# HOWTO Make Your Own ECU Communicate with TCU

## What is this about

As of Feb 2024 it has been _years_ since I've on and off started to work on getting an open source ECU to communicate with a transmission control unit via CANbus. So far I have not accomplished the goal which makes it a great point to write an article on the subject!

I have a [2006 Volkswagen Passat 2.0 Turbo GDI PG35 non-DSG automatic complete running and driving test mule](https://rusefi.com/forum/viewtopic.php?f=2&t=1631) idling on rusEFI open source ECU. Attempting to engage transmission while posting only minimal dashboard CANbus messages causes something violent to happen and the engine would usually stall see https://youtu.be/rTobt4l-iEs

Original ECU which I am looking to replace for no good reason is BOSCH MED9.1. Conveniently the relevant Bosch Funktionsrahmen Ea827 TSI is available on the internet. CANbus traffic seems to mostly align with https://github.com/commaai/opendbc/blob/master/vw_golf_mk4.dbc DBC file. I've used google translate to translate some of interesting field descriptions those are already merged into the file.

## Toolset

I have PCAN USB dongle to record CANbus traces and a few Proteus units for Man-in-the-middle research. I've also created my own suite of CANbus trace processing utilities https://github.com/rusefi/can-log-tools/

## First experiments

First let's grab a trace and a snapshot of CANbus traffic while engine is not running, both with OEM ECU and with OEM ECU disconnected. We get https://github.com/rusefi/rusefi_documentation/commit/722e81d2b0dd2b1f9661a3e9fd6383087ba4418a#diff-457155f3530fe11dbca1083aa1860f4e3cccc8f2d8b78d81634def58fbc38d9f

Comparing these two files we confirm which packets are clearly originating from the now disconnected OEM ECU https://github.com/rusefi/rusefi_documentation/commit/8575c7292d975c2477ccf070634883f5a9423c7e