Hey guys!!!

My head is swimming here! I purchased a great 1983 300SD and after repairing EVERYTHING that needed fixing I finally decided to tackle the inoperable air conditioning system.

I replaced the air conditioning compressor, the expansion valve, the receiver drier, flushed the system out, vacuumed the system down (it held, no leaks) added oil and charged the system with 3 lbs of freon. I also followed the directions from the link referencing the resoldering of cold solder joints in the Climate Control Unit!

That said, the compressor kicks right on and the suction line gets cold as does the high pressure and condensor get hot...The low and high side pressures are right on the money. After about 15 minutes the ajuxillary fan kicks in......The monovalve has voltage on it.......My problem is that there is NO COLD AIR BLOWING INTO THE CABIN!!!!! I really am at a loss here, could it be that a certain flap or two is not opening????? I do know for a FACT that there are vacuum leaks on the vacuum side of the Climate Control System.....But, shouldn't I at least get a LITTLE cool air??????

I am at my wits end, so ANY replies would be more than appreciated!!

Thanks, a lot guys!!! seriously!!!

__________________
Currently Driving
2006 E320 CDI
1999 E300 Turbo Diesl
2002 ML500
1995 E320 Station Wagon


MBs I've owned
1997 E320 Assassinated by Pine Tree
1987 300E Wife Killed Engine
1981 300D Stretch Limo Total Loss
1970 250 Coupe 212,000 mi.
1974 450sel 184,000 mi.
1974 240D 377,000 mi.
1977 300D 204, 000 mi.
1979 280se God Only Knows!
1983 240D 130,000 mi.
1972 220D 280,000 mi.
1983 300SD 244,000 mi.

How old is that monovalve?

If it is broken it could be letting hot coolant into the heater core and making your cold air warm or hot.

Just how hot is the air? or is it more of an ambient temperature?

Are you saying it blows really hot, or the blower is not blowing at all, or the air is the same as the outside temperature?

Thanks for your replies, guys!!!

I get ambient to warm air out of the vents, it's intermittent and when the "DEFROST" mode is activated I get HOT air out of the vents under the front windshield.

__________________
Currently Driving
2006 E320 CDI
1999 E300 Turbo Diesl
2002 ML500
1995 E320 Station Wagon


MBs I've owned
1997 E320 Assassinated by Pine Tree
1987 300E Wife Killed Engine
1981 300D Stretch Limo Total Loss
1970 250 Coupe 212,000 mi.
1974 450sel 184,000 mi.
1974 240D 377,000 mi.
1977 300D 204, 000 mi.
1979 280se God Only Knows!
1983 240D 130,000 mi.
1972 220D 280,000 mi.
1983 300SD 244,000 mi.

Try this. If you get cold AC on initial start up of the engine (engine cold and has not been run all night etc.), and then you get hot air, it's definitely the monovalve. I had this problem, and as the engine started warming up, and hot water moved in the system, then hot air overwhelmed the AC cold air and I got hot air. I replaced the monovalve assembly, and it solved the problem. Part was about $30 and change.
Gene

The condition of hot air out of the defroster nozzles is normal. The system uses both the compressor and the warm coolant to provide hot and dried air to the windshield.

If the condition persists in other modes, either the monovalve is faulty or the CCU is faulty.

Make sure you try the system with the temperature dial at the full cold stop and see it it improves the situation.

A bad ETR switch can cause that condition, also.

Hey Tango!!

If the ETR switch was defective, wouldn't it inhibit the compressor from activating???? My compressor stays on and cycles.....though, I must admit that I have yet to see it cycle off, even when the auxillary fan is running for over 15 minutes!!

__________________
Currently Driving
2006 E320 CDI
1999 E300 Turbo Diesl
2002 ML500
1995 E320 Station Wagon


MBs I've owned
1997 E320 Assassinated by Pine Tree
1987 300E Wife Killed Engine
1981 300D Stretch Limo Total Loss
1970 250 Coupe 212,000 mi.
1974 450sel 184,000 mi.
1974 240D 377,000 mi.
1977 300D 204, 000 mi.
1979 280se God Only Knows!
1983 240D 130,000 mi.
1972 220D 280,000 mi.
1983 300SD 244,000 mi.

