"91 300CE - A/C problems - vent temp air then cold air?? - PeachParts Mercedes-Benz Forum

rgnprof · #1 05-29-2009, 03:59 PM

OK, I bought this car today and driving it home the air didn't get cold until after 15 - 20 minutes or so. I drove it around quite a bit yesterday and I noticed it going to vent temps a couple of times, but for the most part the air was pretty cold. Today, it is blowing a kind of smelly vent temp air. Checked the pressures once I got home - High side 270, Low side about 40 - at 1500 rpm and about 90 Deg ambient temp. So, it's not a gas problem - anyone have any hints?

ryan

suginami · #2 05-29-2009, 04:25 PM

There are 3 rubber diaphragms behind the dash that open and close, and they modulate their position to control heat and a/c, which vents the air blows out of, and fresh / recirculation position.

One of these might have a leak.

Now let me get on my soap box on how you should run the HVAC system:

If you have any button on the HVAC panel pressed other than the EC button, the a/c is always "on". It functions this way by design.

On W124 cars, it is best to not set the wheel in the fully hot or fully cold position, and also to not set the fan blower speed to continuously on "high" or "low", but rather use the "Auto" button.

Drivers have the feeling that full heat or full A/C will get the passenger compartment to the correct temperature fastest.

This doesn't allow the system to work as designed.

To get the longest life from the automatic climate control system, allow the system to exercise all components.

If the temperature inside the car is more than 4 degrees away from the setting on the temperature wheel, the system will use full heat or full A/C regardless.

As the interior temperature approaches the setting at the climate control panel, the automatic functions will slowly reduce blower speed.

Allowing the system to work in its automatic mode, with all components functioning, will prolong the life of the system.

When flap controllers and valves (vacum operated) don't move for long periods, they tend to stick in position and ruin the component.

rgnprof · #3 05-29-2009, 04:35 PM

Where are these rubber parts at? Do you have to tear out the dash to get to them? Any diagrams that might help?

Ryan

suginami · #4 05-29-2009, 06:55 PM

Quote:

Originally Posted by rgnprof

Where are these rubber parts at? Do you have to tear out the dash to get to them? Any diagrams that might help?

Ryan

Behind the dash, and yes you have to take out the dash.

I'm sure pics have been posted on this forum if you're good with the search function.

rgnprof · #5 05-31-2009, 09:14 AM

OK, I don't think it is a vacuum problem. I have been doing extensive reading and have reached a tentative diagnosis - bad monovalve, or what is actually called the heater valve in this car.

The suction pipe is always cold, even when the vent temps are warm, and the heater hose that runs into the cabin at the driver's side firewall is hot. The problem is sporadic and seems to increase under load - if you sit and idle the A/C generally kicks in. It seems that hot coolant is counteracting the effects of the A/C system. A sticking valve, or some other electrical problem is the only thing that makes sense to me giving the sporadic nature of the problem. All of the vacuum stuff seems to be working right - if I push DEF then the air blows out the defrost vents, the Bi-Level setting works right and if I switch the temp wheel to 85 deg, then the floor vents kick in and no air out the center vents.

I suppose it could be the compressor cutting out - slipping belt or a temp sensor, but I haven't checked that yet??? It is harder to tell in this car when the compressor kicks on and off....

What do you guys think? I should also point out that this car has not been driven much in the last several years. I know for a fact that in the past 2 years, the PO put 400-500 miles on the odometer.

ryan