MTH Premier 2-8-0 speed issue

If the board is in the engine, replace tach reader.  If board is in a tender check the harness wire from 7 pin to the tach reader with tether plugged in.  (Blue/org/gray).  May go through the mux boards if you have them.

Normally this is a tach reader or tether issue for PS-2 5V.  G

Well I get good continuity from the tach reader to the 10 pin plug in the engine for the orange and gray wires.  The blue wire goes to another 8 pin board in the front of the engine that seems to get  track power(red) and wires for all the lights in the engine. What does the blue wire do?  Is there an ohm reading to look for between the orange, grey and blue on the reader itself?

Still trying to get to the orange wire in the tender, had to cut off a zip tie. Yes this engine has a MUX board.  

The blue wire is PCB Ground.  If you can see the orange wire in the 7 pin connector you can touch its terminal to get the reading.  If the continuity is good, the issue is the tach reader.  G

I got good continuity with the orange wire in the tender so it must be the tach reader. I think I have one somewhere here, so I guess it's time to change it out and see what happens.  

Thanks GGG for all your help and amazing amount of technical expertise.