A/C Diagnosis

[cbreez]

Hi,

I'm looking for some guidance on pinpointing exactly what the issue is with my A/C. I've reached the point where I need the help of those most familiar with this chassis in specific.

Some quick details on my predicament, the car is a 1994 E500. I've verified that the climate control is working and that my compressor is in fact turning on when the climate control is set to the proper settings. I've extracted the refrigerant and added dye to system when I recharged it, to pinpoint a leak, if there is one. No leaks detected thus far.

I should note that the system had the correct amount of refrigerant in it before I started this process. Under no circumstance do I get any cold or even cool air for that matter out of any of the vents. The air coming from the outer vents on the dash are at a significantly higher temp consistenly than the center dash.

Any thoughts or guidance is appreciated. I'd like to try and repair this on my own before I'm forced to turn this over to a shop. This is peak driving season here in the northeast and I'd like to make the best of it, but without A/C that wont happen.

 

[gsxr]

Are you getting heated air out the dash end vents? Meaning, air temp noticeably above ambient? If so, your monovalve may not be energizing... this allows hot coolant to flow through the heater core, counteracting any cooling done by the AC.

If the monovalve is closing and you have no heated air, and the compressor is running, the next diagnostic is to connect manifold gauges to low AND high side. See video below, your pressures should be similar-ish... under 40 low side, between 150-250 high side, varying as the compressor cycles on/off.

 

[cbreez]

Are you getting heated air out the dash end vents? Meaning, air temp noticeably above ambient? If so, your monovalve may not be energizing... this allows hot coolant to flow through the heater core, counteracting any cooling done by the AC.

If the monovalve is closing and you have no heated air, and the compressor is running, the next diagnostic is to connect manifold gauges to low AND high side. See video below, your pressures should be similar-ish... under 40 low side, between 150-250 high side, varying as the compressor cycles on/off.

Thanks for the quick response. I have not determined that. I’ll try to check this out tomorrow and report back.

 

[gerryvz]

In addition to measuring system pressure, have you pulled a vacuum on the system to see if it is tight and holds vacuum? This will tell you baseline whether you have a leak or not. The dye will just tell you (hopefully) WHERE the leak is.

 

[Alphasud40]

Another quick and dirty method of checking the operation of your A/C refrigerant system is to grab onto the suction (larger of the 2 pipes from the compressor). A system that is working the suction line will be cool to cold to the touch and if you are running in a warm and humid environment condensation will be found on the suction line.

 

[cbreez]

Are you getting heated air out the dash end vents? Meaning, air temp noticeably above ambient? If so, your monovalve may not be energizing... this allows hot coolant to flow through the heater core, counteracting any cooling done by the AC.

If the monovalve is closing and you have no heated air, and the compressor is running, the next diagnostic is to connect manifold gauges to low AND high side. See video below, your pressures should be similar-ish... under 40 low side, between 150-250 high side, varying as the compressor cycles on/off.

I think this might be the issue, the air coming out of the center vents seem to be cooler air. The air coming out of the end vents are definitely heated when compared to the center vents or even ambient temperatures. Now to locate and replace the monovalve.....

 

[gsxr]

I think this might be the issue, the air coming out of the center vents seem to be cooler air. The air coming out of the end vents are definitely heated when compared to the center vents or even ambient temperatures. Now to locate and replace the monovalve.....

Monovalve is located near the CAN box. There's an early & late style and the break point is during 1994 USA model year production. The VIN break in the EPC is wrong... if you need to replace the monovalve, you'll have to eyeball it (take a photo, post on the forum) to verify which one you have. Also look in your driver door jamb for the production month/year, that will help too.

 

[cbreez]

Monovalve is located near the CAN box. There's an early & late style and the break point is during 1994 USA model year production. The VIN break in the EPC is wrong... if you need to replace the monovalve, you'll have to eyeball it (take a photo, post on the forum) to verify which one you have. Also look in your driver door jamb for the production month/year, that will help too.

I was just reading your response to a someone's write up on the replacement of it. I'm going to determine if I have the late model one, which I'm hoping so as my car is a 1994. Dying to get this fixed so I can enjoy the car more.

 

[cbreez]

Based on the post on this thread and some of the other threads here I’m going to start with the monovalve. This evening ambient temp was 92*, temp out of the center vents 78*, and the end events were 127*. Also appears I have the late model unit as research has indicated the 4 hose unit is the later model unit. I’ll report back once I’ve sourced the part and have installed it. Thanks for the help so far!

 

[cbreez]

Thanks to everyone for their guidance! I was able to source the monovalve aka heater control valve within a few days and installed it this evening. Immediately noticed the difference from the moment I turned the system on. I went for a 20 min test drive and got consistent cool temps throughout all the vents. The repair was a success. My car luckily did have the late model unit so I was able to find a brand new one. Part #001-830-25-84. Side note Mercedes online parts stores had it listed for $412, pelican parts has it discounted to $329, I found a MB dealer selling a new one on eBay for $287.

Now on to the newly discovered power steering leak.