Thanks All,
It's not the decoder that I'm questioning but the locomotive interface board. With the newer plug'n play models the DC dummy board or supplied interface board have a clearly marked GND connection. I did run across a well done schematic of the K27 board/ function connections etc. wired to a decoder and it sheds very good light! I'll have to trace out the Shay, as it has 6 screw terminals but no indication of what is what.

Regards,
Len

Thanks Tony.
Now I'm perplexed as to where the ground of the decoder gets wired to the Shay board? Bachmann should supply schematics!

I'm preparing to install a decoder in my Shay. There is a pair of blue wires tied together and going nowhere?? No indication from the supplied wiring diagram as to where they go or what function.
Len Jaskiewicz