The J12 jumper on OBD1 Honda ECU circuit boards controls the operational mode of the ECU's internal serial port.

Jumper Functionality#

• Stock Configuration (Jumper Installed): The ECU serial port operates in half-duplex mode. In this configuration, the receive (RX) and transmit (TX) lines are shared, which is sufficient for basic diagnostic communications using a standard diagnostic connector.
• Full-Duplex Modification (Jumper Removed): Removing the J12 jumper disconnects the RX and TX lines, allowing for full-duplex communication.

Datalogging Requirements#

For reliable, high-speed datalogging, the J12 jumper must be removed. Full-duplex communication allows the PC to send commands to the ECU and receive responses simultaneously, enabling higher sample rates and more robust data transmission without command collisions on the serial line.

Note: Removing J12 will break communication with the factory diagnostic connector located under the dashboard.