I am going to give you short history on the vehicle. It's a 1999 Dodge Ram 1500 5.2. L. Last summer my a/c went out due to bad heater core, so after everythings done I ended up replacing, radiator, water pump, Thermostat and the heater core. And I had my A/C back working, and that was a good thing. Well ever since the end of summer, I have no Heat only cold air from all vents. AFTER THE TRUCK REACHES OPERATING TEMP. UPPER RADIATOR HOSE IS HOT AND SO ARE BOTH HEATER HOSES GOING INTO THE FIREWALL. The Blower is working. When I turn the heater on I noticed today that the air compressor will engage and disengage over and over. The compressor does the same if I am running the A/C. The truck does not overheat. I am surgging forwoard in my troubleshooting. I think I will start looking into the climet control unit, when I go from a/c to heat I donot hear any door open or close. Should I hear a door close?
If anyone can shed any light that would be great. This is my 1st posting I appreciate your input.

 

If the cooling system is all brand new, then yes the issue points to the climate control unit

 

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