redlinegw

I just bought a very clean well maintained 2005 E320 CDI from the original owner and I am in CA so I actually had to use the A/C for the first time yesterday and while I was cruising down the freeway I noticed a hissing sound that seemed like it was coming from the dash behind the gauge cluster. The sound would come and go as I accelerated and decelerated. It is very annoying and quite loud. When I turn the A/C off is seems to go away. Anyone experience this and what was the fix?

N_Jay

Sounds like low Freon.
The expansion valve will hiss as it cycles if the pressure is low.


Have a good AC shop vacuum and fill the system and see if that fixes the issue.

redlinegw

The a/c is still cold, wouldn't low freon cause the a/c to not be cold? Or is this coomon for Mercedes?

N_Jay

When the system is just a little low gas can form on the high pressure side (normally all liquid) of the expansion valve and make a hissing noise.


It will still be cooling since it is just some gas and mostly liquid.


It is a common problem on almost every car with AC.

redlinegw

The hissing sound comes and goes as I accelerate and decelerate and is there even if the a/c is off and just the fan is on. As soon as I turn off the fan it goes away. It almost seems like a vacuum operated door or something is leaking. I have owned many cars thru the years and when they get low on Freon the usual issue is that it is just not very cold and the compressor cycles, I have never heard a hissing sound like this on any car I have ever owned but I have never owned a Mercedes before. I am taking the car to a good mechanic on Monday to have the rotors turned and I will have him check to see if the Freon is low.

N_Jay

If the noise is with AC off, then it is most probably is not the Freon. (Maybe I misunderstood your first post)


I have had this noise on VW, Honda, Chrysler and other cars specifically when there was slightly low Freon.

redlinegw

Still annoyed anyone else have this problem or have a solution?