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Old 06-15-2011, 11:49 PM   #1
abqsaturn
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Question 99 SC2 knocking sound with A/C on and idling

2002saturnsc2 explained my car's current situation with A/C triggering a knocking sound in exact detail in a previous thread (May 2010)- but I never saw or read what the final diagnosis was and if and how it was resolved.

So, the problem I'm having is identifying exactly where the noise is coming from.

Here are the conditions when it happens:
-When I turn the A/C on, or if the A/C is already turned on and I start the car, there is a loud knocking sound coming from the A/C compressor or engine area.
-In gear (stops vibrating in Park)
-Stopped/idling
-A/C on or A/C and recirculate on (not just with the fan on)

When I put the car in park and let it idle, opened the hood, and turned the A/C on and off to locate the noise, I noticed it was obviously connected in some way to the silver pulley/wheel beside the compressor, which would start spinning each time I turned on the A/C, then stop when I shut it off.

Any suggestions or experience with this specific issue? Is it possible the compressor itself just needs to be readjusted? (The more cost effective the better. I'm not averse to trying a reasonably easy fix myself, but I'm not an expert mechanic by any means.)

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Old 06-15-2011, 11:54 PM   #2
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1998 SC2
Default Re: 99 SC2 knocking sound with A/C on and idling

I would suspect the belt tensioner. You can look down right below and in front of the alternator and see the tensioner center bolt. If it is the problem you will see it bouncing up and down in time to the clank.

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Old 06-16-2011, 12:02 AM   #3
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Default Re: 99 SC2 knocking sound with A/C on and idling

Thanks, OldNuc. I'll check it out tomorrow in better light and see what I can find. (Iowa native here, btw. Go Hawks!)

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Old 06-16-2011, 01:18 AM   #4
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Default Re: 99 SC2 knocking sound with A/C on and idling

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Originally Posted by OldNuc View Post
I would suspect the belt tensioner.
+1

I had the same problem with my belt tensioner. There was nothing wrong with the tensioner itself--the belt was too long and the tensioner was reaching the end of its travel (there are two different size belts for our cars--one of them is too long when used with aftermarket tensioners).

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Old 06-17-2011, 03:43 PM   #5
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Default Re: 99 SC2 knocking sound with A/C on and idling

Hello All, abqsaturn, which part was your problem? I have a 1997 sc1 doing the same thing, car lot I bought it from claimed they changed tensioner, and it's still knocking when a/c on and in gear, also, idles rough during this time. Someone told me they suspected the a/c compressor causing both. Any ideas, anyone?

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Old 06-17-2011, 03:51 PM   #6
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Default Re: 99 SC2 knocking sound with A/C on and idling

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Originally Posted by killervol View Post
Hello All, abqsaturn, which part was your problem? I have a 1997 sc1 doing the same thing, car lot I bought it from claimed they changed tensioner, and it's still knocking when a/c on and in gear, also, idles rough during this time. Someone told me they suspected the a/c compressor causing both. Any ideas, anyone?
Induce the problem, then take a look. If it's the tensioner, you should be able to see it vibrating back and forth, in rhythm with the sound. As I said above, it's often the belt causing the problem with the tensioner--if the belt is too long, the tensioner will bounce off the end of its travel. There are two belts listed and sold for the car (I believe something like 778 and 780). If you have the longer one, particularly with an aftermarket tensioner, that may be the problem (it was for me). I know the gatorback belts from autozone are the longer kind and the re-branded NAPA belts are the correct, shorter ones (I think they're either gates or dayco, rebranded as NAPA).

If you can see that it's the tensioner, try a new (shorter version) belt before you replace the tensioner or anything else, even if your belt is new.

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Old 06-17-2011, 03:59 PM   #7
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Default Re: 99 SC2 knocking sound with A/C on and idling

Thanks, Plastic, I'll head down to NAPA and grab a belt, hopefully this will eliminate the knock, will let you know how it goes. I wonder if abqsaturn got his issue resolved?

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Old 06-17-2011, 04:13 PM   #8
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Default Re: 99 SC2 knocking sound with A/C on and idling

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Originally Posted by killervol View Post
Thanks, Plastic, I'll head down to NAPA and grab a belt, hopefully this will eliminate the knock, will let you know how it goes. I wonder if abqsaturn got his issue resolved?
Before spending the money, it's easy to see if the tensioner is the problem. If you can recreate the sound (for me, it was a/c on, in drive, not moving), just look at the tensioner and you'll see if it's vibrating back and forth in rhythm with the sound.

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Old 06-24-2011, 02:31 PM   #9
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Default Re: 99 SC2 knocking sound with A/C on and idling

Well, changed the belt with the NAPA brand belt, and it still makes the noise, just not as loud as before. It does look like the tensioner has been replaced, so not sure where to go from here. Don't know if I should look at replacing tensioner with original part, or change belt again. Any ideas???

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Old 06-24-2011, 02:46 PM   #10
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1998 SC2
Default Re: 99 SC2 knocking sound with A/C on and idling

Tensioner. Aftermarket is ~50 and OEM is 100.

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