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  #16  
Old 06-14-2001, 01:28 PM
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I guess I have been sleeping. Actually the laptop I take home is getting its brains reworked so it can attach to my home DSL line and I haven't had it for all this week.

There are a couple issues here that I will comment about since no one has writen technically about this post.

First issue is temp control. All MB systems till around 1997 used compressor cycling to control evaporator icing. All climate control versions uses system fixed, cycling temperatures whether they used a mechanical switch (as in 123/107/126 chassis - early) or electronic temp probes and logic control. The concept here is to allow the evaporator to reach close to 32 degrees then shut off the compressor and restart when the temp has risen about five degrees. If measured at the center duct the temp is usually from 35-45 degrees when cycling occurs (this often is affected by the temp of the ducts if one is measuring).

I guess the seond issue is what happens around 1997. At this point variable displacement compressors started being used. Its hard to explain this simply. I suppose I will have to write a book. Let me first describe how constant compressor systems have worked since the sixties as all of this is based on the thermodynamics of change of state and pressures ounder the ideal gas laws.

In the cycling example (above) the pressures inside a R12 system drop to around 28psi on the low side. At this pressure freon is boiling and the temp is below 32 degrees. The compressor is developing 180- 250+psi head pressure (depending on outside temp and design). The compressor is shut-off and the low side goes up and the high side goes down. The high side is the pressure that comes over the PBU. In early GM systems a device called a STV valve (suction throttling valve) was placed at the exit from the evaporator. It allowed the pressure to be capped inside the evaporator and as a result the temp was stabilized right above the freezing point of water. In other words the boiling freon which would thermodynamically continue to drop the temp, has its boiling capped similar to a pressure cooker reducing the boiling of water. The later POA (pilot operated absolute - suction throttling valve) valves were used on a number of domestic and Jappanese cars.

But leave it to the Germans to do it differently (actually GM did it first), they control the temp in the evaporator by reducing the pumping capacity of the compressor ). In this case as the evap temp hits 30psi the wobble plate in the compressor is driven by the internal POA valve to a near verticle position. In this position the pistons of the compressor hardly move. The point is to use the least amount of effort to achieve an evap temp just above 32 deg. This amount of effort changes with rpm and air temp and the amount of heat needed to be removed.

Anyway if the pressure on the PBU is cycling then the car has an early compressor. If it slowly changes or just stabilizes at a given road speed then you have the new compressor.

Lots more could be said and some of it I have said. Here are is an articles I wrote on MB A/C for the trade publication "Import Car". There is another that I will try and give an address to but it didn't work here so I removed it:

http://www.continentalimports.com/ser_ic60046.html



[Edited by stevebfl on 06-14-2001 at 12:32 PM]
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Steve Brotherton
Continental Imports
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  #17  
Old 06-14-2001, 02:54 PM
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John,
Well, # 23 is vehicle speed on mine, as I said, they probably will be slightly different. And thanks for making me aware that the exterior temperature plays a role in the pressure! It makes perfect sense. I am 'beginning' to understand...

Steve,
Thanks for helping us out on this thread! So you're telling me that I have compressor cycling on my 1995 C280, and anything 97 and later has variable displacement compressors? But it seems to me that my pressure via PBU changes slowly...? Well I for sure will have to read this one or two more times but I've got to get going for now. Thanks again Steve for coming!
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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)
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  #18  
Old 06-14-2001, 06:06 PM
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I am at a loss for words seeing the response to this. I thank everyone who took the time to reply.

My '96 has variable displacement, as this is the year MB made everything OBDII compliant (fully). '96 seems to be the break year on the compressor part #. My readings from the head unit are quite different from those posted here and when I get things working right, I will post a list of them. The only one that has me truly puzzled right now is 43, which in my car reads 'bEn'. ?? The rest I think I can figure out. Except instead of 9 in mine, it is a -9 and the reading is in degrees Celsius. Between 51 and 54 is a reading labelled -2 which says 10 degrees Celsius. A little playing may tell me more this weekend.
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  #19  
Old 06-14-2001, 08:21 PM
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Benzfan,
No problem! I'm glad we could help... we even helped ourselves along the way. I will get out the sheet that Steve faxed me some time back and see if I can figure out what those numbers are for you. I have quite a few lists, 3 I think. Maybe I can peice together what your other codes might be.
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2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)
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  #20  
Old 06-15-2001, 08:56 AM
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Join Date: Mar 2000
Location: Gainesville FL
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Send me your name and where you are from and your year and model and a fax number and I will send a list for your car.

