How do you reset 1991 Jeep Comanche ecu?

To reset the ECU of a 1991 Jeep Comanche, disconnect the negative battery terminal and leave it disconnected for about 15 minutes. This allows the ECU to clear its memory and reset. After that, reconnect the terminal and start the vehicle, which should reinitialize the ECU. It's also a good idea to drive the vehicle for a short distance to allow the ECU to relearn its settings.