Vince,

Before you replace the ECU, send off a mail to GASPAR for his valued opinion.

Try this:

Be sure all signal cables are inserted into ECU with proper color code orientation. Unlpug the Turbine RPM cable from the ECU.

Now power everything up and see if you get a "ready" on the data terminal (I assume this is what your calling the Gameboy)

Now initate the start sequence and see if the starter engages and the data terminal then indicates "ignition on". When the starter does engage, it will NOT pull back out when reaching 5,000 rpm as intened as yo do not have the RPM cable inserted in the ECU. Power everything down.

The point of this test is to confirm that everything else appears to work correctly leaving only the RPM as the issue. This will isolate the RPM sensor or ECU as the issue.

Good luck and let me know what you discover.