You did not mention if this was for hoisting or traveling, but you are describing a "Wound Rotor" motor which uses resistance in the rotor windings to control the speed of the motor. The resistor banks are connected in a "wye" configuration to the M1, M2 and M3 motor leads. The more resistance the slower the speed. There is not much unique in troubleshooting a wound rotor motor, most of the problems can usually be found in the control circuits.
Most crane system use a single set of shorting contactors for controlling the speed whether moving forward or reverse. In some cases there is a timing relay between several speeds, such as a delay between 3-4 when hoisting and 4-5 when lowering.
If you have speed control in one direction but not in the other then you have problems with the control circuit in your Master Switch. Do any of the shorting contactors work in the "bad" direction?
