I dont consider myself an "expert", but at the time I did the AC work on my 85 300D, I was skeptical about the effectiveness of R134, and made various decisions to maximize the effectiveness of R134. I cant guarentee that other configurations wouldnt work as well, or if all of the "mods" i did were necessary. Here is what I did:

Vertually every component of the AC system, EXCEPT the evaporator, was replaced with new stuff. So the evaporator was the only thing that needed flushing. I intended to use PAG..the prefferred lubricant for R134, but in order to use this, you must flush all mineral oil out of the system. Otherwise you need to use ester as the lubricant.

I flushed with lacquer thinner, and coleman fuel (extremely volatile (and flammable) in order to ensure easy removal). Dont use low volatile mineral spirits....more difficult to remove.

To flush, I drilled out the old expansion valve and reconnected to the evaporator (using it as a service connection) and the two old hoses leading to the engine compartment, and flushed via these hoses...pour in a cup of solvent with a funnel and blow out with compressed air. Flush forward and backward several times, blow out both ways, then pump air through over night for a couple nights to ensure removal of solvent. Done.

All AC components (except evaporator) were replaced with either OE equipment (NEW R4 compressor, exp. valve, rec.drier) or aftermarket (most hoses and condenser). The condenser I used was an aftermarket universal parralel flow condenser, mounted with "L" brackets....I used the biggest one I could fit in. One of the limiting factors in using R134 is the capacity to condense, so I went with a parralel flow unit ensuring good condensing and low "low side" pressures.

Because the fittings on the condenser were not the same as OE, I had the two hoses that connect to the condenser custom made to fit.

Another mod was: I wired the aux. fan via a relay to the compressor, so that whenever the compressor is on, the aux fan goes on.

ANother mod: since the temp/pressure profile for R134 is different from R12, the temp cutoff switch on the rec./drier would not necessarily work properly, so I place of the OE low pressure cutoff switch (also on the rec./drier) I replaced it with a hi-low pressure cutoff switch (I forgot the pressure cutoff points)....this gives me an appropriate high pressure safety cutoff instead of a temp cutoff designed for R12 (this is only a safety feature, and does not effect performance).

The only performance-related mods were the par. flow condenser, and wiring the aux. fan to engage when compressor goes on.

I put in 5 3/4 oz. high visc. PAG into the compressor when I installed it. I lubricated all green o rings with mineral oil. Brought to AC shop, evacuated system under vacuum, then charged with 24 oz. R-134.

Center vent temps go down to 39 deg. F, when outdoor temps are in 90s, and fairly high humidity. When humidity is at 100%, center vent temps go down to 43-45 deg. F.

A major limiting factor in this AC system is the relatively small evaporator. Its just not big enough to blow snow like the good AC systems in good old american cars!!! It seems that the German philosophy here was similar to other systems (radio, dash lights, AC....not necessities and not safety features, therefore underdesigned)

Good Luck,

Mark

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1984 300TD Wagon, 407,800 mi (current daily driver)
1985 300DT Sedan, 330,000 mi (gone to that great autobahn in the sky)