Mercedes-Benz Forum banner

1 - 9 of 9 Posts

·
Registered
82 300D
Joined
·
111 Posts
Discussion Starter · #1 ·
1982 300D.

I have posted about this before, but the mystery still continues.

The ACC unit makes a loud clicking sound. When at idle, it is not too bad. When accelerating it is quite loud. When up to highway speeds, it is much quieter or goes away entirely.

I have read, and been advised that the clicking is coming from the switchover valves that are located behind the acc unit. I replaced one that I could feel airflow from the hole in the bottom. The clicking still continues.

Thinking that the sound is caused by a bad vacuum leak, I now disconnected all 6 lines that go to the 5 HVAC pods and plugged them, thinking the pods are the most likely source of the leak. Also when tested with mighty vac, the pods do not hold pressure.

But the clicking still continues. The sound is absolutely not coming from the switchover valves, but is coming from inside the HVAC unit itself. I have also replaced the unit as well, thinking the original was bad. The both make exactly the same sound, indicating that neither is bad, and the sound is caused by something else impacting the HVAC unit, or the unlikely possibility that both HVAC units have the exact same thing wrong with them.

Please make the noise stop! Think of the children! Can anyone help?:crybaby2::crybaby2:
 

·
Registered
82 300D
Joined
·
111 Posts
Discussion Starter · #3 ·
When it is doing it (which is most of the time) it is essentially constant, like bacon frying.
 

·
Registered
1982 300TDT 150,000 miles 1985 380SE 130,000 miles 1991 560SEL 81,000 miles (a/k/a the nightmare)
Joined
·
1,993 Posts
This is my.02:

You've already switched PBUs to no avail, and have changed one vacuum switchover valve. If the clicking is absolutely part of the HVAC system, so to speak, even though you've disconnected things, plugged them, etc, I'd recommend checking all the other switchover valves. (They're electrically operated, and if you're absolutely sure your PBUs are not at fault, the source of your frustration would most likely be another switchover valve... or loose/broken connections of some sort... how shaky is your diesel? Additionally, I'd think it's the switchover valves since when vacuum is building during acceleration the clicking is quicker/more pronounced. vacuum related elements are notorious for dying out in close succession within various systems. You could also try to figure out which vacuum valve it is by trying the different functions of the HVAC.)

Now, I'm under the impression that one must do a bit of dismantling to get to the switchover valves. To make your driving and maintenance experience more enjoyable, I'd do the following:

Replace the diaphragms for the pods... contact George Murphy for replacements. Cost: ~$60.

Replace the switchover valves.

Replace all the rubber connectors and check valves.

Replace the insulation tubing for the temperature sensor. You can just get a piece of pipe insulation from Home Depot or something, and then use a good spray adhesive to mount it inline.

While you're at it, just go ahead and replace the brushes for the blower motor, and do a full integrity check of everything that is now exposed while you have the dash out. And to make sure your aux water pump (if one is installed in your car) isn't frying out your PBUs, check it, then as maintenance, take the white plastic cap off the end of the unit, and lubricate it with fresh, clean ATF.

I know this is probably not the most economical way of taking care of what seems to be a simple problem, but if you think you'll be driving the car for some time to come, you might as well go ahead and approach the project with a high calibre auto rather than a slingshot.

Also, I recommend this course of action as the owner (or rather, son of the owner) of a W123 wagon that I've done quite a bit of work to. As it pertains to this problem, I have a similar one, and as I have checked electrical integrity of HVAC units, and have gone ahead and rebuilt the vacuum system in its entirety (i.e. every vacuum connection or valve (yes, really, it needed just about all of them done)) I'd rather not have to revisit any vacuum problems for a good 20 or so more years...
 

·
Registered
82 300D
Joined
·
111 Posts
Discussion Starter · #5 ·
The switchover valves are not difficult to get at. Is there a way to check the switchover valves before I go ahead and replace the remaining 4 that have not been replaced?
 

