First we need to determine if the heater core is hot. Feel both heater hoses going to the heater core. If both sides are hot then the core is getting good flow. Once this is determined, we will move on to the heater control inside the truck. There is a TSB that deals with the lack of temperature control. You will need to check fuse 24 and if it is blown follow the TSB. If it is not blown we will need to diagnose the blend door motor. Let me know either way.
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If there is a heater valve inline to the heater core, make sure it is open. If the heater valve is open or if there is no heater valve, the heater core is plugged up. The valve is vacuum operated and cuts the flow of coolant to the heater core when the A/C is in max mode. I do not believe this year and model had a water valve inline, that would mean the heater core is plugged. You could try to backflush the heater core with a water hose, but it may not work. I hope this helps you. If you need more assistance, just reply back.
If the heater hose is hot on one side and cold on the other, the water is not flowing through the core. If the core is open, then it has to be a flow problem going to it. You may need to pull the heater hoses off and run a hose into the coolant recovery tank and see if the coolant is actually flowing. If it is not, the water pump may not be pumping the coolant. The motor is what moves the blend door inside the heater case. This door lets the air to flow through the heater core or not. Thus blending the hot air. If the coolant is not flowing to the heater core, there is no hot air. We still need to check the flow of coolant to the core.
What engine is in this 1999 F250?