Quote:
Originally Posted by Riveted1
So does the orifice tube replace the expansion valve?
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They serve the same purpose but aren't interchangeable. An orifice tube works like putting your finger over the end of a garden hose. Liquid refrigerant sprays out in small droplets. The orifice is a fixed size so an "accumulator" is placed on the outlet of the evaporator core to prevent liquid from getting to the compressor.
The expansion valve is a variable opening that corresponds to the temp of the core. It works best when fed only liquid refrigerant. That's why the drier is placed before it. Early systems like yours did not cycle the compressor and the POA valve helped to prevent freezing at the evap core.
There's a possibility that the mix of parts on your truck prevents it from working effectively.
I didn't mention it earlier but an uncommon, but possible mistake, is connecting the condenser core backwards. The liquid line should exit the bottom and the compressor line should attach to the top.