When I turn dial for heat it blows cool air. I checked coolant its ok. Tks

Check to see if both htr hoses are hot to touch when engien warmed up. If it is it may be an actuator problem. If not then your thermostat may need replacing.

They both should be about the same temp. You also have a heater control valve on yoru truck. It is operated by vacuum
see if it is getting vacuum and working. If not getting vacuum you'll have to replace the valve. I'm sending a pic along it is inline with the htr hose. That mayb e your most likely cause right now.

Make sure the heater control was on heat and if still no vacuum then check for a vacuum leak leading to the heater control or a hose off. Hose off could be at engine or control. If you have vacuum there then it's the vacuum switch on control head.