Sounds like the low aux fan circuit works when A/C pressure triggers the switch on the drier, but the coolant temp sensor may not trigger the high fans. If that's right, seems like either the engine temp doesn't actually reach the 105C cut-in point for those high fans, or the temp sensor is malfunctioning and doesn't produce the low resistance values that the circuit requires to trigger the fans. If you can you verify engine temp, you might be able to rule out a sensor malfunction. I don't recall ever seeing what the diagnostic test for that sensor is, but it must involve backprobing the sensor and watching the resistance fall to near zero as the motor heats up. Replacing the sensor with a known good one obviously works too.
