Truck heaters are extensions of the vehicle cooling system. Inside the cab is a small radiator--also call a heater core-- that allows hot coolant to flow from the engine, circulate through the core, then return to the engine. A fan blows air across the core to heat it, then on through the ducts and finally into the interior. The entire system consists of: a valve to control the hot coolant flow into the core, a blend door to regulate the air discharge temperature, and a fan to circulate the heated air. These may be used in various combinations depending on the manufacturer.
Check the blower-motor circuit first. It's easiest and simple to test. With the key on, turn the blower switch to all positions. If it works only on high, the blower resistors have failed and need to be replaced. They are usually located in the blower case to keep them cool. If the blower does not work at all, check the fuse, and check for power at the blower motor. If the fuse is good and there is no power at the motor, the switch is defective and needs to be replaced, or the resistors have failed. If there is power at the motor, it is defective and needs to be replaced.
Start the truck, raise the hood and let it warm up. Locate the heater control valve installed in a heater hose. Two heater hoses run from the engine to the firewall where they attach to the heater core. The valve will have a vacuum line or cable attached. Set the heater to full heat. Feel the heater hose before and after the heater control valve. The temperature should be hot on both sides. If it is cold on the inlet side to the heater core, make sure there is vacuum or the cable operates properly. If so, replace the heater valve. If not, repair the vacuum circuit such as a vacuum leak or defective heater vacuum switch in the dash. The same is true for a defective cable--a defective dash lever may need repair.
Operate the blend-door by selecting full heat and full cold. On many late model vehicles this is all that controls the temperature; no heater control valve is used. These doors are operated by a slide lever that controls a cable, or a small electric motor that is controlled by a digital switch. Often the blend door seal becomes stuck, the cable breaks, or a plastic lever that operates the cable becomes old and cracks. Replace the climate control panel if this is the problem. Digital dash switches often fail, and it is common for the small, blend-door motor they control to also to fail. Digital climate control panels have a self-diagnostic feature. Follow the manufacturer's service procedures for these units.
Test the engine cooling system. If the temperature of the engine is too cold, the heater will never get hot. A thermostat could be stuck open and needs to be replaced. Check to see if the radiator is low on coolant. If there is not enough to allow any to flow into the heater core, it will not work.
Feel both of the heater hoses going into the firewall. If one is hot going in, but the other is cold coming out, the heater core is blocked and crusted with mineral deposits. It will have to be replaced.
