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Discussion Starter #1
I've posted a couple of generations of this story and not gotten much feedback or interest. But I think I've got enough info that people should have some ideas to kick around. Here's my history with the climate control on my '95 E420 (154k):

1) Almost immediately after buying car, climate control stops working. Blower strip fuse replaced, system then works 100%. Probably not relevant, but what do I know.

2) A few months later, I notice that heat--only heat---will not blow from the middle two vents as you sit in driver seat. Heat still comes from side vents and defrost.

3) One day, suddenly nothing blows at all. Hot, cold, nothing. Instead, I get this awful high-pitched whistling noise. Hard to say the duration of this condition, but I assume my system is out for 1.5-2 wks, and I test and get the whistling/non-function result at least 5 times.

4) On a whim, I give the air a shot, and suddenly it works. That's the current state, and it's now been that way for another 10 drives or so.

The only thing I've done during the last two stages of this timeline is put gas in the car and drive it around. That's it. I opened the hood once to make sure the strip fuse wasn't cracked again. My CEL is on, in case that's relevant.
 

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Warm/hot air should never come out the dash center outlets. Those outlets open only when the system brain determines that cooling is necessary.
Where is this awful high-pitch whistling sound coming from -- engine compartment or dash?
 

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The whistling sound could be as simple as a disconnected vacuum line :eek: or a cracked vacuum diagphragm inside your vent pod(s):crybaby2:.

#1 is easy to diagnose. Just follow the sound to the proximity of the noise and then diagnose with a vacuum gauge. Also inspect the yellow check valve by the brake booster. One of those is reponsible for the center vent.

#2 is a royal PITA since it requires dash removal, which requires seat, center console and steering wheel removal.

Hopefully you get lucky.
 

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Did you make sure the vents are open?

If they are closed down they make a horrendously awful high-pitched whistling noise.
 

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I guess hot air won't come out of the cener vents that's the way its designed it might blow intermittent but I have never seen air coming out in the center
Also if engine is cold and outside air is colder than the inside blower won't run in auto mode ( I guess smart design...) but once engine is warm you should see air
May be the blower contacts need cleaning down the road.
Keep us updated....
Cheers
 

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Discussion Starter #6
Yeah, I meant to add that heat still won't come out the center vents. Good to know that it's not supposed to. The whistling noise seems to be coming more from the dash--admittedly, I spent more time cursing than I did diagnosing :mad:

Vents are open. If the vacuum line were disconnected, how it would it get reconnected and suddenly work again? That might diagnose whistling but I'm more curious about why it worked, didn't work, and now works again. :confused:
Also, the whistling is NOT happening now that the air is functional again.
 

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The whistling may imply that one of your flaps isn't closing completely and you are getting high speed air move across it. Perhaps one of the vacuum pods is "stuck" intermittently.

You can test the integrity of the vacuum system by using a MityVac gauge on each of the vacuum lines coming out of the junction. The junction unit is behind the passenger-side dash just slightly towards the middle. It can be accessed by removing the glovebox in earlier models (easy) or removing the passenger-side airbag on the later models (more involving).

You can test the integrity of each vacuum pod this way and whether they are moving freely. You should also be able to test each conduit on the vacuum junction to see if the junction is leaky.
 

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check your intake manifold vacuum hose (feed line, grey colored) if you follow the line you will see several vacuum valves with it, be sure the rubber hose are not damaged.
 

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Discussion Starter #9
Alright, I've got some things to check out. Still, I wonder which of these is most likely to cause it to work, not work, work again. Several drives later the air is still working fine.
 
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