Hello all.. I hear a loud hissing sound coming from the center vents in the car when the ac is on. Could this just be the sound of the Freon working it's way through the system, or one of the vacuum pods leaking. Has anyone had this problem or heard this sound, coming from the vents.. I am a little low #7 is at 6bars I know I need some more freon.. Has anyone added oil NOT from Mercedes How much did you add?

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Peter L. House
1998 Sport E430
Azure Blue
110,000k Miles

1987 Oldsmobile Cutlass Salon
V8 Bucket Seats
Factory Moon Roof

You cannot judge coolant level from the pressure reading. The system has variable pressure and it might only require 6bar to keep the evaporator at the proper temp. You'll need to look at the pressure readings when the system is in a high demand state for rapid cooling, and if it maxes at 6bar, then you have a problem.

If you have lost coolant (R134a) then you probably have a leak.

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John Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

Coolant is not = freon,coolant is Antifreeze for your engine cooling system,freon is for air condition system R134 or R12 is freon,you may have low on freon in your air condition system. hope is help.

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MB STARS Master Guild Technician,17 years MB tech
14 years independent shop owner

Actually, "freon" is also slang. I worked around HVAC engineers for years, and they referred to the refrigerant as "freon" "coolant" and other names. R134a is not freon, if you want to get picky about it.

I was not talking about engine coolant. I think that's clear in my post.

Also, I would like to know how you diagnosed the low R134a level? Simply by the high-side pressure reading? Do you know how the variable pressure systems work? It is not an acurate indicator of R134 level unless you know other factors.

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John Shellenberg
1998 C230 "Black Betty" 240K

http://img31.exs.cx/img31/4050/tophat6.gif

John
I not try to picky about you post,I am here just try to helping members to understand their vehicles. from my understanding in an automotive business coolant is referred antifreeze.....freon is freon they are two different things.in your HVAC engineers world may call it 'coolant' or other names .I just don't want members to misunderstanding between coolant & freon. I think most the members referred antifreeze=coolant.
thanks to pointing out coolant also call freon in different business. we learn somethings everyday

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MB STARS Master Guild Technician,17 years MB tech
14 years independent shop owner

MB Start,

Freon is the registered trademark for several different chlorofluorocarbons or CFCs - trademarked by Dupont.

As blackmercedes pointed out, the refrigent in question for the 1998 E430 is R134a.

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Zafar
94 E320 58000 Miles

Wow.. Can we get back to the questions... LOL.. The 6bar was from the ACC unit and I got a reading from the low side of 39. the compressor clutch was engaged while I got those reading.. I probably figure I need some Freon .. But what about the hissing sound coming from the center vents,,, Has anybody heard this..

Thanks

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Peter L. House
1998 Sport E430
Azure Blue
110,000k Miles

1987 Oldsmobile Cutlass Salon
V8 Bucket Seats
Factory Moon Roof

AutoZone loan gauge sets. The connectors are under the hood and under the front grill.

When mine made a hissing sound it was accompanied by a slight sweet smell (that was the oil). If you have a leak you can add dye, there are two drain tubes that go from the air handling box (the evaporator sits inside this box, the box is inside the dash) and drain to the outside of the car above the transmission. If your evaporator is bad you'll see the dye coming out of those drains.

Good luck!

Peter

I have found on these systems which use an expansion valve that loud hissing generally does correspond with low refrigerant level. In this circumstance the evaporator is generally colder on one side than the other. You may be able to observe this by checking the temperature of the heater core right & left sensors (#3 & 4).

That said, the 39 degree evaporator temp is spot on, though 6 bar (~90PSI) is quite low for the high side pressure. What was the outside temperature at the time? I'm guessing the a/c wasn't under much load when you read the values.

- JimY

The pressure I read from my gauge was 39 on the low side. Outside temp was 71. The inside reading was 6 bars..

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Peter L. House
1998 Sport E430
Azure Blue
110,000k Miles

1987 Oldsmobile Cutlass Salon
V8 Bucket Seats
Factory Moon Roof

Ah, 39PSI, 39degrees, what's the difference?

My recommendation is to wait until it warms up a bit more and see if the performance is adequate. You need more heat load to really test the a/c system. If it's not cooling like it used to, then top it off.

The only way to accurately charge the system in moderate temperatures is to evacuate it and then put in a full charge by weight.

- JimY

I have this same hissing noise. Did you ever figure out what it was?

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2008 S550
2010 GLK350
1997 E420 (retired)
2005 CLK500 (retired)

I would check for a vacuum leak around your a/c conrtol center.

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'02 CL500
'91 Lincoln Mark VII
'76 Corvette

If the left center vent is significantly warmer than the right center vent, accompanied by a hiss coming from these vents, you are most likely low on refridgerant HFC134a.

To check the fill without evacuating the system, you need to look at the charts in the air-conditioning system diagonstic manual. You'll need a good thermometer, hygrometer and air-conditioning pressure gauges to get the proper readings.

I continue to have some hissing coming on (probably expansion valve) ONLY when the AC is on, but ac seems to cool fine...
Without arguing about safety and the like...
How should I add refrigerant on the low side? Can upright, or upside down? Should I have a guage on both low and high side -- and if so what should it indicated when perfectly charged? What about those hoses that only provide a guage on the low side and show you whether you are in the right range?

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2008 S550
2010 GLK350
1997 E420 (retired)
2005 CLK500 (retired)