Regularly driven. New battery in late 2019. That's when it started.

 

[live2pull]

Was the rear AC control module replaced recently? As far as I know they have been discontinued by GM and the only new replacement available is by Dorman. I replaced my rear AC module with the Dorman and it has a mind of its own occasionally.

 

[swathdiver]

Hoping to get some insight/help.

Getting my winter beater 04 Tahoe LT ready for the season. Last year,.my rear HVAC would randomly turn on once the car was off. Ended up draining my battery multiple times. Took to my mechanic and he said there's something going on with the aftermarket stereo that was installed. So I pulled the fuse and haven't had it happen again.

Today,.I took apart the dash to inspect the wiring for the stereo and any loose connections. When I did this, the rear HVAC came on, key out and engine off. No radio fuse either.

Any idea what's going on? I'd really like to use my stereo this winter. Here's a timeline.

1. New ATOTO A6 stereo professionally installed Dec 2018 and rear HVAC actuators replaced.
2. New stepper motors installed April 2019
* HVAC issue started summer 2019*
3. Checked the cluster connection and it's ok
4. Replaced the RP5-GM11 harness thinking it's the problem. Didn't fix it.
5. Replaced the HVAC fuses. Didn't fix it.

I have no idea how to use a voltage meter (but have one somewhere)

There is a feature called "Afterblow" that turns on the fans under certain conditions to dry out the evaporators and it is activated by a Tech-2. Don't know if this is the case but if you have a bi-directional scan tool you can tell.

 

[Fless]

There is a feature called "Afterblow" that turns on the fans under certain conditions to dry out the evaporators and it is activated by a Tech-2. Don't know if this is the case but if you have a bi-directional scan tool you can tell.

I don't think the GMT-800s are capable of Afterblow; I've never seen that option in my Tech 2.

I'm assuming the HVAC head is the AUTO one that can set the temp to a numerical value. The head unit and/or the overhead control should be checked for updated calibrations, easily done with a Tech 2 and TIS2000 on a PC. Or an advanced scan tool or a trip to the dealer.

I would also suspect the radio installation based on when this started, especially if it wasn't the standard harness extension from the dash harness. Any extra wires added or spliced outside of the factory harness? Also carefully inspect wiring behind dash -- apart from the radio harness -- to see if a wire has been scraped and is grounding out. Look around the HVAC head, and maybe pull it out to inspect.

 

[swathdiver]

I don't think the GMT-800s are capable of Afterblow; I've never seen that option in my Tech 2.

Thanks for the correction!