I think (and don't quote me on this because I have never owned a pre 1981 truck), that on a 1979 the blower cannot be completely shut off.
The electrical control switch only has 3 positions (HI, MED, & LOW) correct? So that switch would not provide a means for totally securing power to the blower.
And, as far as I know, even though the distribution lever has an OFF position, it has no effect on blower speed. It only serves to move the various dampers - that direct the air flow into the cab - to their fully closed positions.
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On the other hand, if the truck is equipped with an A/C system, there
is an interaction between the directional lever and power supply to the blower.
However, it is the opposite of a shut down; when the lever is shifted to MAX AC, the blower is supplied with full battery voltage. Even in the OFF position the blower still runs on LOW. With the lever in any other position, all blower speeds are available.
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