Loud bang when switching between vent modes...

[rush91]

I've noticed this ever since I've had the car. A loud bang / thump / pop whenever I switch between vent modes. And doesn't matter if I'm using the A/C or heat. But for instance if the unit is set on the floor vents, and I hit the button for the dash vents, there is a VERY loud bang....Like something is hitting the underside of the dash. It only happens when I hit the button for the dash vents, or the dash/floor vent combo button. I haven't really dug into this yet, as I hardly use the A/C or heat, which both work fine. The unit works fine, all vents blow out hot or cold air.

[DRM500RUBYZR-1]

Your description indicates that you are able to divert the airflow successfully as commanded, to defrost, upper and lower vents, Correct?

Further, it seems that you are able to command temperature changes and all responds as requested, Correct?

Does the noise happen on every selection of airflow every time, or just the first change?

It sounds as if the long HVAC tube connecting the passenger side to center may not be properly secured or fastened, has the heater core been replaced?

It would be helpful to determine what immediately precedes the noise, i.e.
does it happen after you invoke the diversion of air, or as it is changing?

The blend door could bang, but we need to understand is it temperature invoked or outlet invoked?

I am suspecting outlet invoked.

:cheers:
Marty

[rush91]

Your description indicates that you are able to divert the airflow successfully as commanded, to defrost, upper and lower vents, Correct?

Further, it seems that you are able to command temperature changes and all responds as requested, Correct?

Does the noise happen on every selection of airflow every time, or just the first change?

It sounds as if the long HVAC tube connecting the passenger side to center may not be properly secured or fastened, has the heater core been replaced?

It would be helpful to determine what immediately precedes the noise, i.e.
does it happen after you invoke the diversion of air, or as it is changing?

The blend door could bang, but we need to understand is it temperature invoked or outlet invoked?

I am suspecting outlet invoked.

:cheers:
Marty



Yes. I can divert the air flow to any vent and the air comes out fine. Also the climate controls work fine. I can set it to auto for any temp, and works fine.

It happens when I switch the vents on the 1st change. And doesn't happen if I switch back after that. There really is no noise that follows the slapping / banging noise. As far as I know the heater core is original, and the I have no issues with the heat.

This blend door you speak of lol....That to me sounds like what is happening. Like that door, when switching vent modes, slaps against something. Or bangs loudly when switching. Is this a part that wears out over time?

[billschroeder5842]

Is there any way that you can make a video with sound of the problem?

Sounds are tough to diagnose as one mans bang is another mans smack, thud or whack.

Vocabulary is a tricky thing.

[Mystic ZR-1]

Mine does the same thing after start up.
Believe it's the blend door, does it in every mode except off.
Pretty sure it's supposed to move after start up,
but it shouldn't bang? Just started doing this recently.
All AC/heat controls appear to work OK.
Sounds like the same problem rush91's having?

[BigJohn]

Mine does the same thing after start up.
Believe it's the blend door, does it in every mode except off.
Pretty sure it's supposed to move after start up,
but it shouldn't bang? Just started doing this recently.
All AC/heat controls appear to work OK.
Sounds like the same problem rush91's having?

Does it only happen when it is off the jack-stands???

:confused:

[rush91]

Is there any way that you can make a video with sound of the problem?

Sounds are tough to diagnose as one mans bang is another mans smack, thud or whack.

Vocabulary is a tricky thing.

I will try to get a video of it, or sound clip somehow. But the noise sounds like someone is hitting the dash.....Makes me winch when it happens, same as someone who gets in/out of my Beast and slams the door lol.

[rush91]

Mine does the same thing after start up.
Believe it's the blend door, does it in every mode except off.
Pretty sure it's supposed to move after start up,
but it shouldn't bang? Just started doing this recently.
All AC/heat controls appear to work OK.
Sounds like the same problem rush91's having?

Sounds like the same issue to me. I'm going to go through the books tonight and see about this blend door. But that is what it sounds like, its smacking up against something.

[Z51JEFF]

My 91 does this every once in awhile but I do know the sound. It's almost like the door is either stuck and then releases with force or it my be getting too much vacuum.

[MickeyD]

My black 90 does that. Only has 2200 miles. I wouldn't worry about it.

[DRM500RUBYZR-1]

IF:
Nothing has been disturbed or come loose, and is being buffeted by airflow,
then; there are only really two movable features within the system.

1- the diverter valve and actuator- this does exactly as it sounds like it would, i.e. it diverts the airflow to the outlets invoked by the selection made through the control head, then said selection is passed to the programmer to execute the command, then on to the actuator and the valve, which then re-directs the air.

2- the blend door aka the air mix door- again does what it says, i.e. mixes the ambient air with heated or cooled air to deliver it and achieve the temperature requested at the control head. Temp selection is made at the control head, then communicated through the programmer, which instructs the blend door actuator to move the door to a position that will yield the best mix of air to obtain and maintain the selected temp.

You must determine which sequence is in process when you hear the noise.

I suspect the blend door actuator.

When the car is first started, the system cycles the blend door from fully closed to fully open, one time.
This process conveys info back to the programmer so that ot knows the travel possibilities of the door aiding in determining the soon to be requested commands.

It would seem to be either sticking, or unable to hold itself open, and slamming shut, which although it can operate, it does not have enough strength to hold the door fully open.

While I am sure there are electronic testing procedures in the FSM, it would be far more valuable for you to evaluate the operation of both variables of each system, and determine what is requested immediately before you hear the noise.

Hope I have helped.
:cheers:
Marty

[billschroeder5842]

My black 90 does that. Only has 2200 miles. I wouldn't worry about it.

I would tend to agree. The loud noise (while disconcerting) actually means the system is fully functional, confident and "positive" in its execution.

An aggressive, positive transmission shift will have a "bang" as well.

[henryr]

ditto

scares the crap out of me every time

[Hog]

I've noticed this ever since I've had the car. A loud bang / thump / pop whenever I switch between vent modes. And doesn't matter if I'm using the A/C or heat. But for instance if the unit is set on the floor vents, and I hit the button for the dash vents, there is a VERY loud bang....Like something is hitting the underside of the dash. It only happens when I hit the button for the dash vents, or the dash/floor vent combo button. I haven't really dug into this yet, as I hardly use the A/C or heat, which both work fine. The unit works fine, all vents blow out hot or cold air.

Does the "bang" seem louder the higher the fan speed is selected?

[GOLDCYLON]

Its the blend door slapping

[Hog]

Its the blend door slapping
This happens on a few GM vehicles.

[geezer]

I was having the same HVAC 'bang' issue as Rush 91 initially described. HVAC system operated normally in all modes but when first activated experienced the load ,BANG,.
I removed the knee bolsters Drver side & Pass. side to check HVAC vacuum system.
On pass. side found the "orange" vacuum line to the recirculation outside air control pot
had been the victim of 'mickey or minnie'. Sealed the chewed area. 'Bang' issue resolved!!