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73-87 Chevy _ GMC Trucks => Interior & Equipment => Heating, Ventilation & Air Conditioning (HVAC) => Topic started by: runnergeorge on August 26, 2020, 12:16:49 am

Title: Ac blowing from dash and floor vents
Post by: runnergeorge on August 26, 2020, 12:16:49 am

Hey everyone! I have a 75 C10 350/350. I recently got the ac system working again, running 134. The air is cold, however I am having an issue with the air coming out of the vents. When the ac is on air is being blown out of the dash and floor vents, I only want it coming out of the dash vents. I am wondering If anyone has/had this issue and if there is a fix? Thanks!

PS. The climate controls has options to select different modes, but there is only one option for ac.

Title: Re: Ac blowing from dash and floor vents
Post by: mdws1987 on August 26, 2020, 12:54:41 pm

I believe that I have the same problem. The ac is not cooling the truck as it should because warm or hot air entering the truck out of the heating duct. Could it be one of the doors on the box under the dash is not working????. Might it be a vacuum control issue???

Title: Re: Ac blowing from dash and floor vents
Post by: runnergeorge on August 26, 2020, 02:37:33 pm

I have read on here and other sites that it could be either a vacuum issue. The blend door for the heater and ac works relative to when I choose hot only hot air comes through and vice versa. My issue is air is blowing through both the floor vent and dash vents.

Title: Re: Ac blowing from dash and floor vents
Post by: MIKE S on August 26, 2020, 02:45:06 pm

The hot cold mode is controlled by a cable. The mode doors of which vent the air will come out of is controlled by vacuum. Verify that you have a vacuum supply that is not leaking first.   

Title: Re: Ac blowing from dash and floor vents
Post by: runnergeorge on August 26, 2020, 03:50:44 pm

That’s another issue I’m having. I cannot for the life of me find a vacuum diagram for my truck. I’ve gone through the manuals on here (all few hundred pages). Do you know where I can find one?

Title: Re: Ac blowing from dash and floor vents
Post by: Rapid Roy on August 26, 2020, 04:00:52 pm

My 74 blows at floor on vent and heat setting. Does not on bi-level, defrost and AC. Does not blow hot unless using heater. Blend doors may need adjusting and/or may have some broken parts.

Title: Re: Ac blowing from dash and floor vents
Post by: bd on August 26, 2020, 08:20:41 pm

1974 Service Manual, page 1A-93 (http://www.73-87chevytrucks.com/techinfo/7387CKMans//Service/ST_330_74_1974_Chevrolet_Light_Truck_Service_Manual.pdf#page=117).

Title: Re: Ac blowing from dash and floor vents
Post by: runnergeorge on August 26, 2020, 11:48:48 pm

Thanks bd! I never thought to look in the 74 manual since mine is a 75. But now that I think about it, duh! Should have looked there. I’m going to get into the vacuum issue as soon as I can and I will update as I go. I have a feeling there is an issue with the vacuum not controlling the doors to only blow through the vents I want.

Does anyone have a picture of what their vacuum ball looks like on the passenger side? I’m guessing that is the one used for the climate control system?

Title: Re: Ac blowing from dash and floor vents
Post by: runnergeorge on August 27, 2020, 04:29:12 pm

I found a few minutes today and dove into the truck and the manual. I found a diagram of the hose going from the vacuum tank above the blower motor to a port on the intake manifold. Checked my truck, there was a huge gash hidden from view! Got a new hose, slapped it in, vents work great as they should. Now I need to follow some others advice from here and upgrade to a 90s blower motor 😎

Thanks for the help everyone.