heat in 1978 vette
Race Director
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From: Dayton, Ohio
78 Stan , you said in your OP that the heater controls didn't work.
Have you checked that out ? A vacuum leak would cause the controls to not work and also would allow free flow of coolant through the heater control valve.
Have you checked that out ? A vacuum leak would cause the controls to not work and also would allow free flow of coolant through the heater control valve.
Melting Slicks
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Your '78 may be different but what determines the size of the heater hoses is the size/sizes on the heater core. On my '77 the inlet to the core is 5/8" from the intake and the outlet which hooks to the side of the water pump is 3/4". mike...
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On another post I see they used a 3/4" valve.
http://forums.corvetteforum.com/c3-g...nder-40-a.html
I will do my best with the 5/8"... hopefully I don't have to return it.
http://forums.corvetteforum.com/c3-g...nder-40-a.html
I will do my best with the 5/8"... hopefully I don't have to return it.
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Should I turn the turnbuckle to the right or left? I have my passenger side trim panel off and I am looking at it. It appears to be connected but I'm not sure what else to do, or if I should remove the glove department.
My headlights work by vacume and it seems fine! No vacume issues I can detect.
Last edited by 78stan; May 17, 2011 at 07:26 PM.
Le Mans Master
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From: Oxford MA-----You just lost the game!!!!
Whats the benefit of putting in a manual shut off valve up on top from rather than fixing the heater control valve?
As you can see I think the hose is a 1/2"... I would need to get a fitting that fits this.
I get that the red circles is where I would cut the hose and put in a ball valve from home depot... something that could fit an 1/2" piece fitting, but what about the heater control valve that I assume is deeper in the vette... whats the purpose of that?
The heater control valve for a 1978 vette is hard to get to right? I mean its right up on the passenger side near the firewall.
As you can see I think the hose is a 1/2"... I would need to get a fitting that fits this.
I get that the red circles is where I would cut the hose and put in a ball valve from home depot... something that could fit an 1/2" piece fitting, but what about the heater control valve that I assume is deeper in the vette... whats the purpose of that?
The heater control valve for a 1978 vette is hard to get to right? I mean its right up on the passenger side near the firewall.
I know this is off topic, but I'd be more concerned about that sketchy lookin' fuel line.........
Scott
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From: Buena Vista Colorado
Should I turn the turnbuckle to the right or left? I have my passenger side trim panel off and I am looking at it. It appears to be connected but I'm not sure what else to do, or if I should remove the glove department.
Here's a link to a replacement vacuum valve so you can see what it looks like:
http://willcoxcorvette.com/product_i...ducts_id=26130
Le Mans Master
Joined: Apr 2008
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From: Powassan ON
Might be worth re-routing the lines, or using a braided type line for that.
FWIW, my '78 has 5/8 inside diameter hose running between the manifold and the HC.
Dont forget to shut the fan off, the switch only goes to a "low" setting. Unplug the fan. Its near the pass side hood latch, in the top of the heater box. A 3 or 4 wire L shaped connector.
dodosmike
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From: Dayton, Ohio
I don't remember which way the turnbuckle must be turned. Yes, you need to remove the glove box (4 phillips screws and the light assembly) so you can see what's happening with the plunger in the vacuum valve. As I said before, the plunger must be fully depressed in order to supply vacuum to the underhood valve. That's how the underhood valve is held closed so coolant does not flow through the heater core. The temperature control lever must be fully to the left (cold) as you adjust the turnbuckle.
but sometimes the cable resistance wont hold the vacuum valve fully depressed. If thats the case I'll share what I do to help with that.edit-i found an old thread with what i do to help with the valve
http://forums.corvetteforum.com/c3-g...-question.html
Last edited by ...Roger...; May 18, 2011 at 01:00 PM.
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Pro
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From: Roseville CA
Agree with this except that only one shutoff valve is needed to stop coolant flow through the heater core. If you go this route, install the manual valve in the smaller hose. It is the heater core supply line, and coolant flows through it from the intake manifold to the heater core. That's what I did on my '78 with the exact same "heat in the footwells" problem described by the OP.
Turned out I had 2 problems with my stock system: (1) A badly miss-adjusted temperature control cable, and (2) a broken vacuum hose.
The temp control cable not only controls an air mixing flapper, it also actuates a "bypass valve vacuum switch" that cuts off or supplies vacuum to the "heater hose bypass valve" installed in the coolant supply line between the intake manifold and the heater core. The miss-adjusted temperature control cable was not depressing the plunger of the bypass valve vacuum switch; thus the heater hose bypass valve was always OPEN (the fail-safe position) and hot coolant was ALWAYS flowing through my heater core. So I had HOT footwells regardless of where my temperature control lever was set.
