Problem with AF 312 not running forward

[Laurastom]

You have done a thorough job of reworking and testing the engine and tender. Based on that we know when the motor is direct wired it runs in both directions. I will assume there are no cold solder joints and all the fingers on the Reverse Unit are making firm contact with the copper on the drum. Many replacement finger sets are incorrectly manufactured and require some creative bending to make good contact with the copper on the drum. The final problem item that can cause your symptom is broken strands in one of the conductors in the four wire harness. These wires have very fine copper strands and with flexing over the years some of the individual wire strands can break. This is hard to find because the conductors meter fine when continuity tested with an Ohm meter. The meter uses almost no current so even one remaining strand will pass the test current. When the engine tries to draw its 2A the wire will not pass that much current and the motor will not run. The easiest way to test for this problem is to connect a different and known good 4 wire tender to the 312. It can be from any style engine. No need to even connect the drawbar, it performs no electrical function. Replacement harnesses are readily available.

[AFL Boy]

I have an AF 312 that will run forward and backward when power is hooked up directly to the engine, but not when I have it hooked up to the tender that has a reversing unit.  The wiring is correct for both the engine and the tender.  I have cleaned the drum in the reversing unit and replaced the reversing unit fingers in the tender.  Power is reaching the armature.  The reverse unit cycles as it should - reverse, neutral, forward, neutral.  All parts have been cleaned.  New brushes and springs.  In the reverse position it runs fairly fast.  When I get to the forward setting you can see it want to move forward, but stops. You can feel that it wants to run but it seems to be binding and will not run, even if manually helped along.  Any ideas as to what is the cause?