Trane TTX036C100A1 condenser fan is cutting out after 40 sec while condenser unit is still running. Does anyone know why?

It is a possibility the condenser fan capacitor is defective OR the fan motor itself unless your system has a ambient temperature controller on it. That control allows the unit to run in cool temperatures which we are not having now in my location but sometimes that device becomes defective or out of calibration and can cause the same problem. Sorry but there are a few different items that can cause your problem. I would check the control circuit then if all is OK there I would replace the condenser fan motor capacitor first since that is the cheapest part.