I have a 2-8-8-4 that runs and sounds great when going forward.  When in reverse the sound functions but the engine will not move.  Have I burnt out something in the decoder or some other problem?  If I can't figure this out I may end up running it forward all the time.  Any ideas?

I was given an HO 2-8-8-4 with no tender, so ordered a tender and decoder from Bachmann.  Since almost all the wires from the engine are black, can someone tell me what sequence the 4 pin and 2 pin connections under the engine are and how they connect to the decoder?  Thanks.