Help: Clacking/Knocking When A/C Kicks In

[xvvvz]

Over the last two days, most of the time (but not all the time) when the air conditioning compressor kicks in I get a rapid clacking or knocking sound from the engine compartment for a short time that varies with engine speed (not vehicle speed). This usually occurs when the car has been sitting and is first started if I had left the a/c on when the car was last turned off (a/c kicks in immediately since it is so hot here). It can also occur as the compressor cycles on and off (not always though) while driving or when I first active the a/c while driving (via the EC button). The sound only lasts for a few seconds and then dies away.

The car is a 99 C230 Kompressor. I have tried to find a source for the sound but it is very difficult to determine because it disappears so quickly and I have to run from the front seat to the engine bay to try and catch it. I believe I have the source narrowed down to either the air conditioning compressor or the tensioner assembly.

Looking at the tensioner’s pulley, the V belt and the a/c compressor’s pulley, I cannot see any significant wobble or movement.

Looking through the archives here, it seem that a worn rubber bushing on the shock/damper that is attached the tensioner assembly is a common source for a similar sound. I went out and tried to see all sides of the tensioner assembly and I cannot find anything that resembles a shock/strut! If looking at the tensioner assembly while standing at the front of the car, shouldn’t the shock/strut be attached to the upper right side of the tensioner assembly?

Can the tensioner assembly itself be making this noise? Is there any way to test it? I hate to just throw $$ parts at the problem. The car is 7+ years old but it has less than 31k miles on it.

Any suggestions from people who have experience with these components/problems would be appreciated.

Thanks.

[xvvvz]

Looking through WIS, I just found a troubleshooting article named "Knocking or rattling noises from belt tensioning devices."

It basically says the cause can be the "stud/bolt and/or collar nut for fastening pulley is loose."

Ok, I can determine where the stud bolt is but does anyone know where the "collar nut" is on the tensioning device??? Is it under the cover on the front of the pully located on the tensioning device?

Any help would be appreciated. I am going to go try to find some female torx sockets tomorrow since the stud is a freakin' torx type. Grrrrrr...

[kevinzorz]

I think i have the same problem. Its really loud from the outside right? and barely noticeable inside the cabin

[SLAMMED_C]

does your a/c blow cold.. or just cool? maybe a/c compressor cycling casue you are low on freon. or as you have mentioned it can be a tensioner. I believe you wont find a belt tensioner shock.. our W202 Kompressors dont have them. just the tensioner.
I have heard of the tensioners going. may want to keep an eye on it for a lil longer to make sure its the tensioner.

[xvvvz]

Yes, thankfully the air-co is still nice and cool. I am just hoping the problem is not the a/c compressor. Refrigerant was just filled up in December after the dealership found a leak in the a/c system and replaced the dryer, including a full evacuation, leak test, recharge and a few other parts. I will take another look in the daylight to see if I can find the sight glass to see if it can give me an indication on level of refrigerant.

Thanks.

It just seems odd that the tensioner would go out at only 31k miles but would rather have it be that than the a/c compressor.

Anyone else with thoughts or ideas? Nobody else is familiar with the location of the "collar nut"?

[SLAMMED_C]

Originally posted by xvvvz
Yes, thankfully the air-co is still nice and cool. I am just hoping the problem is not the a/c compressor. Refrigerant was just filled up in December after the dealership found a leak in the a/c system and replaced the dryer, including a full evacuation, leak test, recharge and a few other parts. I will take another look in the daylight to see if I can find the sight glass to see if it can give me an indication on level of refrigerant.

Thanks.

It just seems odd that the tensioner would go out at only 31k miles but would rather have it be that than the a/c compressor.

Anyone else with thoughts or ideas? Nobody else is familiar with the location of the "collar nut"?
Ive seen a 1999 C230K with a faulty front hardy disc at 40,000km.. so anything is possible!

[xvvvz]

>>Ok, I can determine where the stud bolt is but does anyone know where the "collar nut" is on the tensioning device??? Is it under the cover on the front of the pully located on the tensioning device?<<

For those who in the future read this thread, it appears the collar nut is under the black plastic cover. You will probably want a closed/box wrench that has the end angled to reach a recessed nut. A deep (but unfortunately not normal) socket will reach it too, but the advisory says to keep counter pressure on the torx stud, so that means you have to use a combo of torx socket and wrench.

Be advised, the plastic cap is a bitch to get off and you aren't going to get it fully seated back on without removing the fan to get access to bang it with a rubber mallet.

[xvvvz]

>>faulty front hardy disc <<

What is a "hardy disc?" Same as a flex disc?

[jnenad16]

you said the dealership recharged the ac? the first thing that comes to my mind would be that your ac is low on oil. check the part of the invoice where it says that the ac has been refilled. does it state how much freon has been used and how much/what type of oil was added. every time you evacuate and refill the ac, you should add 1 ounce of oil to it. every time you replace a major part, such as compressor or evaporator, you add a full recommended dosage of ac oil to the system, usually about 4 ounces, depending on the system's size. if all this checks out, then you turn your attention to the compressor clutch. you will need a second person helping you by turning the ac on and you placing an end of a long screw driver or a metal rod against the compressor and other end holding in your hand and put your ear against it and listen for the noise. if the noise isn't coming from the compressor or the clutch, you do the same for the tensioner, but be careful not to touch the belt or any other moving parts. this way you'll be able to hunt down the problem more accurately.

nenad

[xvvvz]

>> check the part of the invoice where it says that the ac has been refilled. does it state how much freon has been used and how much/what type of oil was added.<<

Here is what it says:

"Leak found at rec dryer, recover freon, rplc rec dryer & switch, evac and chrg system, retest, all normal"

Here are the parts used:

202 830 00 83 Tank, AC
140 830 00 72 Temperature
140 997 06 45 Seal Ring. VL
140 997 11 45 64 Seal Ring
702 Frigi-Quiet (quan = 2)
R134a Freon (quan = 22)

Is Frigi-Quiet the oil?

Can I somehow manually check the oil?

If it is the clutch, will a shop replace the a/c clutch by itself or do you have to get the whole a/c compressor?

Thanks for the help!

[jnenad16]

I dont know what frigi-quiet is. it might just be an additive. ac oil is usually labeled as pag-90 or ester. there is no way of manually checking the oil amount in the system. the clutch can be changes separately from the compressor, but dealer tries to sell you the whole assembly, so you might want to be firm with them on that.

nenad

[cbswartz]

My C280 Sport had a loose plastic shroud behind the grill that was making contact with an auxillary fan, that only seems to run with the A/C. Could be a possibility...