It sounds like the low speed function of fan is not being controlled by the PCM as it should be, but instead by the high pressure cutout switch. That switch is intended as a redundant safety measure to keep the system from exploding/discharging from excessive pressure.
Have the fan control system checked by somebody who understands it if you aren't certain as to how it works. Avoid using the A/C system in the meantime until the primary problem is repaired.
If you have a meter and a schematic, the circuit isn't that difficult to diagnose and I can walk you through it.
Steve
