I would look for the pins on the plug that show evidence of the wires having been ripped off.
Then, this may help:
http://www.amplicon.co.uk/manuals/pc27e.pdf
If you are right about the yellow wire being analogue ground, it should go to pin 3.
I would take a guess that the other two wires are channel 0 and channel 1, so pins 4 and 5. But that is simply a guess as I have no way of knowing how the board channels have been programmed.
If the evidence from inspection of the plug is that two pins other than 4 and 5 originally had connections to them, try with red on one and blue on the other. If that doesn't work, swap them round.
I am assuming that the computer needs two analogue signals - one for drum RPM and one for load - but I could be completely wrong about that too!