Any updates on this issue? I am having a similar issue with a C6000. The rear wheel will not respond to input from the directional switch. The front out rigger wheels respond normally, (90 degree or 0 degrees depending on the placement of the switch). The relays all seem to be ok. I did notice that if I unplug the steering solenoid under the operator compartment, that I lose functionality to the front wheels and gain function back in the rear.
Now... if I drive the machine normal, I do have steering abilities, This issue seems to only be associated with the directional control switch. I did reach out to Combilift today but haven't been given much insight yet.