Matt:

You may have a vacuum problem -- the climate control is a Frigidaire unit, same as the GM ones, so a GM guide will help. The usually default to defrost when there is no vac, so check vac lines from the engine compartment to dash. It is fairly easy to pull one loose, and I'm sure the rubber is shot on the connectors, just like MBs are.

You may also have a bad temperature sensor, so the system thinks it is alway cold in the car!

Note that you don't have an expansion valve, you have a CCOT (cycling clutch orifice tube) system. There is a fixed orifice at the evaporator in the pressure line, and there is an acuumulator on the low side, to which is attached a pressure switch. This switch opens as the pressure drops (the evaporator gets cold) and opens the clutch on the compressor. When the air is cool, the evaporator floods and the liquid freon accumulates in the accumulator, and the vapor is picked up from the top with a U tube. Ford uses a similar system. Biggest problem is clogged orifice tube screens, as there isn't a receiver/dryer/filter between the condensor and the evaporator.

Larry:

Most new systems with R134a have larger evaporators, and you can usually get one to fit your car. I don't know what the job is like in a W123, but I know my sister had a larger one put on their Volvo 240 when the moved to Florida -- the mechanic said "black car, black interior, Florida, you need more AC". You DO NOT want to put one in a W124 or W126 just to get more AC! On the W124 the book time is 16 hrs!

The difference between S Indiana and Texas is only lower heat (upper 90s this week) and higher humidity -- we will be getting late night fogs soon, and will have them periodically until late September when the air goes below dew point at night. Very sticky. Removing humidity uses a considerable amount of the cooling capacity, resulting in higher air temp. Probably makes us even, since my Volvo is black on black... bad choice, never again!

Re: Expansion valve thermo sensors -- the MB is internal, measures the temp of the gas stream coming out of the evaporator, I think. They don't have a bulb or "wire", a least. Very easy to change. Chrysler used to put the senor in a fitting in the low pressure line, and if you didn't have the conductive grease in it, the evaporator froze. MB has a temp switch to cut the compressor off if the evaporator gets too cold, and if it fails, it can mimic no cooling as the evaporator freezes up.

__________________
1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!