1998 mercedes 320, ml , a/c blows freezing cold air,then hot air - Benzworld.org

bevleblanc · 08-23-2005, 02:15 PM

The a/c on my 1998 ml, 320, is really having a problem. First it blows freezing cold air. After a while, it blows hot and cold air at the same time and then just blows heated air. The following items have been replaced: the compressor, high pressure hose,and the AAM.

Now they want to replace the fuse box. I have spent over $3,000 already. Neither the dealer nor the mercedes mechanic whom I have used for 18 years can find the problem. They just keep charging me. I am in need of help with this problem.

Bevleblanc@aol.com

Noodles · 08-23-2005, 03:36 PM

I would plead with the SM at the dealer to get MBUSA involved at this point. I can only imagine how much they will want to change the fusebox. On top of that I am doubtful that it would fix the problem.
I did a search and did not find a similar issue like yours at this forum.
I hope you get a swift and fair resolution to this problem seeing as it has already gotten so expensive and taken so long.

bevleblanc · 08-23-2005, 09:56 PM

Now they are taking it all apart and bleeding all the freon, dye and oil and cleaning, replacing the valve and drier and making sure that I don't need a condenser.

So far, this trip will be another $1300. Now, i am up to $4,300 to fix this problem. I do not thing that this will fix it.
Beverly

**cmitch** · 08-24-2005, 04:40 AM

Quote:

bevleblanc - 8/23/2005 5:15 PM

The a/c on my 1998 ml, 320, is really having a problem. First it blows freezing cold air. After a while, it blows hot and cold air at the same time and then just blows heated air. The following items have been replaced: the compressor, high pressure hose,and the AAM.

Now they want to replace the fuse box. I have spent over $3,000 already. Neither the dealer nor the mercedes mechanic whom I have used for 18 years can find the problem. They just keep charging me. I am in need of help with this problem.

Bevleblanc@aol.com

So let's think this through. You say it's blowing hot and cold air at the same time?? Are you saying that when you feel the air come out of the vents, you can feel hot and cold air mixed? That would not be possible if the fault were with your a/c itself. If your a/c compressor or other a/c components were malfunctioning, it would be one way or the other. It has got to have something to do with your air getting by something hot like the heater core.

The next step would be to find a more competent mechanic because it's obvious they do not know what they are doing if they replaced your compressor, high pressure hose and ... the AAM??? The fuse Box???! I'm really lost on that one but maybe somebody else knows something about that. The fact they didn't replace your dryer cannister when they replaced your compressor tells me loads about these guys.

I would be looking at the possibility of vacuum lines (if the ML uses them) or what else could be causing the hot air to pass by the heater core.

BTW, didn't benzboyz say something about a diverter flap in another thread about this?? That would make more sense to me than to continue to throw money at this with the same people working on it.

Methinks that you are being taken advantage of and I wouldn't stand for it!

geordie · 08-24-2005, 06:47 AM

This used to be (and still is) a fairly common problem on the W124s (1988 - 1995 300Es etc)
The problem was usually traced to a faulty Monovalve which diverts the coolant through the heater.
I don't know much about the M class HVAC system but it does sound similar. Have an EXPERIENCED MB tech check out the system.

Wolfgang · 08-24-2005, 07:17 AM

Quote:

geordie - 8/24/2005 6:47 AM
The problem was usually traced to a faulty Monovalve which diverts the coolant through the heater.

MLs don't have a mono valve, so at least that can't break. [:)] Here's a schematic for 98 to 01 MLs which might be useful for troubleshooting.

http://www.whnet.com/4x4/climate.html

jfreezn · 08-25-2005, 04:32 PM

Not mentioned so far is the little aux blower behind the glove box, that draws air across the in-cabin temperature sensor. If that has locked up (somewhat common) or become disconnected, then the sensor is sending the wrong command to the control chain. There will be large swings in cabin temperature due to the sensor (up by the mirror) not responding quickly enough. Failure of this auxilliary blower is often preceded by strange squealing noises coming from the dash.

Regards,

Jim in Phoenix

Wolfgang · 08-25-2005, 07:23 PM

Quote:

jfreezn - 8/25/2005 4:32 PM
Not mentioned so far is the little aux blower behind the glove box, that draws air across the in-cabin temperature sensor.

Added some pictures of the in-cabin temperature sensor, an NTC under the low range switch. Like you said there's a tube connected to the rear of it which draws air across the sensor. If disconnected the reading might become unreliable...