Hi, my 1991 300E "hisses" when I have the climate control set to A/C (I don't think it does it on EC). It sounds like a sucking straw sound about every minute or so. It's not very loud, but is noticeable. I recall my 1985 380SE used to do this too. Is this normal? Has anyone else out there experienced this?

The climate control and A/C works great for the most part. It did shut off on me 1 time in the last 6 weeks - actually, it didn't shut off but it switched the fan to the lowest setting and turned off the compressor briefly, then took about a minute and a half to slowly build back up to normal A/C operation with medium fan and compressor cycling on and off. I'm interpreting this to mean that the pushbutton unit may need to be resoldered soon - can anyone verify this?

As for the hissing - is this normal?

Thanks in advance!

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

I, too, have a "hissing", but it is related to the gas pedal (although it might require the HVAC to be on...)

It seems that when the HVAC is on, whenever I give it a little pedal, it hisses.

I wonder if yours is tied to the gas pedal?

-David
92 500sel

Nope, mine is not related to the gas pedal. Actually, on the way home from work today I realized it isn't really occurring quite as frequently as I originally stated; more like once every 3 or 4 minutes if I recall correctly (it's hard to concentrate on these things and keep track while driving). It's a hissing noise that lasts about 2 or 3 seconds; starts out strong and then fades out. It is truly cyclical so it appears to be intentional. I'd like to know what it is...

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08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

Hey,
I noticed the same thing and I traced the sound to the AC expansion valve. On my 300SD it located behind the panel just above the gas pedal.

As for normal..as long as your getting cool air why worry?
Actually, I'm not sure if it's normal, I would asume that it is but who knows?

Danny

The hissing sound is normal. You will hear it whenever the A/C compressor cycles on...

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

Thanks for the input - I suppose as long as it's normal and isn't a sign of something about to go bust, then I'll just not worry about it. I think it's odd though - I've never heard that sound in any other car, just my Mercedes. Seems odd that Mercedes would use vacuum locks because they're quieter than electric locks, but would allow a noticeable hissing from the A/C?

Oh well...

__________________
08 W251 R350
97 W210 E320
91 W124 300E
86 W126 560SEL
85 W126 380SE Silver
85 W126 380SE Cranberry
79 W123 250
78 W123 280E
75 W114 280

The 124 models have the most vocal expansion valves - it's in front of the instrument cluster, just on the other side of the firewall. Other models have it buried deep under the dash (123, 126). I'm not even sure where it lives on 210 cars. Hopefully I'll never have to learn

My observation is they are louder when converted to R-134a or when low on charge. Cooling can be fine in either case, just more noise. If you have R-12 in the system, check the sight glass on the dryer to see if it's fully charged.

- JimY