Mark:
Try this: Use a jumper wire in the diagnostic port connector, between the black wire and the light green with red tracer wire. Jump them and then turn on the system, and the codes will flash out. If there is just a really rapid flashing too fast to count, that's the 'intermission', the marker between the repeated sets of codes, and if that's all you get, then there are no codes in the system.
Maybe if you get a code #, you can figure what's wrong.