This is PS-3 BB correct? John's picture is PS-2 3V and not correct for PS-3. No polarity switch. The Engine Right side wheels pick up via spring plungers. The Left Side are chassis ground. Only right side wheels and center pickup go to the 6 pin drawbar to send power to tender. The left side of engine can only make connection via the wire pin along the drawbar.
Tender wheels have to provide ground through the axle wiper, friction contact of truck and switch.
If you have a non 3R-2R engine and it runs fine on layout, I would go into engine and connect the engine right side wire to chassis ground. Now Engine both side wheels and the center pickup have AC track power via drawbar to the tender. Now you have better continuity of power for the enigne for 3R. Easy to reverse if you want to run 2R at a later date or sell engine. G
