Speedometer Reverse Engineering

I reverse engineered the data stored in my Mitsubishi FTO Speedometer in order to re-calibrate it to suit the Mini face. Both faces show 180km/h, but the mini face spreads it over a slightly wider angle than the FTO does. The data is stored in an OKI 16811G 128 byte Serial EEPROM. This is quite an old part which uses Microwire (a predecessor and subset of SPI) to communicate. I had some trouble finding the data sheet for this part so I tried to sniff the communications between the odometer's CPU and the EEPROM with an Arduino running gillham's SUMP compatible firmware. Unfortunately the Arduino only has 2k of ram so recording all the state changes wouldn't fit. The communication is clocked at about 1MHz and I wasn't able to write a parser fast enough to stay in sync.

There is a fair amount of questionable information about odometer EEPROMs on the Internet, mostly related to tools for sale to wind your odometer back. One of these references claimed the part was compatible with a 93C46 so I wrote an EEPROM reader for that part and read out the data. Unfortunately this wiped the EEPROM! Somehow my reader code triggered the erase all command. This isn't entirely surprising because the right data sheet shows this EEPROM uses a different protocol to that used in the 93C46.

With a wiped EEPROM the odometer stop working, so I had to get another one. I also got an Open Bench Logic Sniffer which let me observe the entire communication between the odometer and the EEPROM and confirm that the protocol in the correct data sheet was the protocol actually in use. While re-scaling the speedometer for the new face I also reverse engineered the stored kilometre value and re-set the odometer to zero to celebrate the conversion to electric power. I also made some somewhat comprehensible notes about how this data is stored, an Arduino sketch to read and write the EEPROM which includes some tools to re-format the data, and an Arduino sketch to test an electronic speedometer and find the number of pulses per km.