The operational mode of a Honda ECU is determined by an interaction between physical hardware configuration and software logic.

Hardware Configuration#

The physical state of the ECU is determined by the presence or absence of shunts at locations RP17 and RP18. These shunts define whether the ECU hardware is configured for automatic or manual transmission control.

Important

Refer to the Auto to Manual Conversion0 guide for specific instructions on modifying the PCB shunts to change the hardware state.

Software Logic#

ECU firmware is responsible for polling the hardware shunts to determine if automatic transmission control should be enabled.

• Manual-configured hardware: If the ECU is physically configured for manual (via shunts), it will operate as a manual ECU regardless of the software.
• Automatic-configured hardware: If the ECU is configured for automatic, the software determines the behavior. It is possible to force an automatic-configured ECU to operate as a manual unit by using a calibration that has the automatic transmission circuit check disabled.
• Factory Variations: Some factory calibrations (such as specific EDM P30 variants) do not perform a check on the automatic transmission shunt configuration. Conversely, most USDM calibrations (e.g., P74, P75, P28, PM6) perform a mandatory check of these shunts.

Summary of Procedures#

• Converting Automatic to Manual: You may modify the hardware shunts according to the Auto to Manual0 guide, disable the shunt check within the software calibration, or perform both actions.
• Maintaining Automatic Control: To ensure the ECU successfully controls an automatic transmission, you must use a base calibration that includes the necessary code to poll the transmission shunts. The USDM P28 is a standard, reliable choice for automatic applications.