DIY Diagnostic Interface

Diagnostic Port

An ECU diagnostic port is provided to allow communication with a Windows PC. If the engine is not running, the ECU must be powered by a 12-volt battery in order to communicate with the PC. Older bikes have a 4-pin diagnostic connector and power must be provided via a separate connector. Newer bikes use a 6-pin connector on which power is applied along with the communications signals.

Diagnostic Interface Fabrication

OSSA's original $800 USD diagnostic hardware/software utilized a computer's RS-232 (+/- 12V serial communications) port. However, since the ECU requires logic-level (5-volt TTL) communications signals, the original diagnostic hardware shifted those RS-232 signal levels to TTL levels.

As computers transitioned from RS-232 ports to USB ports, many would-be tuners were forced to employ a USB to RS-232 converter which then connected to the original RS-232 to TTL level-shifter. This is complicated wiring for a simple task (a bit like sending a letter from the US to the UK via Australia).

Direct USB-to-TTL converters are available for a few dollars on eBay. I started with a converter that used a Prolific 2303HX chip, but it suddenly stopped working after a Windows 7 update. Apparently, the Prolific product was being counterfeited in China. Prolific's solution was to write new drivers that not only invalidated the cloned chips but their own older products as well! There was no way to roll back to old drivers. Prolific wanted everybody to buy new hardware. Not wanting to reward behavior like that, I got a converter based around the Silicon Labs CP2102 chip. It has been working fine despite many subsequent updates to Win7. Such a converter is shown below.

In addition to the aforementioned converter, you will also need some Sumitomo HM090 series connectors. These can be purchased from Joe Scavone at Cycle Terminal.

You can see my solution in the photo below. It works with both early and late-style diagnostic connectors via a supplemental adapter. The translucent blob is hot-melt glue surrounding the USB-to-TTL converter (not elegant, but expedient). The blue cable is shielded to prevent ignition interference from disrupting communications.

Complete wiring needed to communicate with the ECU via K-Scan

This is not intended to be a step-by-step tutorial so I am specifically not calling out any part numbers. Building this requires some electronics knowledge and experience, but here are a few tips:

Be 100% certain you have identified the correct connector terminal and mating connections on the bike. Use an ohmmeter to verify that the pins marked “GND” and “BATT -” show continuity to the bike chassis.

A good place to start is by simply connecting the battery to the bike's wiring harness. If the fuel pump does not run for a second, something is wrong.

The pins marked “TXD” and “RXD” in my photographs refer to “transmit data” and “receive data.” These terms are from the computer's point of view. I intend them to be connected to the pins on the converter with the same names. (Transmit/Receive nomenclature has caused confusion in the computer industry ever since the early days of serial communications. By design, one device's transmit pin is the mating device's receive pin and vice versa.)

Note that although the homemade diagnostic cable works with K-Scan, you cannot load a map with it. Loading a map requires the official Kokusan Denki Communication Unit.

Battery connector (needed for 4-pin diagnostic connector)

4-pin diagnostic connector (early)

6-pin diagnostic connector (late)

4-pin to 6-pin adapter

Second DIY attempt, a bit tidier.

Official Kokusan Denki Communication Unit