A/C only works on hi BUT heat,Vent, Def positions have all speeds?

[MikecLA]

So i got my 74 C10 factory a?c all up and running cold finally. in A/C position blower motor fan only works in HI position. But when on Vent , Heat Def all speeds work.
Factory  Wiring diagram isn't clear at the switch whats going on but think its up here, anyone else run into this?  Resistor should be fine due to all other functions have speed control. thanks

[Rapid Roy]

Slide the temperature arm from all the way cold  about 1/4" toward hot. You should feel a slight indent. Then all speeds will work.

With the arm all the way cold is the same as "Max AC".

[MikecLA]

Wow Roy that's some Jedi mind stuff right there! Thank you, I love that function!, cant believe i didn't notice it yet.

[Rapid Roy]

You are welcome. I did not know about it till I had the truck 4 years. I happen to find out from someone else.

Enjoy

[Rapid Roy]

Also, with all the climate control settings to off, the fan still blows on low. It defaults through the ac vents. The switch speeds are Lo, Med 1, Med 2, Hi. I found out a way to eliminate the Lo speed fan from blowing. It was especially bad during winter when it would blow cold air in my face when starting. The upper wire in the resistor connector can be removed and Lo speed is eliminated.

[MIKE S]

 If you like the way the fan works that is fine. It used to irritate me that you could only have recirculate with high speed. There is a single wire on the control head you can remove that will allow you to have any speed while on recirculate.

[VileZambonie]

It should default to defrost and floor in the off position and run the fan anytime the key is on. This is actually a good thing, especially in the old heater box design. It keeps the mice strong trying to run the wheel 

[MikecLA]

The always on low speed can be annoying,blowing hot or cold air depending on the season. i might wire in a switch perhaps. thanks again.

[bd]

"Annoying" is driving a vehicle that has mildew permeating the cab.  Drying the ductwork to prevent mildew formation is the real reason the fan circulates air continuously ... that and keeping the mice healthy.   

[frotosride]

For the longest time I thought I just had a constant blow through heat and AC system. I even thought I may have had a new rust hole I didn't know about... Imagine my surprise that is was all just a simple engineering design for my health and comfort! 

[blazer74]

If you don’t like the high blower all the time when in max mode, lever all the way cold activating the internal switch.
Remove the single orange wire from the blower speed  switch and it will have all the speed settings.
The extra wire defaults the switch to high when in max on your 74 year.

[VileZambonie]

Max is intended to be used to get to the desired temp more rapidly and then you should switch to the fresh setting.