My 1968 GMC Fan works no heat through the ductwork🙁

[Howell1988]

So I bought this 68 GMC. She is a great running truck and solid for only 1800 bucks. The only issue I'm having is I hooked up the box to the firewall that was sitting in the guys shop when I bought it. Got my hoses ran. They're hot and the fan is blowing on all speeds. Now the lines were not hooked to the inside box to open and close the vents but I physically moved them open and close. Also open and closed the flap on the box on the fire wall. Also note it is an A/C cab so this box is A/C equipped and seems in very good shape. Now the box under the dash is very warm but no air is coming out of it. Seems the fan would be blowing it in well by the sound of the fan on high. But....I got nothing. Also one more thing there is a gap where I'm sure a seal needs to be between the box and firewall where the fan is blowing in. I'm new to this heater stuff although this is my third old truck and a keeper....tis the season for heat. Aaand a few pics added. Thanks for any help. Also I think I may need to switch my hoses first thing in the morning they are both hot...but I think they may be wrong.

[Howell1988]

Loading it on the trailer just 6 days ago🙂

[Astro-Balls]

Pull the heater Box out from under the Dash, its not that difficult, you probably have a bunch of leaves and stuff plugged up in there.

[RustyPile]

^^^^^ What he said.. After you get the duct work and boxes cleared of debris, replace, repair, and properly install the control cables and vacuum lines that control the diverter doors..

[Andy4639]

Surely you cleaned any debri out of it before you installed the box. That shouldn't be the problem.

Get the cables all hooked up right and make sure the doors and flapper's are all working correctly. The diverter door in the bottom is the most likely one not working. You can move it by hand all you want but it doesn't mean it's moving the flapper. The rubber comes apart and the rod is just spinning inside but the flapper is closed I bet. When you put it together if you got the end of the rubber caught in the gap it want move either and close off the heat.

[Howell1988]

Thank you for all the replies and I'm starting this morning by taking the box under the dash out. The box on the firewall is clean and clear fan blowing strong so I'll start in the cab. Thanks again. I'll be getting to it now.😁

[Howell1988]

Originally Posted by RustyPile

^^^^^ What he said.. After you get the duct work and boxes cleared of debris, replace, repair, and properly install the control cables and vacuum lines that control the diverter doors..

What's the importance of this lever and should it be turned right keeping the box flap closed?

[Astro-Balls]

Originally Posted by Howell1988

What's the importance of this lever and should it be turned right keeping the box flap closed?

You should have a cable going to that arm, I think that controls the door between the AC and Heater core.

[Astro-Balls]

Also the side vents can get filled with leaves and debris, the oval vents on both kick panels are easy to remove, stick a vacuum in there and get all that stuff out too..

[Howell1988]

Cleaned out the passenger side it was loaded with debris. Now for the driver side. Also placed the gasket seal between firewall and blower. Hoses are correct. Opened ductwork under dash nothing jammed or closed. Heater core is now exposed and with the blower on I still feel very little air passing from blower through the core... maybe let her warm up and run down the road see if it start coming through or should I pull the box entirely and buy a new core� it doesn't show signs of leaking but maybe clogged?

[Astro-Balls]

Yep, Now is the time to pull the core out and put in a new one or have your original cleaned out.

[randy500]

That valve selects heat or cooling.
Lever towards drivers side for heat.

That area of the outside box is usually full of leaves and other crap.

[Howell1988]

Originally Posted by RustyPile

^^^^^ What he said.. After you get the duct work and boxes cleared of debris, replace, repair, and properly install the control cables and vacuum lines that control the diverter doors..

Found something right before getting ready to buy a core. Whoever heard of someone welding a cover over the hole the air comes into the cab through.. guess it was just a summer truck for the guy? Wanted to keep the heat out of the cab? So yeah....the air wasn't getting in to the core that isn't leaking or clogged its getting heat but no blow....because the hole is closed. Tomorrow opening that hole up and ordering all new duct hoses tonight since they weren't in it when I bought it. Thanks everyone. Hopefully I'll post success tomorrow. Next project resealing the cab roof.

[Howell1988]

Took a picture for y'all. Can you see it back there welded up?

[Astro-Balls]

Wow, Never seen that before..

[Astro-Balls]

Check to make sure your AC Control switch is OK, Its located inside the cab behind the plenum box.

[randy500]

Originally Posted by Astro-Balls

Check to make sure your AC Control switch is OK, Its located inside the cab behind the plenum box.

I think he meant to say “relay” there always bad which means no high speed fan setting.

[randy500]

And the welded up hole....the stupidest thing i have ever seen.

[randy500]

And when you get the outer box off....the easiest way to clean it out is to remove the evaporator case half and the evaporator, debris hides behind that flap door with the lever for hot and cold.

[Astro-Balls]

No, I meant to say Switch, If that switch is bad your AC Clutch wont turn on.
Mine was missing..

http://67-72chevytrucks.com/vboard/s...d.php?t=787771

[Howell1988]

Thank you and yes the outer box was off when I got the truck. I will have to pull it in the morning and get the welded plate off and I can't believe I never noticed the plate I was like wow how could I miss that...though I didnt know what I was looking for since this is my first this model. My last was a 66 lwb I left to my dad. Too much rust and things wrong for me to afford. So hopefully all goes smooth in the morning my friends. Thank you all so much for replies and until next update I'm out.

[Howell1988]

Got the firewall opened up guys. Both sides were welded shut luckily it came out fairly easy with a little grinding and light tapping popped right out.

[Howell1988]

Got her buttoned back up and blowing hot really hot air boys just in time for a cold front. Now order the duct work and she'll be wrapped up on heater work woo hoo thank you all for all the suggestions and interest in my post. Cheers.

[Stocker]

Yes, there is a well-hidden switch that controls the A/C compressor's clutch (thank you for linking that thread). However it doesn't affect the fan's airflow, just whether or not the compressor engages so the air can be cooled.

[Stocker]

Good to hear it! These old trucks might be dinosaurs but the heaters are very effective.