lexrex

Hi Everyone,

I sold my '80 450SL a few months ago, and am the proud new owner of a 1998 E320 4Matic sedan w/103k miles. I have a question about the air conditioning. When I turn on the A/C, I hear a hissing noise for a few seconds. Is this normal? I searched the archives and found someone who reported hearing a similar noise, but there wasn't any further discussion.

Thanks.

CHATMANR

lexrex;
Did you ever find out the source of the hissing?

dcoleman11

mine did the same thing. your low on freon.

lexrex

not anymore

RickyE430

So it was low on freon?

lexrex

yes, "hissing" coupled with a right/left temperature differential is virtually diagnostic of low refrigerant charge

lxl_Jeffro_lxl

What PSI was the high side of the freon line set to, if you know? I'm going to take my car to my schools auto shop and do it myself.....I would ususally fill it to about 220 PSI. But yeah, if you can tell me what PSI you filled yours to maybe I can it with a little extra freon. Do you know what it was previous filled to in PSI maybe like 90-100?

Racin_fool

why not just refill it with the correct amount of refrigerant? pull a vacuum on it and get the rest of it out and then put the correct amount in instead of trying to mess with pressures.

lxl_Jeffro_lxl

do you know the specs of freon needed for a 1999 E300 TD? I have yet to find it....good point about not bothering to mess with pressures, don't want to blow up anything!

lexrex

To do it right, you need to vacuum out all the refrigerant and refill with the correct amount (1 kg). Our cars have variable displacement compressors, which means that there is no way to know when the system is filled based on pressure. On a cool day, high side pressure may run 6-8 bar. On a hot day, it can be near 20. The high side pressure varies considerably depending on load. If you're insistent on recharging it by the seat of your pants, I'd suggest setting the fan to a low to medium speed and adding refrigerant until hissing noises disappear.

lxl_Jeffro_lxl

Alright I'll fill it up on Mon. and give an update on what I did! Thanks lexrex for the info.

Darkmann

It was probably me. I had the hissing noise in my 01 430 and it drove me nuts. It would hiss continously when I first turned A/C on and thereafter would hiss intermittently, almost periodic in nature. On hard acceleration on hot days, it would also hiss more. I took it in several time while in warranty and they said there was no problem - I couldn't believe it!!! A week after warranty expired, I took it in again and they found a leak in the line from the evap to the condenser. I was so irrate that I asked them to just refreon the thing and let me think about it. $320 later, it refilled with freon, no noise, but that was just temporary. I finally asked them to get the parts, called MBUSA and had a $200 discount on labor - coil and receiver drier were replaced. No noise since last summer!!!

lexrex

I'm rather amused... This thread gets more attention now than when I posted it >2 years ago.

michakaveli

IT's very easy to add refrigerant ot the system. First off getting a simple pressure guage is essential to measuring the high pressure end of the system. I found a usable unit @ Walmart. Then make sure to have a can or 2 of freon, depending on how low your system is. Then, on the climate control panel, set the diagnostic to the display that shows the refrigerant pressure, I forgot what itme it is, 7 I believe - On the gauge when adding the refrigerant you may see it go to the upper end of the safe zone which is ok. While adding the refregerant I always have the AC set to MAX cool while performing this process. If you add too much refrigerant you can always relieve the system. On the climate control panel I've found that the correct amount of refrigerant yields a pressure value, in the middle to lower teens BAR. When the compressor is first engaged after starting the engine and the car sitting the pressure reading shoudl reach the upper teens maybe low twenties. I have found from a prior experience that when the compressor engages and if the pressure value reaches 30, the compressor itself will cut-off. If this happens you know you need to relieve the system of some refrigerant.


Hope that assisten some in their quest of keeping cool :-)

michakaveli

Gotta let the thread simmer on low heat you know...

Darkmann

And you think this easy to do for anyone out there? You have to be joking. Unless you write up a complete procedure, there's no way any newbee out there can use this - they'll probably blow a hole through themselves before completing the job. Also isn't the some kind of law in the US regarding purchase/handling of these refrigerrants - I don't know, just asking.

BTW, I think he has a leak. I don't believe any amount of refrigerant added will permanently solve the problem - unless he subscribes to an annual refill program. These systems are not supposed to hiss under any condition, period. If they do, there's a problem somewhere.

michakaveli

That was the "Cliff Note" version :-)

The refrigerant is available @ Wal-Mart. Older R-12 I believe refrigerant is banned, well very difficult to get....

Depends on the age of the sytem, etc.... I have to add some refrigerant to my system, not a lot, but a top off is required. I can tell as the system doesn't cool down as promply and there is that audible "hiss"

lxl_Jeffro_lxl

Where are the two locations of the fill caps? I only see one near the right side of the intake...

michakaveli

High pressure is on the driver's side, near the shock tower area in the engine bay....

Racin_fool

depending on the age of the car, the locations for the 2 fill ports are in different areas. There is the one by the shock tower and the other may be down by the compressor itself, possibly down by the receiver/drier, or behind a small plastic panel infront of the radiator/condenser by the drivers side of the fans.

lxl_Jeffro_lxl

Found it thanks!