Quote:
Originally Posted by 87tdwagen
You mentioned the use of the correct expansion valve. The one my car calls for 140 830 04 84 is the same for the 1987 R12 300td as it is for the 94 e320 R134a car. At least in my look-up the expansion valve is used for many models pre R134a and post. I assume that there is no functional difference based on the gas used and the (New) valves are compatible with either system?
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The Montreal Protocol banned the use of R12 in 1993 and newer model vehicles. The expansion valve available is a superseded part number that retrofits to the older cars. On 1986+ cars, the coils and systems were identical between R12 and R134a, the expansion valve itself changed (and became a retrofit item) so that the orifice size and power head spring rate were correct for R134a which runs a higher gas pressure and has a smaller molecule size than R12.
From personal experience converting multiple MB's of this vintage and several BMW's, there is definitely a difference in the expansion valve. Lots of armchair mechanics like to say there isn't (because they read so on the Internet), but you'll run a performance penalty if you don't change it when switching gasses. Running the original R12 valve with R134a requires you to undercharge the system to allow the gas to boil to vapor in the evaporator, the result is a reduced system capacity. Overcharging with the wrong valve leads to a flooded evaporator which reduces system capacity even further (and will wreck your compressor from trying to pump liquid). It's a lose-lose situation. Spend the $25 or so and change the valve if you're running R134a, it's the single best investment you'll make in the system since you're already having it open. You'll be able to charge the system so that you have a full liquid column at the expansion valve and gas in the evaporator (translated into English: You'll have nearly the full rated capacity of your system with the replacement gas in it).