Troubleshooting a Ford Truck Heaterby Don Bowman
The heating system in a Ford truck starts at the water pump, which supplies hot water to the heater core through a hose attached to the top of the water pump. The water and antifreeze mixture is directed through the heater core and returns to the intake manifold through a hose attached to the engine side of the thermostat.
The heater core is enclosed in a plastic case under the instrument panel. This case contains the heater core, the air conditioning evaporator and the blower motor. Contained in the housing are a series of flaps that act as doors to open or shut off the airflow through the housing. The flaps effectively direct the airflow through the desired vents. These doors are operated by one of two methods, depending on the year of the vehicle. The older Fords used a vacuum diaphragm to operate the doors. This system's components consist of a vacuum line from a vacuum source on the engine or from the brake booster to a vacuum amplifier. The vacuum amplifier is a bottle that contains sufficient vacuum to operate the system without being affected by the varying vacuum associated with engine RPM. The vacuum hose goes from the amplifier to the control head on the instrument panel. The control on the instrument panel has a series of vacuum tubes running to the different doors on the housing. The main vacuum line supplies the primary vacuum and as the control is rotated it directs this vacuum to the different doors.
The present day vehicles use electrical servo motors which simply move an arm back and forth when voltage is applied. This voltage is controlled by the control panel as well. The blower motor function for the heating and air conditioning starts at the fuse block. From the fuse, the electrical power goes to the blower motor relay. From the relay the power goes to the blower motor resister. The electrical fan speed switch not only operates the blower motor relay, but the fan speeds as well, through the blower motor resister. The power then goes from the resister to the blower motor.
If the heater is not getting sufficiently hot, check that the engine thermostat is working and that the engine is warming to normal operating temperature. Make sure that neither of the water lines from the engine to heater core is crimped. Remove the glove compartment and operate the air level control and make sure all the vacuum or electrical motors are working and moving the doors. There may be one defective and not opening a door.
If the blower motor is not working, check the fuse and the relay first. Use a circuit tester and check that there is power to the relay on one terminal with the key off. With the key on and the fan switch on, there should be two terminals with power. If not, the fan switch is bad. If the fuse and relay were alright, check to make sure that there is power to the blower motor resister which is next to the blower motor to the left about 2 inches away toward the back. If it is getting power, check the blower motor two-wire connector for power. If there is no power at the connector, the blower motor resister is bad. If there is power, the blower motor is bad.