1991 chevy truck - AC knocking

When i start my truck or try to turn on the AC, it makes this knocking sound for about 30 seconds and then stops. Usually it doesnt blow cold air. But if i disconnect my battery and then reconnect, it will blow cold for a random amount of time(maybe an hour, maybe a couple days).

The knocking sound is coming from the front-right side of the dash, sounds like its in the dash.

Does anyone know what this is? Or how I can fix it??? Thanks in advance!!! :)


edit: i uploaded a recording of the sound. http://media.putfile.com/ac-knock

might be your blower motor. its located under the dash right above the floorboard.

might be your blower motor. its located under the dash right above the floorboard.

if the blower motor was bad, wouldnt it just not blow?

the vents always blow, but not always cold.

the knocking occurs when:
1) i start the truck
2) turn the AC on or off
3) turn the truck off

i recorded the knocking sound. it sounds like its coming from the passenger side under the dash.

The recording is kind of quiet.

http://media.putfile.com/ac-knock

Sounds like something is hitting the cage that the blower motor spins (I call it a squirel cage). It it pretty easy to get the blower motor out. Take it out and look for something loose or hitting the cage.

//2000CAYukon

Try the temperature control motor. It is located directly behind the right edge of the center dash panel (where the tape deck usually is if you have one) You can get access to it by removing the glove box insert (4 bolts). Try putting your hand on the servo when you turn on your A/C and see if its the culprit. I had A/C troubles recently, I never got cold air, only heat and the compressor would turn on and off every 5 seconds. I found the motor behind the glove box and its connector had vibrated out.. Easy fix for me and ice cold A/C now. The guy who owned the truck before me was quoted $700 bucks from Chevy Stealership to fix it, so he let it go. His loss is my gain

Sounds like something is hitting the cage that the blower motor spins (I call it a squirel cage). It it pretty easy to get the blower motor out. Take it out and look for something loose or hitting the cage.

//2000CAYukon

it only does it for the first 40 seconds or so, and then it stops. but the air is still blowing.

if something is hitting the cage, wouldnt it do it the whole time the ac is on?

also, whenever i have my ac OFF, it still makes that noise when i start and stop my truck.

it only does it for the first 40 seconds or so, and then it stops. but the air is still blowing.

if something is hitting the cage, wouldnt it do it the whole time the ac is on?

also, whenever i have my ac OFF, it still makes that noise when i start and stop my truck.

The noise sounds like it is coming from the cage. Just because it goes away does not mean that something is not in there.

That is where I would start.

//2000CAYukon

I still think its a servo motor. It could be worn out and trying to open or close a duct, or possibly just sticky. There are a few of them down there.... just unplug them one at a time and see if the noise stops.

I still think its a servo motor. It could be worn out and trying to open or close a duct, or possibly just sticky. There are a few of them down there.... just unplug them one at a time and see if the noise stops.

I agree. Because your vents change everytime you turn your truck on, or off. Or it could be your A/C control sending the wrong signal to one of the servors...this is going to take some trouble shooting on ur part. I would start with the servo.

Also, why your at it check the cage...I don't think its because if it knocks like that when the blower is off, I don't think something is trapped in the cage... because it would rattle diffrently with out the blower forcing it into the cage, could be faster, quiter...but wouldnt be the same. But maybe you will get lucky and it will be a simple solution! As they say, start small and get big.

OK, I did some poking around today, and made some progress.

Basically the noise is coming from the actuator that moves the flapper(valve?) for the air intake. The thing that makes it take air from the outside or inside.

Directly behind the blower motor there is that white door/flapper/valve (i dont know what you call it) that moves when you select "normal" or "recycle". It looks like that door is raising about halfway and then getting stuck, and thats when the knocking sound starts.

I'm not sure where to go from here. Suggestions????
Thanks for all the help so far!!!

Hrm, disconect the servo from the flap, and see if it still makes that noise...if it does, the servo is probably bad, if not...see how easy the flap moves...

2 years later and I wanted to say I have the exact same problem (and my truck is 2 years older than yours, so there ya go). Thank you for helping me isolate where the tap tap tapping is coming from. The trouble is that particular servo is the hardest to get to. It's up under and behind everything and there is this big cluster of wires that run through the firewall that's in the way too. I played with the rec button and when the flap is down it taps and if I just push up on it with my finger it stops. I'm thinking about rigging it with just a bulldog clip or something to hold it slightly open. It also makes that noise when it gets to the fully open position but that's something I can live with for now because just like the orig. poster it does stop after 30 secs or so.

I already took the blower out and put it back. I wish I checked back there from within to see if there was something I could do. But it is NOT the blower or the cage.

Happy New Year