How was the conversion done? Correctly or a quick fix. Correctly is a complete flush, new Orings, new expansion valve if needed, new oil and the freon.
Quick fix. Add the R134a can with oil, Add more freon and it will blow cold. BUT your compressor will die in a year or so. Then your doing everything list above and more!

Second. Can you put the fan switch on high and it blows cold and at a high fan speed? If you can't then your AC control unit may be bad.

More info please?

Dave

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1970 220D, owned 1980-1990
1980 240D, owned 1990-1992
1982 300TD, owned 1992-1993
1986 300SDL, owned 1993-2004
1999 E300, owned 1999-2003
1982 300TD, 213,880mi, owned since Nov 18, 1991- Aug 4, 2010 SOLD
1988 560SL, 100,000mi, owned since 1995
1965 Mustang Fastback Mileage Unknown(My sons)
1983 240D, 176,000mi (My daughers) owned since 2004
2007 Honda Accord EX-L I4 auto, the new daily driver
1985 300D 264,000mi Son's new daily driver.(sold)
2008 Hyundai Tiberon. Daughters new car