I will do that for the first ten people that reply to limit my time and long distance costs.

Remember, like all other testing on the road, one person should drive and the other monitor the readings. I would not suggest doing this while talking on your cellular phone on the Santa Monica Freeway (bg).

PS: unless of course you are stopped, which I guess is more often the case on the Santa Monica Freeway.
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  #21  
Old 06-16-2001, 03:38 AM
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Location: Taiwan
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Steve, thanks for your insightful technical explanations. So my 94 C220 have a fixed, cycling compressor. Back to my original question, the compressor seems never visually disengaged once AC is on but the PBU #7 reading does change back and forth from 10.x to 16.x in engine idle condition. Besides, the lowest evaperator temp is only 39. It's not as cold as you said 32. Does it mean I have to flush and recharge the system then the compressor may work the way it should be.

Thanks a lot

CC
94 C220
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  #22  
Old 06-16-2001, 03:41 AM
David C Klasse's Avatar
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CHarles,
Your readings should be between 35 and 50 degrees on LO, depending on fan speed and engine coolant temperature. 32 seems a little low.
__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)
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  #23  
Old 06-16-2001, 03:51 AM
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Posts: 21
Wow, a fast reply in 3 minutes. What a quick service. ^_^
Thanks a lot, David.

I set the temp to LO, fan speed to level 2, coolant temp is about 88-90 degrees C. So 39F is OK for evaperator.
I feel more comfortable.

CC
94 C220
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  #24  
Old 06-16-2001, 04:03 AM
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Charles,
YEs, your evaporator sounds perfect! I wouldn't worry about a thing. Your temp sounds fine. Now, how to detect leaks in the system! I seem to have a general feel for it now so I will monitor it occasionally.

BTW, anyone know how often we should have our AC system checked?
__________________
2006 E350 w/ 155k miles (Daily Driver)

Previous:
1993 300E 3.2L Sedan w/ close to about 300k miles
2003 E500 Brilliant Silver (Had 217k miles when totalled!)
1989 300E with 289,000 miles (had for <1 yr while in HI)
03 CLK 500 cabrio (Mom's)
2006 C230k (Dad's)
1999 S420 (Mom's/Dad's)
2000 C230k Sport sedans
2001 CLK320 Cabrio (Mom's)
1995 C280 My First Mercedes-Benz... (155k miles. EXCEPTIONAL AUTOMOBILE. Was Very hard to let go of!)
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  #25  
Old 06-16-2001, 03:38 PM
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Location: Gainesville FL
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The 32 degree temp is what has to be avoided. Ideally the evaporator would be held just above the barametrically calculated freezing point of water (32 deg F at sealevel). When your evaporator is at 33 deg the air blowing over it is never made as cold as the actual metal of the evaporator. The sensor is measuring the temp of the air right as it comes through the evap. I would guess that in your case where the air is 39, that the evap is very close to 32 if not cooler.

This relationship (the increase of temp related to distance from the evaporator)is further realized if one watches the calculated temp on the PBU and also monitors it with a digital thermometer at the center vent. If you are cycling at 39 then you will see 42-47 degrees at the center vent. This changes as the car cools off. The first cycle from at dead start with a hot car can sometimes look like 50-55 degrees. That is because that 39 degree air is heated by the 130deg temperator of the dash and ductwork.

It also must be noted that in hot southern climates the A/C may never cycle around town, ESPECIALLY if run in the high blow position.
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33 years MB technician
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  #26  
Old 06-17-2001, 04:02 AM
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Join Date: Oct 2000
Location: Taiwan
Posts: 21
Thanks master Steve. You clear my mind. The reason why I care the AC system is because my compressor and dryer were replaced last summer and got whole system flushed and recharged. The job was done by my uncle, and he is not so familiar with MB. Through your and David's explanations, I believe I can have a comfortable summer this year. Thanks again to all of you.
BTW, I live in Taiwan and it's hot now.

CC
94 C220
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  #27  
Old 06-18-2001, 06:41 PM
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Location: Toronto
Posts: 277
I just retrieved my car tonight, and all is well. The A/C works as good as new. We did, however, have to replace the dryer as a final piece. Total bill: $2700.00CDN. The shop (a well-known chain) insisted on giving me a rental for the weekend. They gave me an unlimited-mileage Lincoln Navigator. Interesting but thirsty vehicle. There is a warranty on parts and labour for my A/C and they are going to try to help me figure out all my head unit's codes and parameters for nothing later this week. I'm happy with their work and service.
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