How much vacuum should be supplied to the heater/AC vacuum motors at idle?

The document says supply vacuum should be at least 10 inches of vacuum (Hg) with the engine idling; less than 10 inches indicates a supply or engine problem.

How do I test a vacuum motor for the outside-recirc door or heater water valve?

Turn off the vehicle and apply 18 inches of vacuum (Hg) to the motor. If the motor will not hold vacuum, replace the motor.

What are the vacuum line colors in the Hi-Lo heater system?

White (outside-recirc), brown (water valve), yellow-top and red-side (panel-floor partial/full), orange-top and blue-side (heat-defrost partial/full).

What are the major components of the heater and A/C vacuum system?

Vacuum actuated push-pull motors, the temperature selector valve, the 8-port vacuum selector operated by the function selector lever, a vacuum reservoir, and a vacuum check valve.

What should I check if vacuum at the engine is over 10 inches but the system still fails?

Check the supply hose to the vacuum reserve tank, the hose from the reserve tank to the vacuum selector valve, and the in-line vacuum check valve; if those are OK, search for a leak or blockage in other vacuum systems.

Home/ Registry/ Ford/ Bronco/ 1978-1979/ 1979 Ford Bronco/ Photo supermotors.net/registry/media/1032661

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

1978 F-350 and Bronco heater-AC vacuum system with vacuum motor test chart

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Heater/AC vacuum system operation plus a vacuum motor test chart listing applied vacuum (inches) per function/temperature lever position for outside, water, panel-floor, and heat-defrost motors.

AI caption August 2014 · Photo 42 of 48

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.

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Frequently asked questions

How much vacuum should be supplied to the heater/AC vacuum motors at idle?: The document says supply vacuum should be at least 10 inches of vacuum (Hg) with the engine idling; less than 10 inches indicates a supply or engine problem.
How do I test a vacuum motor for the outside-recirc door or heater water valve?: Turn off the vehicle and apply 18 inches of vacuum (Hg) to the motor. If the motor will not hold vacuum, replace the motor.
What are the vacuum line colors in the Hi-Lo heater system?: White (outside-recirc), brown (water valve), yellow-top and red-side (panel-floor partial/full), orange-top and blue-side (heat-defrost partial/full).
What are the major components of the heater and A/C vacuum system?: Vacuum actuated push-pull motors, the temperature selector valve, the 8-port vacuum selector operated by the function selector lever, a vacuum reservoir, and a vacuum check valve.
What should I check if vacuum at the engine is over 10 inches but the system still fails?: Check the supply hose to the vacuum reserve tank, the hose from the reserve tank to the vacuum selector valve, and the in-line vacuum check valve; if those are OK, search for a leak or blockage in other vacuum systems.