·
Registered
1982 300TDT 150,000 miles 1985 380SE 130,000 miles 1991 560SEL 81,000 miles (a/k/a the nightmare)
Joined
·
1,993 Posts
I think you can just check them by connecting leads to the proper point off the battery (This should be done with the valves disconnected from the system. I'm no expert, but you can do a search to find more comprehensive data. As a very pertinent aside, I have read that those switchover valves have a tendency to click when they go bad, as well as well there is a vacuum leak to the line(s) supplying them. If you know your diaphragms are leaking, you should probably just change them out now, along with the other vacuum related items that will probably fail in line) ... offhand, I don't know what is pos or neg (Or whether you'll need a Mityvac to supply vacuum to the valve while testing), but I think you can try them out either way, and you should be in little danger of ruining them before their time.

However, as a matter of convenience, most people switch out all of the vacuum valves at once, thinking it a "while you're in there" procedure. On this matter, I have opted for amassing replacements for all of the switchover valves for when I finally get to operate on my own system. Additionally, as I alluded to, since I've gone to the trouble of checking all the vacuum elements in the car, and I know there are vacuum leaks under the dash, I am rebuilding the entire vacuum system under the dash when I proceed, and the parts are on hand. I'd rather do the entire thing now than have to revisit the issue. That's just how I'm approaching this car of mine, please excuse me if you believe I am suggesting that this is the only course of action for you. I figure that with all of this work done, the only thing I might have to worry about in the future is the Evaporator or Heater core. As a note, I have ordered the washers pertinent to the heater core as well, just in case, as they are cheap.
 

·
Registered
82 300D
Joined
·
111 Posts
Discussion Starter · #7 ·
I have got the ACC to quict clicking. Here is what I did, but also uncovered a new problem. First I disconnected all of the lines to the pods and plugged. That did not do it entirely. The I capped of the line going to HVAC in the engine compartment. That did it. I then replaced the diaphragm on the pod right behind the glove compartment. I couldn't get a george murphy diaphragm, so instead I used a balloon around the original diaphragm, sealed off with a zip tie. I hooked up the main vacuum line and it worked, and held a vacuum. I then hooked up the three other pods that hold vacuum, leaving the one on the drivers side (Defrost?) disconnected as it does not hold vacuum.

Now when I operate the ACC and turn on the air (which doesn't blow cold anyway) i can hear a much fainter, very regular clicking sound coming from where the switchover valves are. I am assuming this is the clicking that I have read most about. Faint enough, that with the radio on you can't really even hear it.

However here is the new problem. When inspecting the pod that controls the register to the feet I noticed that There is a hard line the stops about 2 inches (sorry 5 cm) above the floor. Looks like there once was some soft material that bridged the gap between this hard line and the hole in the floor. Now I can feel air rushing out the hard line. What is this?

Also, the pod conrtolling air to the feet still does not open the flap to the feet, but it holds vacuum (testing simply by pushing in, then pluggin hole, and it stays pushed in until I release my finger over the hole where the vacuum line attaches. Does this indicate a bad switchover valve, or bad ACC unit?
 

·
Registered
1982 300TDT 150,000 miles 1985 380SE 130,000 miles 1991 560SEL 81,000 miles (a/k/a the nightmare)
Joined
·
1,993 Posts
the switchover valve might be worn, but you must stem the vacuum leaks around it to make sure. Additionally, while you've got the PBU out you can take a look at its innards to see if the board looks burnt at all... you can also do this with the amp behind the glove box... if you ever figure out how to get that thing out.

Did you figure out exactly which switchover valve is clicking? Cause if you did, simply try it in another position and swap a known good one in its place, then you can figure it out from there.
 

·
Registered
82 300D
Joined
·
111 Posts
Discussion Starter · #9 ·
NO haven't figured out which one as of yet. Need to wait until I have a long weekend so I can tear it apart and be without a car for a couple of days. Any idea on the hard line going to the floor carrying a high volume of air (between the firewall and the "foot" HVAC Pod)?
 
1 - 9 of 9 Posts
Top