Dear neiler
I have a cab heater on my tractor as I maintainance my tractor, if you follow the hose from your heater to the engine, you will notice that there is a valve switch which is to turn on or off for the summer and winter. If you look at the valve switch that is 90 degree from your hose flow, you will need to turn the valve to the hose flow which will be open and then let your tractor run for a while to see if you got heat going. Remember, don't forget to turn on and off while working on your tractor through seasonal. I hope this will help to solve your problem.