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:
[Edited by stevebfl on 06-14-2001 at 12:32 PM]