I recently acquired a 1947 Lionel “R” Type 110 watt transformer.  While checking it out, I found all terminals function correctly, except the variable A – C terminals.  All the internal wires are connected and I can find no broken or disconnected wires.  I have cleaned the contact area on the coil and the contact arms. The contact arms are making good contact with the contact area on the coil.  The other 3 variable (A – F, B – C, B –F) terminals are working correctly. 
Does anyone have any ideas why the variable terminals (A – C) are not working?