I have a E420 95. At 25mph my a/c registers show just above 50F with sustained movement. At 50mph+ it will go down close to 40F.
However at idle it will go up to about 90-100F. Outside temperature during this test is 90F+

I have tested for leaks and tested my R134a on a warm engine out of the sun. Its at 45psi, top end of the "full" range but not into the overfill range.

Both electric fans cycle on and off depending on engine temp which ranges between 80c-90c. Belt driven fan moves freely when engine is off and turns normal when engine is running.

I feel that I can eliminate the compressor due to it working properly when not idle. I do not appear to have a leak and also appear to have proper R134a. Airflow appears to be ok

What else can cause this? My controls when set to auto and 65 will throttle down the fan as the car cools off and they otherwise appear to control heat, cool, high/low/auto fan, etc.

I see a constant swishing about of R134a in the sight glass. Not a constant flow nor bubbles. looks more like a swishing flow

I am now thinking evaporator? Condensor? something within the dash? I am not understanding how it can blow perfect at driving speed and blow air as warm or warmer than outside air
at idle? I can see if the fans were not functioning properly and it was an air flow issue.

Any pointers would be greatly appreciated. Its approaching constant 100 here in Texas.

 

I found something on a search about Duo Valves possibly causing this? I am not trying to find/figure out what a duo valve is.

I played around with the controls and I do get heat out of the dash vents which one post said should not be happening because it would make the driver sleepy?

 

Hi,
Pardon me but I didn't undertsand your opening sentence.

"25mph my a/c registers show just above 50F with sustained movement. At 50mph+ it will go down close to 40F.
However at idle it will go up to about 90-100F. Outside temperature during this test is 90F+"

What / displays 50F? The A/C control panel? I not familiar with a 95 E but I guess it is be similar to other MB's and the display on the A/C controls should (I believe) display the temp you wish the unit to TRY to achieve. Are you stating that this display shows 50F whilst driving yet 90 at idle?)

or are you measuring the temperature of the air off the evaporator (the vent in the fascia) somehow?

If the system works some of the time and not others it is unlikely to be the evaporator or condensor (subject to no leaks)

However the evaporator control valve and the compressor control circuit can cause all sorts of problems. Likewise the vacuum operated flap valves on the AC unit (behind the dascia) can also cause problems. Hot air when you want cold etc.

Can you please try to clarify my understanding of the problem before i make a suggestion..
Stuart

 

I am sorry. I have an a/c thermometer in the center air vent while performing the test. The actual temp control wheel on the car is set at the coldest level (the dot on the blue section of the wheel)

I am hoping that it is not a major component but rather a switch or valve. What I am really puzzled about is how it is only present at idle. I went down the trail of thinking maybe a vacum line was leaking and thus when not at idle it works(higher rpm giving more vacum). However, I can sit in park and rev the motor up to about 2000 rpm and hold it for several minutes and that does not cure it. The car has to actually be moving. This brought me back to airflow being a problem. What has me puzzled there is that all 3 fans are working and I feel alot of air flow

 

I have more information with a brief diagnosis from an MB tech.

He quickly ruled out the major under hood components. He said at this point he would need to connect it to the diagnostic machine to see what the problem is but suspects a control and more particularly he was pointing in the area of the passenger side firewall saying there is a valve that controls the flow of hot water to try and maintain the temp you set on the dial.

I cranked the car in his lot and the center dash was blowing 40F. Within 5 minutes of sitting idle on this 95F day the dash was at 80F.

The fans were running, compressor running, site glass flow looked good and R134a was proper level. We drove down the street and back and he witnessed it dropping back to 40F at the dash within 2 blocks.

Is there a way to locate and test this valve without the diagnostic test? Any temporary fix? Price of the valve?

