Defroster problems 1972 Chevrolet PU

[w6tractor]

I am confused/frustrated wth my A/C-Heater. Everything works perfectly, except the de icer. When I move the controls to the position to de ice, the A/C compressor does not come on. I have had all the A/C/Heat ducting etc. off and replaced. The cables go to the correct places. The switch on the underdash heater box that controls the compressor works, but not when set for de ice. Everything else switches from vents to floor to windshield outlets. How do I get the compressor to come on in the de ice position? What have I missed? Thanks

[Camper Power]

the compressor shouldnt come on when you have it set to heat. it should only come on when you set it to A/C. the heater uses heat from the engine coolant to warm the air.

[leddzepp]

The A/C compressor is not supposed to be on in the de-ice position. You are using heat from the heater core to de-ice. Do you mean that the fan does not come on?

[red71c10]

Modern cars and trucks do have the A/C kick in when the controls are set to defrost (deice, defog), but older rigs do not. The A/C acts as a dehumidifier and helps remove the excess moisture from the windshield.

[w6tractor]

I didn't mention that this, but the truck is factory a/c. It has a position for both defrost and de ice. I believe that on this model the compressor is supposed to run to dehumidify the air as you stated. Just can't figure out what's wrong to keep it from working. Thanks for the comments.

[leddzepp]

I have factory A/C on both my 72's. When you slide the control lever over, it hits the button on the back of the box to shut off the compressor. It is not supposed to run in our trucks to dehumidify the air, unlike newer model cars.

[w6tractor]

That's exactly what it does. That seems strange to have that capability and not be able to use it. Thanks for clearing that up. Thanks to all of you that responded. On to the next challenge.