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Keep this in mind when wiring a modified control board to the existing tstat. The outdoor fan speed is controlled by whether one or two compressors are called for. The tstat controls the indoor fan speed . So in wiring the mod board you must make provisions that the lo signal to the out door fan is interrupted when the second compressor comes on.

I accomplished this with a relay for the outdoor fan connected to the same control wire as the compressor 2 signal from the tstat. The 120v for the fan comes from the compressor 1 120v source through a normally closed contact, so that when compressor 1 comes on it also starts the fan on low speed, when the 2nd compressor kicks it changes the fan relay to change to high speed.  

See the yellow highlights on my drawing: