buzzing noise on a 560SL
 

Hello All

when I first open either door or trunk lid on my 1987 560SL, I hear a very low volume buzzing noise coming from the vecinity of the rear stowage area behind the seats. It only lasts for maybe 30 seconds.

any ideas what could be causing it? I've only noticed it the last couple of days or so. I've only owned the car for about two weeks.

am thinking it might be a remnant of the car alarm this car apparently had at one time, the horn assembly is still in the engine compartment but I've not found or looked very hard for the alarm module itself. I've not managed to set it off accidentally so I think it's disabled.

dom chang

Hmm, it *sounds* like you have a vacuum leak and your vacuum pump continues to run, trying to get proper pressure to unlock the doors. I say this because you say it's coming from the trunk area, and happens right after you unlock the doors...
I would get in the car, be very quiet, and lock, then unlocking the doors, then try and find a hissing sound (like air rushing) to see which of the door actuators might be leaking. Are you having any other problems with your door locks? One not unlocking, persay, etc., etc.
Good luck!

found the source?
 

it's a module in the trunk of the car right next to the battery and under the jack, it's got several wires leading into it and at least one vacuum line. Am thinking it's the alarm unit for the car which has become defective over the years? It's got a rubberized cover on it and I've not taken the time to try and remove it to see what it looks like underneath.

anyone know how to disable the alarm unit so I can see if the buzzing noise goes away?

further on symptoms....the buzzing noise only occurs at night for whatever reason, I do not get the noise when first opening a door or trunk lid in daylight.

weird

Still sounds like a vacuum leak to me! My leak is worse at night also, something to do with the pressure I'm assuming?

do you know if that module/unit in the trunk, next to the battery is the vacuum pump/generator?

Your vacuum pump is in fact in the trunk. I am not 100% sure where, but mine is on the passenger side behind the trunk lining. I would think that is where yours is... is that the device you have found?

well, the location you're pointing to seems to match what I got, so if it's the vacuum pump, why the buzzing noise? and why only at night? and why does it occur by the simple act of opening up the trunk lid which far as I know has no vacuum lines involved when opened?

thanks

dom

Only at night? Probably because of the air pressure difference (??). It is probably happening all the time, just more pronounced at night time.
One of your door lock (or trunk) actuators is probably worn (has a leak in it). I currently have the same problem in one of my doors.

I am not sure if your trunk lock is electrical or vacuum... I know they changed about 3 or 4 years ago, but I'm not sure if it went to electrical from vacuum, or vice versa?

Do what I suggested earlier and try to see if you can hear a leak inside the cabin.

One more thing, is it much more pronounced on unlock, meanwhile, locking, is fine? I have the same problem, and this is because when it's locking, it's sucking the actuator (rubber, I believe) together, thus "sealing" the leak, while when opening, it blows all the air right out of the actuator, thus the continuing run-on of the vacuum pump (buzzing sound), in an attempt to equalize (or whatever it's doing) the pressure. But it will eventually time-out (shut off) after a certain amt of time.

Keep us updated! AND GOOD LUCK!!:eek: :)

my mistake, the trunk lock is vacuum controlled as well....not thinking when I wrote that last point.

yes, it's definitely a nighttime thing and the buzzing is definitely coming from the vacuum pump in the trunk, next to the battery's location.

so now I've got to find the leak. will try and check tonight to see if can hear the leak from either door lock location perhaps.

Don't forget the fuel door too Dom. It has a vacuum element as well (though not likely bad).

Mike,
VERY GOOD POINT!!

Domchang, of course, remember there is a vacuum lock to the fuel door!

well, spent about an hour locking and unlocking the doors which would cause the buzzing noise to come from the vacuum pump in the trunk of the car....so that's definitely the source, could not find a single point where hissing leakage of air could be detected by my hearing.

what vacuum lines I did find that were related to the locks seemed in fine shape.

am thinking it's the actual actuator mechanism for the vacuum lock on the passengers side door that is not functioning correctly. I guess I'll put it on the list of things to be fixed when I take it in for some other items that I am putting off till the spring.

thanks all for your feedback, will just have to lock the doors from the passenger side for the time being.

a further and final note:

I've resolved the vacuum locking problem for the door locks on this 560SL, details in different thread. Turns out that thread and this one were inter-related.

The buzzing noise is most evident when one unlocks the doors, not when one locks the doors. At least when locking it appears much more muted.

The locks work so for now am going to adjust to living with the buzzing noise when getting into the car, at least it times out pretty quickly.

No other vacuum related problems that I've noticed

Thanks very much to all who responded and helped.

dom

followup
 

turns out to have been a vacuum leak in the vacuum element on the driver side door, causing the pump to try and make up for the leak until it timed out. Once I plugged the hose, the buzzing noise which was the pump working, stopped. Replacing the door vacuum element fixed this.

Glad you found the leak. I had a similar problem on a VW with vacuum locks where the vacuum pump would run for 2 minutes before timing out. All locks worked normal and there was no vacuum leak. Took pump apart and found bad solder joint on printed circuit board at switch relay. Re-soldred joint and fixed problem. Just thought I'd mention it in case someone gets a problem similar. Thanks for telling us the solution to your problem. We all learn a little more when someone fixes something.