I have recently noticed my defrost mode is no longer functioning. To be honest I do not know if it ever worked but I think it did. I have had the truck for a few years and half of the time it was not running because of a major engine upgrade. My orginal 360 is now a happy 360 with new eldebrock intake, cam, headers etc.

The new intake requires a repositioning of the AC compressor and I have not done that as of yet. So here is the question, with the A/C no longer functioning and the electrical and and all hoses removed, will that affect the defrost function? I have vacuum to the heat doors because I can physically move them to the open position and push the heat button and they will close. The dash vents are always open and air is moving through them but the defrost mode does not function. I have hot air at my feet and vents but can't get it up on the window and just wonder if the absence of A/C components is at the base of my problems. Any thoughts? tnx psg

No it is not.

With that then, any thoughts on my next step, is it likely behind the controls or at the defrost door? Maybe you can also identify the module or "can" in the heater return line hose in the engine compartment that has a vacuum line attached to it. It doesn't seem to be doing anything. tnx psg

Have you seen the duct work going to the defrost vents? These could have come apart.

check the back of your heater control. There is a series of small vacuum lines going there, that have a rubber block holding the lines. This deteriorates. I had the same problem with an '86 I had.

Thanks basket case, I guess it is time to pull more things apart...I was hoping to avoid the process but it seems like a reasonable next step. psg

I dunno about trucks, but cars of that era would default to defrost if vacuum was lost to the controls.

I would pull the line off the vacuum can that controls defrost and see if you are getting constant vacuum there, if so the control unit is probably the issue.

I was just gonna suggest that.

That said I have a project in the garage, an 85 d150.part of the project includes adding "factory" ac to a truck that originally came w/o

I have most of 3 factory ac heater boxes to scrounge and cherry pick the best components from to (hopefully) build up 2 "factory ac" boxes among the pile.
I say most o, because one housing is busted all to hell probably from rough removal) not done by me, was a swap meet find as it sits)
My main concern is can anyone tell me if the gaskets and seals within are available anywhere, whether nos or possibly from one of the aftermarket rubber/weatherstrip places for these trucks?

gasket sets are avail for a/c and non a/c. you do not get the foam on the doors. my dad did his and he went to the upholstery shop and bought thin foam to fix the doors. detroit mucsle offers the kits.

I agree.
Then to carry it farther, if the control valve is the issue, you should be able to remove the vacuum from the control and have air coming out of the defroster ducts, then all you need is temp control. I seldom switch off from defrost during the winter, and normally control the inside temp by fan speed, but usually my winter trips are not very long.

For the record, without the AC, the defrost isn't nearly as quick clearing the windshield. The AC removes the moisture from the air that hits the glass, that dry air clears the glass 2x as fast. That was the main reason AC became common on vehicles, faster clearing of the windshield in defrost mode. I believe that the Government required (or mandated) AC to be included at one point in time.

So I dived into the climate control and found the two vacuum line groups at the back of the control module. Everything seemed fine. I followed the left group which are all black lines that lead to a connector which attaches to additional black lines. It turns out there is one line, which is short and not connected to anything, open and does not appear broken or torn at the end. I cannot find anything to connect it to. It is only about 4-5 inches long. Any thoughts? Is there a circuit diagram for these lines?

"I would pull the line off the vacuum can that controls defrost and see if you are getting constant vacuum there, if so the control unit is probably the issue."

I am not sure which can works the defrost. I have only found two behind the glove box.

I think you have a one way valve in that mess of tubing under the dash also