Welp, since Ive owned my truck neither the heat or defrost setting would blow one bit. ac/vent work fine and blow at all speeds ( if only I had a compressor on it...)
Started at the resistor behind the glovebox and ordered it from the dealer directly, matched the vin and all. When the new one got in I noticed THIS new one was a 3 blade as the one in the truck was a 5 blade. Seeing as I dont have AC I didnt see this being a problem, bolted it in and BAM, no changes.
I can see the diaphrams on the heater box moving just as intended. Is the resistor still the culprit here or should/where do I start investigating next? Id really not like to wait on another one if its not the case. Thanks Again!

 

vacuum lines are what switch it from top to bottom

 

The vacuum line usually breaks at the firewall. A piece of windshield washer fluid tubing will joint them back together.

 

On mine the electrical connection was loose in 2 places. first on the back of the control unit in the dash. Then the connection behind the glove box. I was lucky and just had to unplug then plug them both back in and the problem was fixed. My understanding and investigation in regards to the vacuum hoses is the just allows the door on the transmission tunnel to open and close. Good luck...

 

I'm in the same boat with no defrost. The vac controlled diaphragms are what move the air up or down, there's I think 5 total. Behind your radio you will find a vacumme distribution harness that takes the main vac supply from the brake booster, and transitions it into 5 or 6 different color plastic tubes - each going to its own diaphragm. Check the firewall for sure- but there can be other issues. I have 15" vac at the miulti color harness but I still can't operate the defrost. If you open the glove box, then release the bendy clip so it swing all the way out you will see two diaphragms. The one on the left is what diverts air to the defrost vents. You can experiment my manually moving the control rod it onwards the diaphragm. It's usually stiiff - see if it's hardened and see if it's got vac supply to it.

 

#2isgreen is close....there are only 3 vacuum pots on the heater AC box. But there are about 7 lines in the vacuum connection. Only 5 of the lines run to the vacuum pots though.

Check the main heater/AC switch in the dash. The vacuum plug, with all 7 lines, plugs into the back of it. They can sometimes not be seated properly, and therefore loosing vacuum. Or the small nipples that are on the main control assembly can break with age, or from people removing the assembly incorrectly.

 

UPDATE

Turns out it was the dam resistor all along. Ma' has completely different wiring when comparing a AC to just a heater truck, so using just a heater resistor results in what im experiencing. thanks for all the help tho fellas/ladies.

 

Thanks for the update...good info to keep in the back of the old grey matter....