Hi all,

New to the forums, but not new to my 1992 320CE

in fact, I've owned it since brand new



Story-

My C124 has suddenly developed a loud buzz from behind the glovebox,
after taking apart the glove box liner, i've located it to this part 1248001378.

Hoping someone here could tell me what this part does and which vacuum hose goes into each outlet- Google says its a Climate Control Air Diverter Valve, but I have no idea what its function is, since my AC still blows cold and I'm still able to divert AC to different outlets via the twist knob.

more to the story:
Since its a public holiday here in Hong Kong today, I've decided to drop by a scrapyard where they have a couple of w124s lying around to get me another one of this part to test it, replacing it without knowing if it works properly (i was just trying my luck). But unfortunately, the same loud buzzing noise persisted on the scrapyard part.



Would anyone here be able to shed some light to this problem because I haven't seen any posts online about this part failing?

The loud buzzing sounds exactly like a vacuum pump with a torn diaphragm.


I just wanted to make sure before I start throwing $177+shipping into replacing a part that isn't the problem.

I recently fixed a vacuum leak in my w202 that made an annoying high pitched squeal under the dash. But I don’t think that is what is going on here.

So your part is some kind of electrical switchover valve. It takes electrical signals from a controller (hvac) and connects or interrupts vacuum flow likely to flap actuators.

I would start by disconnecting the wires (one at a time if possible) and seeing if it stops. Also disconnect vacuum hoses one at a time and seeing if it stops. This can tell you if it is on the vacuum or electrical side.

My gut feel is it’s on the electrical side. Sometimes a bad driver circuit can make a solenoid buzz like on an old style doorbell buzzer. So your problem may lie in what is driving the switchover valve.

A good way to verify this is when it buzzes probe the wires (with a pin poking through the insulation) and check their voltage. Either with a meter or scope. You might see a jumpy signal. You should get 12v constant voltage or 0. That’s the way my w126 works. When these switchovers fail usually the coil opens. That’s why I think your issue is actually the electrical drive signal.

__________________
79 300TD “Old Smokey” AKA “The Mistake” (SOLD)
82 240D stick shift 335k miles (SOLD)
82 300SD 300k miles
85 300D Turbodiesel 170k miles
97 C280 147k miles