feedback

[mr. gatsbury]

at first i thought it was the radio but recently i have this high pitch noise, sounds like microphone feedback. It seems to start when i slow down and even when i stop (not all the time) and when i accelerate it goes away.... has anyone had this problem before??? I can't get out and pop the hood fast enough to see where it might be comming from....

thanks in advance,
john

[benzfan]

Does the pitch rise and fall with the engine rpm? Does it happen with the stereo off? If only with the stereo on, is it only when playing CDs?

[neema12]

If the issue is as benzfan stated, then it sounds like an alternator wine. Check your grounds. Did you install anything recently?

[mr. gatsbury]

i didn't instal anything recently it just started happening out of nowhere. I happens with the radio off too though. But it's wierd like a high pitch frequency. Microphone feedback was the best way i could describe it...

john

[benzfan]

I've seen it before coming from the speakers, and it turned out to be the CD player connection in the trunk being loose, which is why I asked about the rising and falling of pitch. What you describe can be caused by several of the things that are driven by the accessory belt or even the belt itself. To isolate it, I would suggest buying a mechanic's stethoscope. The one by Lisle is quite good and cheap and you can probably get it at any auto parts place ($29CDN for the one I have-invaluable tool). A socket extension held up to your ear is nowhere near as good. You need to listen to all the components driven by the belt individually. Start with the belt tensioner, as the pulley bearing can wear out, then try the alternator (same bearing issue), then try the water pump. A badly worn belt can sing loudly as well, particularly when the humidity rises such as on a rainy day for some peculiar reason.

[c55m8o]

this happened to a co-worker's Jeep Cherokee ... one day, his radio he had installed just started buzzing @ certain times. but when it was 1st installed, it didn't buzz (for months). won't go into everything tried. turned out being his sparkplug wires. They just sort'a gave out one day, and he started getting all this buzzing. not saying this is it in your case for sure; just something to consider.

[mr. gatsbury]

thanks for the info guys. it's the wierdest thing. I sounds like a noise that would come out of a speaker (it's that high of a note) it's not that loud. But the radio has been turned off and it still happens. Infact you probably couldn't hear it unless you were standing right next to the car when it was happening. Only problem with locating it is it never happens when i'm parked or stopped. I can't stop and get to it fast enough before the sound dies out. I'll try to figure it out but it's not gonna be easy...

thanks again,
john

[Grech]

I logged in right now just to post the exact same problem!

I have a '98 C200, bought 3 weeks ago. Every thing is in perfect condition. But 2 weeks back i started getting this problem. The sound used to come rarely before and used to go away quickly (just like your car), but from the past few days, its been really getting on my nerves as it plays constantly. Thanks to that i have been able to figure out a few characteristics of it:

1) The sound only comes when i turn off the A/C.

2) Only comes when car is idling or decelerating.

3) As far as my car is concerned, the sound is ONLY audible in the CABIN. Nothing outside. Nothing in the hood.

4) It's coming from the right front of the cabin, specifically from AROUND the place where the A/C grill and the tweeter are.

5) While the sound is on, if i turn the AC on, there is a sound of kinda a vent opening, and all of the sudden its gone. This particular thing makes me believe that the sound is mechanical rather than electrical. And it has nothing to do with the audio system.

I am really getting confused about this. Even when im freezing i still have to keep the AC on now so that i dont have to hear the stupid piercing sound. I will take the car to the mechanic, but that can only be done next weekend.

I hope the experienced fellow 202ers will find this post helpful in diagnosing this problem.

[benzfan]

What you are hearing is a vacuum leak, either from the actuator for the fresh/recirc air flap or from a fitting or hose associated with it. You didn't mention whether you are turning off the AC with the EC button or turning it off by shutting the fan down. If the fan is still on when you hear it, try cycling the recirc mode and if it disappears, the problem is likely the recirc flap actuator. I believe that when everything is shut off, the recirc flap is closed to outside air. Actuators for the individual air vents are more centrally located. I think there might be the main vacuum connection block for the ventilation system behind the glove box. Without actually pulling my cars apart I can't say for sure, but try taking the glove box out to have a look in behind and under the dash to see if anything is accessible or visible. Sometimes the connections of the hard vacuum lines to the distributor block will crack at the fitting.

I would think that you have an automatic climate control system that requires you keep the AC turned on all the time though. The system keeps the car at the preset temperature. MB used compressors that are variable displacement, and really need to turn all the time to keep seals lubricated. It is better for them.

[Grech]

Thanks a lot benzfan for your reply!! (Even though it might take me many readings to properly understand what you said, sorry im new to DIY stuff)

How i usually turn off my AC is by clicking the minus (-) button until it shows OFF on the AC screen. Thats what the sound kicks in. Yes i do have Automatic climate control, but i dont keep it on all the time.

Do u think it might be by any chance related to the cabin filter? I dont think its ever been changed, and for now thats one thing i can do on my own.

Thanks a lot again.

[benzfan]

Originally posted by Grech


Do u think it might be by any chance related to the cabin filter? I dont think its ever been changed, and for now thats one thing i can do on my own.

[/B]

Not possible, as what you are hearing is related to air movement, particular to times of high engine manifold vacuum (idling, decelerating), indicating a vacuum control circuit. There would be no air movement through the cabin filter when the fan is shut off. All your ventilation dampers are run using engine vacuum and there is a main distribution block up by your diagnostic connector under the hood, and a switchover valve block which I think is behind the glove box. These will have many hard plastic vacuum lines connected to them. Be careful with the plastic lines and fittings, they can break easily. All you want to do is try to track down the source of the actual leak to see if it can be repaired without replacement and you won't need to disconnect lines to do it. If you're really lucky, you might find a line disconnected.