Hvac Goes Into Defrost (again)

fbachman · June 11, 2011

about 18 months ago, while still under the bumper to bumper, my AC would, seemingly on its own, default to Defrost mode. Nothing I did would get it out, other than stopping and restarting the engine (with almost no wait).

It sure seems like a software issue, as, it seems to me that if something is failing, it would eventually fail and then stay that way.

I plan on pulling the HVAC fuse for 10min or so tomorrow to get the system to reboot from the beginning.

hasn't thrown a code over it.

As I am out of bumper-to-bumper, called GMC customer service, and they seem like they will take care of me as it was fairly recently doing the same thing, and was originally deiscovered under the B2B warranty. Just a perplexing problem. On the previous WO, they wrote that they "recalibrated the AC head and did a software update".

Have had no issues since then until last week. I sure as heck hope the dash doesn't have to be pulled to fix this. Anyone else had this?

If so, what is/was the culprit? I know there have been issues with fan switches and one side hot, one side cold, and such. Both sides remain cold, and the AC puts out air that will cover about a 6-7" diameter of frost to the windshield when this happens.

I have the Auto dual-zone climate set-up...

8.1HD/ALLISON · June 12, 2011

Probably have a mode door actuator gone bad. Don't think the dash has to come out on that one, but not 100% sure.

joe04 · June 12, 2011

about 18 months ago, while still under the bumper to bumper, my AC would, seemingly on its own, default to Defrost mode. Nothing I did would get it out, other than stopping and restarting the engine (with almost no wait).

It sure seems like a software issue, as, it seems to me that if something is failing, it would eventually fail and then stay that way.

I plan on pulling the HVAC fuse for 10min or so tomorrow to get the system to reboot from the beginning.

hasn't thrown a code over it.

As I am out of bumper-to-bumper, called GMC customer service, and they seem like they will take care of me as it was fairly recently doing the same thing, and was originally deiscovered under the B2B warranty. Just a perplexing problem. On the previous WO, they wrote that they "recalibrated the AC head and did a software update".

Have had no issues since then until last week. I sure as heck hope the dash doesn't have to be pulled to fix this. Anyone else had this?

If so, what is/was the culprit? I know there have been issues with fan switches and one side hot, one side cold, and such. Both sides remain cold, and the AC puts out air that will cover about a 6-7" diameter of frost to the windshield when this happens.

I have the Auto dual-zone climate set-up...

Grumpy,

Mode door actuator. I had one go on my 07 Classic, and watched the tech change it in about 30 minutes. Done from under the dash. Hope this helps!

Joe

fbachman · June 12, 2011

Thanks (to both) for the input. I will note that. As I may have to pay for this repair (as I am not sure what GMC will allow), I will jot this down and have that done first, if and when this gets to the shop.