1978-79 Ford F100-F350 Bronco Heater/AC Vacuum System Diagram & Test Chart

This page from a Ford factory reference booklet explains the heater and air conditioner vacuum system used on 1978 F-100 through F-350 trucks and the Bronco. It lists the system's major components (vacuum-actuated push-pull motors, temperature selector valve, 8-port vacuum selector operated by the function selector lever, vacuum reservoir, and vacuum check valve) and provides a step-by-step trouble-shooting procedure for cases where the outside-recirc door or heater water valve does not operate, including vacuum measurements (at least 10 inches Hg supply, 18 inches Hg applied to test a motor) and instructions to replace motors, valves, or repair hoses as required. A large 'Deluxe Hi Lo Heater System Vacuum Motor Test Chart' shows which vacuum motors (outside-recirc, water valve, panel-floor partial/full, heat-defrost partial/full) are applied with vacuum for every combination of function lever position (OFF, VENT left/right detent, HI-LO, HEAT, DEFOG, DEFROST) and temperature lever position (Cool-1 through Warm-5), with a vacuum line color code (white, brown, yellow, red, orange, blue). Diagrams show the dash control panel with its levers and an airflow/vacuum-motor schematic of the blower motor, blend door, heater core, evaporator core, water valve, and outside-recirc door.