1978 Corvette Heater - Temperature Control
Thread Starter
Burning Brakes
Joined: Jan 2001
Posts: 1,217
Likes: 444
From: Loir Valley Sarthe, France
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2024 C3 of the Year Finalist - Unmodified
2023 C3 of the Year Finalist - Unmodified
1978 Corvette Heater - Temperature Control
So, silly question of the week here!
How or what actually controls the temperature of the heat in the C3 heater, specifically, a 1978? I ask following the detachment of the Bowden cable from the control lever on my ’78, which prompted me to think what was at the other end of the cable! I know it goes to a small lever up in the heater box assembly, but what exactly does it do there? 
I also know that the valve in the heater hose in the engine bay is controlled, via vacuum, from the lever in the car, so what it the role of that valve?
I’m pleased to say I’ve re-located and more securely fixed the cable and the heater is now nice and warm or cool as I want it, but I’m just interested to know the “mechanics” which none of my books fully explain.
How or what actually controls the temperature of the heat in the C3 heater, specifically, a 1978? I ask following the detachment of the Bowden cable from the control lever on my ’78, which prompted me to think what was at the other end of the cable! I know it goes to a small lever up in the heater box assembly, but what exactly does it do there? I also know that the valve in the heater hose in the engine bay is controlled, via vacuum, from the lever in the car, so what it the role of that valve?I’m pleased to say I’ve re-located and more securely fixed the cable and the heater is now nice and warm or cool as I want it, but I’m just interested to know the “mechanics” which none of my books fully explain.
Burning Brakes
Joined: Feb 2013
Posts: 1,062
Likes: 28
From: Burkburnett TX
on my 82, I can see this flapper door through the opening for the blower motor resistor under the hood. When my cable broke I was able to use a long screwdriver to push the flapper towards the heat side to warm me up. That reminds me...i need to fix that cable..
Le Mans Master
Joined: Sep 2002
Posts: 5,236
Likes: 898
From: Myrtle Beach SC
The valve turns the flow to the heater core on and off and the cable probably connnects to a door to vary the amount of air flowing through the heater core.
Your temp control switch on the console allows the driver to direct the hot/cold air mixture to different vents. For example, when defrost is selected, hot air goes to the windshield. Full heat selected, if I remember correctly, minimizes cold air and directs it to the windshield. When cabin heat is selected, actuator valves by the floorboard vents allows or limited cold air to mix with the ductwork hot air which comes through those side vents.
If you select A/C, the heater box actuator valve is closed so, theoretically, hot air is prohibited from entering the cabin and you just get cold air from the A/C condensor. But if that valve doesn't close...well than you have hot air that is getting in when you want just cold air and that becomes a common complaint with C3 owners.
Hope this helps
Safety Car
Joined: Jun 2004
Posts: 4,372
Likes: 223
From: Beecher Illinois
When the TEMP control is specifically ON COLD.
The CABLE activates a vacuum SWITCH(above the heater box behind the glove box)that activates the vacuum VALVE in the heater hose.
Safety Car
Joined: Mar 2007
Posts: 3,652
Likes: 49
From: Bridgewater nj
The selector control, AC/ACmax,vent, heat, defrost
The heat position activates the heater hose valve with vac and sends the heat to the floor.
Safety Car
Joined: Jun 2004
Posts: 4,372
Likes: 223
From: Beecher Illinois
Everything else is vacuum operated.
End of cable @ the door and vac switch....
Last edited by KapsSA; Mar 25, 2014 at 11:45 PM. Reason: pics
Corvette Stories
The Best of Corvette for Corvette Enthusiasts
Top 10 Most Expensive Corvettes Ever Sold on Bring A Trailer
Brett Foote
10 Things Every Corvette Owner Needs (2026 Edition)
Michael S. Palmer
8 Most "Only Corvette Owners Understand" Quirks and Problems
Pouria Savadkouei
10 Reasons the C6 Z06 is Still A Performance Benchmark After 20 Years
Joe Kucinski
How Much Horsepower Every Corvette Engine "LOST" in 1972
Joe Kucinski
Top 10 DOs and DON'Ts for Protecting Your Convertible Top!
Michael S. Palmer
Top 10 Most Explosive Corvettes Ever Made: Power-to-Weight Ratio Ranked!
Joe Kucinski
150 hp to 1,250 hp: Every Corvette Generation Compared by the Specs That Matter
Joe Kucinski
8 Coolest Corvette Pace Cars (and Replicas) of All Time
Verdad Gallardo
Thread Starter
Burning Brakes
Joined: Jan 2001
Posts: 1,217
Likes: 444
From: Loir Valley Sarthe, France
2025 c3 ('74-'82) of the Year Finalist - Unmodified
2024 C3 of the Year Finalist - Unmodified
2023 C3 of the Year Finalist - Unmodified
Hi Guys
Thanks for all the info, and the photos! As I say, I was just curious really, but this is all interesting stuff. I'm afraid I failed to take any photos of the fault or my repair! Hopefully nothing further will go wrong - it looks a whole lot of fun if anything vacuum related fails!
Thanks for all the info, and the photos!
As I say, I was just curious really, but this is all interesting stuff. I'm afraid I failed to take any photos of the fault or my repair! Hopefully nothing further will go wrong - it looks a whole lot of fun if anything vacuum related fails!