2008 Suburban defroster noise and delay

[HawkeyeScott]

2008 Suburban LT 1500 5.3L with 129K

Second time in three days this has happened: Cold vehicle (24 degrees out this morning), turn on front defroster (from the bi-level setting) and there is a small but audible grinding sound coming from in the dash, and defroster doesn't doesn't blow. Then, a few minutes later, out of the blue, it starts blowing as it should. It worked fine the rest of the way in to work.

My immediate remedy is to just leave it on the defroster setting at the end of the commute home, and see if not asking it to switch to defroster mode when cold will make it work as it should immediately tomorrow.

Obviously, something underlying is causing this. Any ideas what I might need to replace? Thanks.

 

[HangFire]

Typical old vehicle behavior. There is an air diverter flap in the dash, and some kind of vacuum actuator or low power electric motor to move it. When cold it doesn't want to move but once warmed up it moves and the air goes where it should.

The fix depends on the vehicle and the problem. Sometimes a clean and lube of the pivot points is all that is needed, but getting to them can be difficult on American vehicles. Sometimes you need a new actuator or door motor. Sometimes you need a whole new air box.

In the mean time, I would suggest keep doing what you are doing, and don't ask it to change settings while cold, or you may break whatever functionality you have left.

Someone that knows Suburbans well can get into more detail.

 

[WyrTwister]

Yep .

 

[Dave9]

Does it still blow out a different (the wrong) vent? If not, odds are you need a new blower motor, or possibly the contacts on the circuit to it are fouled somewhere and worsen with temperature, like the switch in the dash or a connector contact or speed controller or relay or fuse. If it does blow out the wrong vent well, it's the electric or vac operated solenoid for the blend door or that door itself sticking or broken as HangFire mentioned. If the solenoid is vac operated it could fail from a vac leak but that typically wouldn't only happen in cold weather and if significant enough might throw up a lean engine code.

 

[NYEngineer]

I have an 09 Suburban and had a different but related problem. Last summer, my AC was blowing cold except out of my vent on the left side of the dash. It turned out to be the blend door actuator. Thankfully, it was the one on the bottom.
I don't believe anything on these trucks is vacuum actuated.

When I posted about this problem, someone on here posted a diagram of the air box under the dash and the locations of the blower motor and all of the actuators. I have it saved in another computer. I'll post it when I get there.