M9000 4WD with CAB, the AC will blow cold but the when I turn the temp dial to heat, I get cold air. Is there a fuse? How do I troubleshoot the heater?
There are many variations of the M9000.
I found an illustration of one unit where the heater valve is in the ceiling and controlled by a cable.
Heater systems with the core in the roof space will be very sensitive to low coolant levels.
Try and find the supply and return heater hoses and feel them to assess their temp.
The same fan powers the AC and heater. is it running when you are calling for heat?