If you can find an aftermarket expansion valve rated for R134a, then go for it. You might consult with the mfg. of your aftermarket A/C system. Make sure the evaporator core is OK as well. Schematics with non factory air will again have to come from the mfg. Factory diagrams may be available from Steve. He's pretty nice about sharing that kind of stuff. Otherwise, get the CD for your car.

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Jeff Lawrence
1987 300e
1989 300e
1987 BMW 325