The expansion valve controls the temperature of the evaporator by controlling the amount of liquid freon passing through. When the evaporator is warn, the valve opens and allows more in, when it cools off it closes down and reduces the amount going through. It should hold the temp at about 35 degrees F and the pressure at 35 psi (R12) and about 27 psi (134a).

The most common mistake made in changing to R134a is to overcharge -- an R12 system must be charged to 85% (by weight) with R134a or the low side pressure will be too high, and it won't cool properly, especially at high speeds. A different expanstion valve may be needed to obtain a lower pressure, but shouldn't be needed if the old one is working.

I've converted several R12 systems now, including two MBs (115 chassis and 124 chassis). They all cool just fine, except the Volvo, and I think that is because I put too much freon in.

Properly done, R134a cools as well as R12, lubrication problems aside.

Peter

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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!