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Old 07-23-2006, 07:22 PM   #1
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Question Tow Vehicle - AC Problem

The are so many knowledgable people who read these Forums, I thought I would give this a try. I've asked this question on the GM TRUCK FORUMS and so far no useful info has turned up.

This one has got me stumpted! There is a clapping noise coming from inside the AC/Heater enclosure, over on the passenger side, just above the foot area. The AC blows cold and seems to be function properly. This is on a 1994, GMC, Suburban, 2500 series with front and rear AC. This problem just started while I was driving down the freeway on my way to pick up my new 76 31ft Sovereign.

Now it claps at about twice a second when I turn on the ignition and lasts for 20 seconds.....and then it stops! It also starts clapping again when I turn the ignition OFF, and last for 20 seconds! While it is clapping, if I press the MAX (recirculate) button on the AC, it stops for about 5 seconds and starts over again and last for 20 seconds!

The capping noise sounds like it is an actuator door rapidly clapping closed, open, close, open, close, etc. The part that really stumps me is that it does this when I turn the ignition ON or OFF, with the AC ON or OFF!! Doesn't matter. When I turn the ignition OFF, where is this thing getting it's power to clap?!! I never start the engine, so there is NO Vacuum to operate it.

I have removed the glove box and you have limited access to the top of the AC/Heater housing. There are 4 little black boxes (electrical switches, relays) mounted there on top of the AC housing and plugged into a bundle of wires. I have unplugged each of them, one at a time, but the problem is still there. After thinking about it, those are probably the power windows relays.

Also, I thought of disconnecting the battery so that the Microprocessor that controls all this stuff would reboot and re-program itself.....thinking THAT activity would surely fix the problem. Well, I disconnected the battery for 5 minutes......but it didn't fix the problem!

OK, all you AC experts.......what is the problem and how do i fix it? Thanks, Bill
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Old 07-23-2006, 08:37 PM   #2
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My son has a '91 C1500 stepside that had the same problem. His older brother tore out the entire dash trying to figure out what was causing it. He finally disconnected the MAX AC hose and it quit doing it. Obviously there is some kind of vacuum motor involved and while he does not have the clapping now, he doesn't have MAX AIR either.
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Old 07-24-2006, 01:30 AM   #3
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It is possible the vacuum can operator has some small pinhole rupture in it
causing the rapid opening or the controller itself as that system can hold vacuum ,under the hood you might find a round ball shaped reservoir that hold vacuum for the a/c ducting .Possibility anyway.

Scott
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Old 07-24-2006, 08:17 AM   #4
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Vacuum Storage Can

Quote:
Originally Posted by scottanlily
It is possible the vacuum can operator has some small pinhole rupture in it
causing the rapid opening or the controller itself as that system can hold vacuum ,under the hood you might find a round ball shaped reservoir that hold vacuum for the a/c ducting .Possibility anyway.

Scott
Tkanks for your comments, guys. There is a large "TIN CAN SHAPED" device under the hood that looks like it's the vacuum storage container. I'm going to unplug the hose from the engine side to stop the vacuum source to see if that stops the "clapping". If it does, then I know it's a vacuum motor causing the noise. I'm not sure why it would do that. And that sounds like a difficult thing to get at and replace. If it still claps, then I know it's an electrically operated divice. Then I will be really stumpt and wondering how and why it claps for 20 seconds AFTER I have turned the ignition OFF....where does it get it's power?!!
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Old 07-24-2006, 02:12 PM   #5
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Wrong Tank!!

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Originally Posted by lebolewis
Tkanks for your comments, guys. There is a large "TIN CAN SHAPED" device under the hood that looks like it's the vacuum storage container. I'm going to unplug the hose from the engine side to stop the vacuum source to see if that stops the "clapping". If it does, then I know it's a vacuum motor causing the noise. I'm not sure why it would do that. And that sounds like a difficult thing to get at and replace. If it still claps, then I know it's an electrically operated divice. Then I will be really stumpt and wondering how and why it claps for 20 seconds AFTER I have turned the ignition OFF....where does it get it's power?!!
Well, that large "TIN CAN SHAPED" device under the hood that looks like it's the vacuum storage container........is NOT the vacuum tank........it's the emission control "fume" collection tank!! And, I have looked everywhere under the hood and there seems NOT to be a vacuum tank anywhere. I don't know if that means rhat I dont have one or not? Anybody know for sure? Bill
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Old 07-24-2006, 05:31 PM   #6
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Bill,
You should have a vacum can somewere. The ones I remember from my misspent youth with Chevy's were kind of a ball shape, typically mounted on the fire wall.