Thanks

 

Hi,
Everything you have added fits in with my own thoughts.
I believe that the problem will be the HOT control valve, or HOT distribution system as you state. I'm not sure how this valve is controlled i.e. electronically or thru vacuum. without looking in the manual.
Diagnostics is always a great way to get a lead on what's up but maybe as an alternative you could try clamping the HOT supply tube off temporarily between the dash and the engine (brake hose type clamp).
This will confirm that it is Hot bleed that is causing the problem.
You then need to decide whether it is the control valve or a distribution flap that is at fault. These components tend to be behind the dash and are therefore difficult to access.
The distribution flaps will be vacuum controlled. Are you sure you don't have a leak in the vacuum supply since this would show up more when the car is at idle rather than pulling load. Before changing the valve I would get a vacuum gauge and see what is happening to vac pressure as the problem occurs.
Stuart

 

If your '95 is similar to the W210 set-up, this sounds like a stuck duovalve. If your car has a duovalve like the W210 E-class, it should be easy to locate. On my '99 E430 it is located forward of the electronics panel on the rear passenger side of the engine compartment. Easy to distinguish, it looks like two metal cylinders about 3" in height sitting right next to each other. There should be a coolant line running into the base of the unit. With the engine COLD, remove the T10 torx nuts holding the upper portion of the valve on. GENTLY use a flat tip screwdriver or flexible putty knife to work the upper prtion of the valve free from the base. A small amount of coolant might drip out - no worry. Carefully clean the cylinders with a clean rag and lube lightly with something petroleum based like vaseline. Check the piston movement - there should be free movement against the springs. If not, clean these and lightly coat with vaseline. Hopefully that will solve your problem.

 

I saw the following description while surfing today. It appears to concur with my testing and what has been discussed in this thread. I will follow up with more results.

Q. I have a 1995 Mercedes E320 wagon with A/C problems. I read your explanation of the 300D Automatic Climate Control (ACC) and believe mine would be similar in the way it works. My systems compressor runs fine as it cycles with the temperature wheel. Air deflects to proper places when heating and vent. What it does is when A/C is needed it will only blow out cooled air through the center vents.


Outer vents seem to blow outdoor air. To further confuse me the system only blows the cooled air out the center vents when it is on low fan. If you set it on ACC (Climate Control) or on high fan it blows outdoor air through all vents. I would appreciate any input you might give me.

A. The door vacuum motors are controlled by the Switchover Valve Unit (Y7). The switchover valves, which are located beneath the dash panel on the heater box, connect the vacuum source to the various vacuum elements.

The switchover valve unit consists of a plastic vacuum manifold, which is fitted with electrically operated solenoid valves.

While the engine is running, vacuum is supplied to the switchover valve manifold. A vacuum reservoir supplements the system during periods of low engine vacuum and a check valve prevents system bleed off. When the operator selects a climate control function the push-button control unit energizes the appropriate electric solenoid of the switchover valve unit. Vacuum from the engine and or the vacuum reservoir is connected via the switchover valves to the corresponding vacuum element(s) opening or closing the flaps.

You can check the door operation by unplugging a vacuum line, one at a time, and applying vacuum to check each door vacuum motor for proper operation. If all the door vacuum motors are working, then the problem is probably in the ACC unit itself.

 

I located the valve which is inline for the heater. I guess my 95 E420 has a UNOvalve instead of a duovalve. It only has 1 3" cylinder on it but otherwise looked as described above. I cleaned and oiled the little stub(valve rod) so that it would depress and return more easily. It was sort of gritty feeling when going up and down. After replacing it and testing, it did not appear to solve the problem.

I will try pinching off the line tonight and see if that temporarily gives good results. If so, my guess would be the 2 wire connector which controls it may be sending a signal but it might not be listening. I could not hear it activing when having someone turn the a/c on and off while I listening. It might be to subtle of a sound though. The device does not appear to be costly. I might just replace it anyway. Closing off the hose and testing will hopefully confirm.

I am assuming the valve fails in the open position which might be why I am having the issue?

 

Hi,
Turning the A/C on/off will not cause the valve to move. However selecting Hot/Cold should ensure that the valve moves.
The reason for this is that the heater is used to warm chilled air during for instance the winter to provide defogging (dehumidifying).
OK?
Stuart.