Purchased my first MB last year. A/c was not working. The clutch bearing was locked up. I replaced the clutch, all seals in compressor (Nippendenso 10pa17c), TXV, and drier. I pulled a vacuum and installed R134a today. The air temp coming out of the vents at idle was 100 degrees. The mono valve is closing. I pulled it out and checked it with a battery. I also clampped the hose shut going to the mono valve to make sure I was not getting heated air. At 2000 rpm the air temp dropped to 80. I know these air systems are not the best but was looking for something better. At 2000 rpm the low side pressure was 37lbs but it would not cool. I added more freon but it did not help. I will list the presures below. Is it possible the compressor is worn out? It is the original one. It runs all the time when the a/c system is turned on. Does not make any noise or grinding sounds. The fans in front of the raditor also come on.

At idle the low side is 70lbs
high side is 210lbs
The sight glass is clear with no bubbles.

At 2000rpm the low side is 50lbs
high side is 325lbs
The sight glass is foamy
The suction hose coming from the firewall is slightly cool at this speed.
Any help would be appreciated.

70 psi on low side at idle is extremely high, and 325 psi at 200 rpm on the high side is also very high. My first guess is that you are overcharged and maybe your belt is not tight enough. Get your low side pressure to between 35 and 45 when at 2000 rpm.

I am assuming that you have adequate oil in the system. I am also assuming that you did not fill R134a in a system which was previously filled with R12 without fluashing the system. Otherwise these pressures are difficult to interpret.

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Saumil S. Patel

I let out some of the freon. The belt is new but when I put it on I could not find the pointer to tell when the tension was right. I found it tonight and loosened everything and tightened it according to the book. At idle the low side is #50 and at 2000 rpm's it is between #30 and #35. The air coming out of the vents is about 60. The belt must have been the problem. How can I fine tune the a/c now? I may have let out too much freon. Thank you for your help!!!

You should have lower than 50 at idle on low-side, the low-side pressure should not change that much from idle to 2000rpm. The belt is still loose. Also are your aux fans working properly ? The pressure is a little low at 2000 rpm, get it to between 35 and 45.

One way you can optimize your a/c is as follows: (it is not perfect but works better than trial and error).

1. You will have to either fix the belt OR keep the car running on 2000 rpm during the test below.
2. Run the a/c for 10 min on re-circulate mode with highest speed for blower, lowest temperature selection and some a/c mode for air routing, e.g. bilevel.
3. Now read the temperature (degF) corresponding to whatever low-side pressure you have from the gauge. There is s seperate dial marking for temperature corresponding to the R134a on the gauge.
4. You will need a infra-red thermometer or a thermometer that you can wrap around the low pressure hose near the low pressure valve. Measure the low pressure hose temp (degF)
5. Adjust the pressure on the low side by charging or discharging until the temp measured on the hose is between 15 to 20 degF lower than what is read from the gauge.

Maybe your belt tensioner is not working and so you cant tighten the belt properly. Also make sure there is no oil or grease on any of the pulleys and the belt.

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Saumil S. Patel

I let off the belt and tightened it again. I am sure that the tensioner is working. I added enough freon so that the low side pressure is #40 at 2000 rpm's.
The car idles @ 700 rpm's and the low side pressure #60. The suctin hose is not very cold at idle speed. The hose is colder at 2000 rpm's but the air coming from the vents is about 75 or 80. This is after the car has been running about 20 minutes. The mono valve is working. I turned the temp dial to max for heat and noticed that the a/c compressor was still running. If it is blowing heat at the min setting would that cause the suction hose not to be cold? The freon is moving, so I don't think I have a blockage. Could the inside temp sensor for the cabin be bad telling the system to blow warm air? The other night it was blowing 60 degree air from the vents. What do I look for next?

Are you saying that the monovalve is open when the a/c is at minimum temp ? How did you check the monovalve ? The voltage across the valve's connector should be about 12V to be fully closed, did you not find a voltage there ? You can remove the wire connector and just connect 12V across the valve and see if it makes a difference. At the same time, you can also disconnect the voltage to the aux water pump. This pump, pumps the hot water thru the valve.

Yes, if the monovalve is letting warm water thru the heat exchanger, it will counteract the cooling and your intake hose will be warmer than what it should have been, and your air temp from the vent will be substantially higher.

It looks like the compressor is not working at all at idle. I am not sure why the pressure drops so much from 700 to 2000 rpm if the belt is tight enough. Make sure there is no slippage due to oil or grease on the pulleys.

If you need more info about the a/c functions, send me an email at saumil@swbell.net, I can send you the manual pages, cannot upload all the material, it is too big.

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Saumil S. Patel

Tonight (Sat) I turned the a/c on as soon as I started the car. The low side pressure is #60 and the suction hose is cold. The air temp coming from the vents is about 63 degrees. At 2000 rpm's the low side pressure is #40 and the temp dropped a few degrees. The suction hose is still cold. The longer the car ran the warmer the air became coming from the vents. The mon valve has 11.8 volts when the a/c is turned on. When it is off the voltage is 0.03. I noticed that the a/c compressor stays on when I turn the temp wheel to max for heat. How can I check the temp sensor that tells the system what the temp is in the car? I know air is being drawn into it because I placed a small paper across the grill and the suction held it there. Is it hard to replace?

This superheat method is only for fixed orifice systems because a TXV is constantly adjusting the flow of refrigerant. Do you know something I don't?

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2012 E350
2006 Callaway SC560

Thats correct, this method does not perfectly with auto air, for one reason you mentioned and another because the air flow is variable.

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Saumil S. Patel