Defroster without AC

[Moodyalaskan]

Yeah, AC, wing vent, same thing on these years, right?

Probably, just want the moist air to get out. Door window is higher than vent window and may let more warm air out.

 

[Dejure]

BUT everybody knows, vertical openings create far better (perfect) vacuums above 27 mph than do horizontal ones.

[Said by a former smoker when such things were still found on new cars]

SIDE NOTE: Always leave a couple holes open in the fire wall, so the vacuum created doesn't collapse the cab.

 

[Trucksareforwork]

Without vacuum doesn’t the Squarebody system default to defrost? Been a while since I ran one without the vac line but I thought that was pretty standard. I might be misremembering my old ford.

If that’s the case, your just need the blend door to be in the hot position, which is a mechanical link, you need coolant flow to the heater core, and you need the fan to blow.

Defrost is the basic setting.

 

[Dejure]

(Since these show up in relative searches for needed info)

About those square body heater control circuits:

1.0 The amount of power (volts times amps (wattage)) used in each of the blower speeds is, exactly, the same.

What changes is, when the switch is in the high speed position, the full battery voltage and power is going to the motor, but lower speeds are attained by dropping some of the battery voltage [and power] across the wire wound heater resistor by taping different points on the resistor.

Simply put, some of the power consumed on the lower speeds is dissipated at the resistor, instead of the motor.

SIDE NOTES:

(a) The heater resistor is on the firewall, next to the blower.

(b) The high speed circuit is, usually connected to the resistor output, which goes directly to the blower motor.

2.0 MY (yours may be/probably is different) 1978 C15, doesn't have a relay for the heater circuit. As such, all the power to the blower motor and resistor go through the switch.

3.0 For rigs that have a relay for only the high speed switch position (e.g., two relay activation terminals, and two terminals to feed power to the blower), the heater control provides power to the relay in the high speed position, rather than to the blower or through the resistor. This activates the relay, which, then, feeds full battery voltage and power to the blower.

3.1 This means, the switch only has to deal with a fraction of the power it would have to handle without the relay when on high, adding years of life to the switch.

3.2 The catch is, the switch still has to deal with the same power the relay handles in the lower speed positions. So the switch still takes a beating the way it would have in the high speed position, when in the lower speed positions,

4.0 Those whose systems do not have a relay or have a relay that only handles the high speed circuit could add a relay, or relays, to take the load off the switch.

4.1 Instead of the wires from the switch feeding directly to the resistor, they would feed the activation circuit of relays (I like the idea of having one relay for each speed, so if one relay fails, the other speeds still work) and the relay would then feed where the switch wire, previous to the modification, went on the resistor plug.