Aaron
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Old 07-24-2006, 05:40 PM   #7
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Sounds like a stripped gear on the blend door. This is an electrical part, not vacuum operated. You should find two wires coming out of the bottom of the A/C assembly, near the center of the dash (on the bottom), follow them to a plug, and unplug it. If the noise stops: 1-you found the right wires 2-that was the problem.
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Old 07-24-2006, 07:21 PM   #8
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Quote:
Originally Posted by wahoonc
Bill,
You should have a vacum can somewere. The ones I remember from my misspent youth with Chevy's were kind of a ball shape, typically mounted on the fire wall.

Aaron
Aaron, I have looked everywhere, it's simply not there! Bill
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Old 07-24-2006, 07:33 PM   #9
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Ah Ha, getting closer to the cause.

Quote:
Originally Posted by overlander63
Sounds like a stripped gear on the blend door. This is an electrical part, not vacuum operated. You should find two wires coming out of the bottom of the A/C assembly, near the center of the dash (on the bottom), follow them to a plug, and unplug it. If the noise stops: 1-you found the right wires 2-that was the problem.
Hey Terry, you sound like you know something about this stuff. Because of the rapidity of the clapping, I have always thought it was an "electrical" door that was clapping. Now, tell me this, 1) how does it clap AFTER I have turned OFF the ignition? And 2) why doesn't it clap all the time?.....it only claps for 20 seconds and then it stops! That makes me think that it's not the device itself, but the thing that is feeding it the signal that makes it clap! Am I wrong? Bill
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Old 07-24-2006, 07:50 PM   #10
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The door opens when you turn the key on, and closes when you turn the key off. When the motor reaches its limit of travel, it stops. This is a very common problem with those vehicles, and one reason they no longer are made that way. Don't get me started about the wonderful radios in that vintage of truck...
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Old 07-24-2006, 10:18 PM   #11
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That ball may not be there as it is hard to know all the changes GM made.
davidz71 said the 91 did the same thing ,with a result of a hose disconnection on max a/c .Haynes manuals have pretty good illustrations
and may help provide you with a photo of the blend door and other components in the heater and a/c section .I have not seen this problem myself on these years of trucks ,also there may be an obstruction as
I commonly find pens inside the defrost/ducting and the blend door of
vehicals I work on if I am doing a heater core or blower motor replacement.

Scott
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Old 07-24-2006, 11:35 PM   #12
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Quote:
Originally Posted by overlander63
Sounds like a stripped gear on the blend door. This is an electrical part, not vacuum operated. You should find two wires coming out of the bottom of the A/C assembly, near the center of the dash (on the bottom), follow them to a plug, and unplug it. If the noise stops: 1-you found the right wires 2-that was the problem.
Terry, I sent a reply to your comments....twice......any idea why they have not posted on this thread? Bill
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Old 07-25-2006, 12:07 AM   #13
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Quote:
Originally Posted by scottanlily
That ball may not be there as it is hard to know all the changes GM made.
davidz71 said the 91 did the same thing ,with a result of a hose disconnection on max a/c .Haynes manuals have pretty good illustrations
and may help provide you with a photo of the blend door and other components in the heater and a/c section .I have not seen this problem myself on these years of trucks ,also there may be an obstruction as
I commonly find pens inside the defrost/ducting and the blend door of
vehicals I work on if I am doing a heater core or blower motor replacement.

Scott
I remember that the vacuum ball is black, looks a little bigger than a baseball, is located on the passenger side under the hood of my wife's '95 Astrovan within easy reach. I'll check tomorrow to see where it is on my son 's '91.
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Old 07-25-2006, 08:08 AM   #14
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AC Blend Motor Functions?

Quote:
Originally Posted by overlander63
The door opens when you turn the key on, and closes when you turn the key off. When the motor reaches its limit of travel, it stops. This is a very common problem with those vehicles, and one reason they no longer are made that way. Don't get me started about the wonderful radios in that vintage of truck...
Terry, if I unplug the motor wires while the AC is in the MAX mode (recirc) will it stay in that mode permanently....same question for the "fresh air" mode? Also when Winter arrives and I no longer need AC, can I get the heat to come out the bottom instead of out of the AC vents? I guess my question is, what functions do I disable with the Blend Motor wires unplugged? Bill
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