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First of all, I have to say it, but replacing a fan motor on a 240 with a used one is like replacing a bald tire with a different bald tire. They're ALL bad from the get-go (essentially).
That said, the A/C may be a combination of a number of factors.
1. If you didn't replace the condenser with a cross flow condenser (1993 style) then it won't work as well.
2. There is a grille condenser fan on the 1991-1992 model 240 that runs based on condenser pressure (high side pressure). There is also a pressure switch that is located on the high side near the oil filter. The harness can be jumped and the effect will be, the fan will run whenever the A/C is turned "on" (snowflake switch is on). This helps a lot.
3. A converted unit not done properly may be weak anyway (compressor weak) and the only way to find that is to test the high side readings with the low side readings to make sure that the conversion was done properly and the pressures are good.
4. The FOT (fixed orifice tube) must have been replaced or the conversion will work poorly.
5. The heater control valve may be operating poorly as well, allowing hot air to enter the mix.
6. Based on what you've said (5) is not likely. The cycling compressor is FINE, as R134a operates on different pressure-to-temperature curve than R12, and it certainly is adequate if the compressor cycles frequently, but the unit cools well also (common). Most R134a conversions are overcharged on the 240 series from what I've seen.
7. A visit to a good A/C shop should identify the problem, but make sure they're reputable because a lot of people promise everything and deliver much less. I have done a lot of A/C conversions for 240/740/940 series models (similar units) and doing a proper conversion in the 240 usually results in an "OK" cooling result. The 740/940 almost always result in excellent conversions. Use the tip in (2) for best results in any R134a conversion in a 240 series car.
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