Modifying the TCU

The TCU uses a surface-mount ROM but there are two lines of solder dots where a socket can be fitted to allow the use of a DIP-28 style rom instead. The replacement chip is a 27SF512 EEPROM which is pin-compatible with the original 27C256 ROM.

Unsolder one leg at a time and bend it up away from the board. When all the legs have been unsoldered, gently pry the chip away from the board. Then use some copper braid to clean up the contacts where the chip used to be.

Thanks to Michael Emery from Ecutune for providing the following information: Instead of completely removing the stock ROM, you can disable it by moving the surface mount resistor from position R432 to R431. The resistor is a small green rectangle with 000 stamped on it, located in the top right corner of the photo above. Michael also recommends using solvent to clean the Conformal Lacquer coating from the parts of the board you are working on.

Clean out the holes with a solder sucker and then solder in the new socket.

Program your modified ROM image into a 27SF512 EEPROM. I use a cheap Chinese Top2004 programmer that I bought on ebay. The size of the ROM image is 16K while the EEPROM chips are 64K. You need to program the 16K image into the top quarter of the chip, starting at address 0xC000. I always append the image to itself 3 times to create a 64K image and then load that into the chip starting from address 0x0000.

Before attempting to drive the car, use the select monitor adapter to download the ROM image and check that everything is working as expected.