1998 Suburban K1500 HVAC issue

[wildncrazyguy]

I picked up a new to me 1998 Suburban back in December 2014. One issue I'm trying to solve is HVAC related. Try as I might, I've not seen my issue discussed anywhere so I thought I would see if anyone here may have an idea to throw at me.

All, as far as I can tell, including rear controls seem to work properly. Air blows where it is supposed to, fan speed correct, heat/cold fine, control unit lights function when pressed on or off. Everything seems ok, except the one issue I have is when the recirculation button is pressed or the AC button is pressed and in the MAX/vent position, the system immediately moves the mode door and prevents vent position and starts blowing the air out the floor vents and maybe some in the defrost windshield area, not 100% sure on that. It closes the vents off, which is of course where you would like to have the AC blowing at you and not on your feet.

Now AC seems to work and cold air blows in your face, just not on MAX AC which automatically lights up the recirc button. Seems to me the recirc button is the root of my issue here and am wondering what direction to take.

With all that said, I do have also a 1998 K1500 pickup with a control unit almost identical to the Suburban, just without the rear window defrost as my truck does not have that feature. It is a real pain to take my dash bezel on the truck off, but can be done. I thought about swapping the control units to see if they acted the same in both vehicles, like the Suburban now works and the truck is now broke. Advice as to whether this route might definitely indicate a bad control unit in the Suburban, worth the risk of breaking? I just don't really want to take the risk of breaking my truck unit which works perfectly fine and/or the Suburban which I think could be bad, but may be perfectly fine as well.

It may not be my issue even at all and I'm hoping someone here may point me to a resistor or relay or something... Someone with experience in this era HVAC. I did look at the mode door on my truck and Suburban and they seemingly move exactly the same, except the Suburban wants to move when the recirc button is pressed and the truck stays in place as far as I can tell. I will do more research on that as soon as possible to confirm this. In any case, thanks for any help or insight.

 

[wildncrazyguy]

So... Nobody? One question. Chances of it likely being a control unit issue more than a mechanical door or actuator issue? I'm thinking of buying a new control unit, pricey, but if it is the issue and corrects it, then I'm ok with spending the money. It's supposed to have a lifetime guarantee, I guess as long as they keep making them, the company states they will swap the defective unit with a new one. Shipping not included.

Another question. Is there any good way to tell if the recirculation door is opening and closing. I have a mirror on a stick and a flashlight and a general idea of where it is to look. Should I be able to hear it if I put my ear to the passenger dash or something? My thought process is the control unit is bad and not actually opening and closing the recirc door, but sending the signal to the mode door instead. Not knowing anything about the HVAC system of course, but maybe even there is a short in the switch or circuit board and it's sending signal to move both doors, including the mode door by mistake. Who knows. This is why I'm here obviously, hoping someone has a pretty good knowledge of these units and how they're supposed to act.

 

[wildncrazyguy]

So. Confirmed the mode door actuator on the Suburban acts exactly the same as the truck when not in recirc mode. When pressing the recirc button on the Suburban, the mode door moves forward. On the truck, stays put (as expected). This leads me to think the mode door actuator works and functions the doors properly, so it must be in the electronics of the control unit somewhere or some wiring at fault. Hoping to hear from someone who might be better at this than I. Thanks.

 

[wildncrazyguy]

so I was poking around some more... found this thread. http://www.gmt400.com/forum/showthread.php?2014-No-AC-from-dash-vents

Same issue, no real resolution. I hate it when people put something out there and never follow up...

This calibration thing regarding the actuators, I've disconnected the battery a few times and still have this issue. Isn't there a recalibration procedure, like turn hvac off, disconnect battery, turn on, etc? Does anyone have the correct info on recalibrating, specifically a 1998 Suburban K1500 if it matters?

 

[wildncrazyguy]

RESOLUTION!!! In my case it was the AC dash control unit. I purchased another control unit, ACDelco 15-72547. Easy to replace, just a few plug connection harnesses to disconnect and reconnect. Now everything works and recirculation button doesn't send the airflow to the floor. Thanks for all the help on this one.

 

[someotherguy]

You're welcome. Are you mad because in a span of less than 2 weeks nobody else that had the exact same problem and fixed it, responded to your post? Sometimes it works out like that. Not like people are ignoring you on purpose..

Weakest point on all 1995-up GMT400 A/C = the controls

Richard

 

[wildncrazyguy]

You're welcome. Are you mad because in a span of less than 2 weeks nobody else that had the exact same problem and fixed it, responded to your post? Sometimes it works out like that. Not like people are ignoring you on purpose..

Weakest point on all 1995-up GMT400 A/C = the controls

Richard

Nope. Not mad at all. A bit disappointed that NOBODY took time to respond. Just looking for a little direction. Came here because the other forum I was in (you're also a member there) I got at least some response, but not really anything helpful. I figured between the two forums, somebody has either had a similar issue or might know enough about how the HVAC system works on this type of vehicle more than me. But not mad at all. Just also hate to find threads with the same issue and never a resolution, at least in their case, which is why I followed up with my resolution.

 

[wildncrazyguy]

RESOLUTION!!! In my case it was the AC dash control unit. I purchased another control unit, ACDelco 15-72547. Easy to replace, just a few plug connection harnesses to disconnect and reconnect. Now everything works and recirculation button doesn't send the airflow to the floor. Thanks for all the help on this one.

To follow up. I a glad I did NOT decide to swap dash control units from my truck to the Suburban. When I pulled the control unit out of the Suburban to replace it, the ears on the main wiring harness I guess it would be, broke off to no end. I was even very careful, but they just snapped. I'm sure my truck would have had the same issue had I tried. 18 year old plastic... Probably no big deal as the harness is pretty tight and if necessary a little tape strategically placed would not allow the harness to suddenly disconnect.

 

[someotherguy]

I didn't see you on CT so you must mean FSC; I haven't been active over there in 3+ years. Gave up on that place.. I see you got some really, uh, "interesting" replies! Low freon! Plugged evaporator! Plugged cabin filter (lol)

But yeah; I would have pointed you to the controls first. Almost always the root of all evil when it comes to weird HVAC behavior on 1995-up. Just didn't see your post here...this site is a bit less active than the other two big ones.

And yeah also, the stupid plastic on those controls is crazy brittle.

Richard