1991 Chevrolet Dually Flapping noise under pass. side dash

[NoxCobra]

Ok, this is really annoying. when I turn the key on the noise will go about 30 seconds and stop. Also after I turn the truck off it will continue for 30 seconds without the key in and stop. It sounds like maybe a flap for the heater vents or something. It is not the blower motor. Any Ideas?

Thank you for your replies!

 

[6.2 turbo]

I heard this noise on alot of these trucks,I think its one of the servos slipping its gears.

 

[DieselCash]

My truck does this on start up and shut down when it is cold. I have not yet figured out what it is.

 

[NoxCobra]

I heard this noise on alot of these trucks,I think its one of the servos slipping its gears.

Something in th heater box?

 

[vegtech]

You can unplug it but it will put your heat to dearest only

 

[trouttrooper]

I can't remember now, are the vent controls vacuum operated on those years? I bet it has something to do with the flapper that controls where the air goes.

 

[GM Guy]

I believe it is called the blend door actuator, it is a little electric motor that moves a vent door. It is designed to move every time the key cycles, so that is why it fails quicker than the others. I have the same issue on my 91 gas (91 diesel is manual controls, IIRC manual controls dont have this actuator)

It is a pain to do, which explains why i havent done it yet. There was a good writeup on Fullsizechevy.com, but I cant find it now.

good luck!

 

[mattthebrat]

(91 diesel is manual controls, IIRC manual controls dont have this actuator)

Unless it is an OBS (burb, dually or CC) it will have the electronic HVAC.

 

[GM Guy]

Unless it is an OBS (burb, dually or CC) it will have the electronic HVAC.

well, it is an 88-98 body style, but it has the OEM option of no air conditioning (cables, only electrical is the fan speed), if that tells you anything.

I need to look, as I am uncertain if it has the blend door actuator or not. Whenever I cycle the key on any of the other non-AC trucks we have, I dont hear anything.