Hey 300DGuy and Brian

This morning I will full blast the AC while the engine is still cold to see if it makes a difference, I will post my results shortly! While I'm out there, is there a maximum voltage that I should be looking for at the monovalve, as to insure that it's completely closed???? I am an electronics tech so I fully understand the concept of pulse width modulation/modification...

__________________
Currently Driving
2006 E320 CDI
1999 E300 Turbo Diesl
2002 ML500
1995 E320 Station Wagon


MBs I've owned
1997 E320 Assassinated by Pine Tree
1987 300E Wife Killed Engine
1981 300D Stretch Limo Total Loss
1970 250 Coupe 212,000 mi.
1974 450sel 184,000 mi.
1974 240D 377,000 mi.
1977 300D 204, 000 mi.
1979 280se God Only Knows!
1983 240D 130,000 mi.
1972 220D 280,000 mi.
1983 300SD 244,000 mi.

I'm not sure of the voltage for a closed valve, but, I'll take a guess that it's close to 12V.

However, if you can reach the hoses that connect to the valve, attempt to determine if the valve is blocking coolant flow into the heater core. If the valve is not fully closing, and you have sufficient voltage to the valve, then it's the likely culprit.

It's not that fancy, just 12V to close and 0V to open. If you set the controls on "max cold" you should see a steady 12V at the valve (to hold it closed). At intermediate temperatures, the control power will cycle between 0V and 12V to maintain the desired temperature. You can hear the monovalve click when it cycles open or closed. It is common for the valve internals to be damaged (torn diaphragm) so that it does not stop the coolant flow to the heater core. Remove the top of the monovalve (4 screws) and take a look.

A bad ETR switch could prevent the compressor from engaging.
But what commonly happens is that the ETR gets "out of cal." It will allow the compressor to run for a few minutes. A/C will work fine for 3-4 minutes, then the switch opens at a much-too-high temperature and disengages the compressor, say at 55 degrees instead of the spec 36 degrees.

I am not suggesting that the ETR switch is the cause of your particular problem. Just that the test prescribed in post #5 might not be as definitive as suggested.

It's not supposed to cycle. The low-pressure switch is on the high-side line and only prevents the compressor from starting if the charge is low. Once the compressor starts, the pressure goes up in that line and the switch stays closed.

If you replaced the switch, the replacement will include a 30-bar cutoff switch (the original didn't), but that should only very rarely activate, if ever.

If the compressor is running and you get high pressure in the high side, low pressure in the low side, and no cooling, it could be the expansion valve.

Ok guys, I took your suggestions and here are the results:

I started the vehicle (engine cold) with the AC on and let it idle for a few moments. The air coming out of the vents was slightly cooler than the ambient temperature. I have 12 volts at the monovalve and felt the heater core lines (Qty. 3 approx 5/8 hose bundled together goiing into the firewall) and they remained cool, though I did feel hot water going through 2 smaller hoses (located on either side of the above mentioned 3 hoses)......One of the replies to this thread raised my curiosity so I accessed the expansion valve and IT DID NOT FEEL COOL TO THE TOUCH!!!! Is it possible that that indicates my new expansion valve is defective?????. It seems to me that the expansion valve should be, at the very least, COLD!

__________________
Currently Driving
2006 E320 CDI
1999 E300 Turbo Diesl
2002 ML500
1995 E320 Station Wagon


MBs I've owned
1997 E320 Assassinated by Pine Tree
1987 300E Wife Killed Engine
1981 300D Stretch Limo Total Loss
1970 250 Coupe 212,000 mi.
1974 450sel 184,000 mi.
1974 240D 377,000 mi.
1977 300D 204, 000 mi.
1979 280se God Only Knows!
1983 240D 130,000 mi.
1972 220D 280,000 mi.
1983 300SD 244,000 mi.

The side of it toward the evaporator should be cold. The high-pressure side sees hot liquid.