Also, the small white or yellowish vacuum hose from the bypass valve vacuum switch to the heater hose bypass valve was completely severed INSIDE the firewall grommet. That hose had turned very brittle over 30 years.
The temperature control cable moves a lever that depresses a plunger on the bypass valve vacuum switch. The position of that lever is adjusted through a turnbuckle in the cable. Remove your passenger seat and the center console side cover, and you'll see the turnbuckle (white plastic, about 1-1/2" or 2" long) in the temperature control cable. (The turnbuckle looks a lot like an in-line fuse holder.) You'll have to remove the glove box liner to see the bypass valve vacuum switch that is mounted on top of the heater box behind the glove box. The plunger on the bypass valve vacuum switch must be FULLY depressed (temp control lever fully COLD) in order to supply the vacuum to the heater hose bypass valve and thus stop the flow of coolant through the heater core.
Here's the best climate control vacuum system schematic I've found so far:
I hope all this helps.
Turned out I had 2 problems with my stock system: (1) A badly miss-adjusted temperature control cable, and (2) a broken vacuum hose.
The temp control cable not only controls an air mixing flapper, it also actuates a "bypass valve vacuum switch" that cuts off or supplies vacuum to the "heater hose bypass valve" installed in the coolant supply line between the intake manifold and the heater core. The miss-adjusted temperature control cable was not depressing the plunger of the bypass valve vacuum switch; thus the heater hose bypass valve was always OPEN (the fail-safe position) and hot coolant was ALWAYS flowing through my heater core. So I had HOT footwells regardless of where my temperature control lever was set.
Also, the small white or yellowish vacuum hose from the bypass valve vacuum switch to the heater hose bypass valve was completely severed INSIDE the firewall grommet. That hose had turned very brittle over 30 years.
The temperature control cable moves a lever that depresses a plunger on the bypass valve vacuum switch. The position of that lever is adjusted through a turnbuckle in the cable. Remove your passenger seat and the center console side cover, and you'll see the turnbuckle (white plastic, about 1-1/2" or 2" long) in the temperature control cable. (The turnbuckle looks a lot like an in-line fuse holder.) You'll have to remove the glove box liner to see the bypass valve vacuum switch that is mounted on top of the heater box behind the glove box. The plunger on the bypass valve vacuum switch must be FULLY depressed (temp control lever fully COLD) in order to supply the vacuum to the heater hose bypass valve and thus stop the flow of coolant through the heater core.
Here's the best climate control vacuum system schematic I've found so far:
I hope all this helps.
Here's some pics from my 1980 vette when I had a heat issue. I replaced the vacuum control valve that is attached to passenger side front fender, temp was still 68F with AC running. I then pulled the glove box and the vacuum control switch button was not being depressed when lever was in full cold position. I made the cable adjustment and the temp went from 68F to 28F when AC is running on my car.
Tony G
Last edited by Yellow1980; May 19, 2011 at 12:31 AM.
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There is no button
I pulled off the glove box, and found that my cable was not even touching the button. I adjusted it but then found that at its most it doesn't even make contact. Then... I found out that the vacume tube switch doesn't even have a button to push! I placed my index finger on the tip and nothing was being depressed.
How would I fix this?
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From: Dayton, Ohio
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Last edited by 78stan; May 19, 2011 at 05:57 PM.
Racer
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From: Buena Vista Colorado
Is this the bolt?
I am assuming there are two...
This valve is a real PITA to get out and replace without removing the dash, but it can be done with the right 1/4" drive small socket + extension + socket driver combination. Removing the A/C duct to passenger side outlet gives a little more space. That duct is just a slip fit at both ends and you can pull it out through the glove box opening in the dash.
If you decide you'd rather remove the dash, here's a link to a paper that will help. It covers heater core removal/replacement, which requires dash removal:
http://www.corvettefaq.com/c3/heatercoreremoval.pdf
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Just to update everyone.
I installed the "custom" shut off valve. I was worried for a bit because the temp passed over 200, but then dropped rapidly. I guess it didnt get a chance to warm up when I was on my way. I am skipping the vacuum tube install... no need for this if all I'm gonna do is shut off the valve. Thanks
I installed the "custom" shut off valve. I was worried for a bit because the temp passed over 200, but then dropped rapidly. I guess it didnt get a chance to warm up when I was on my way. I am skipping the vacuum tube install... no need for this if all I'm gonna do is shut off the valve. Thanks
Melting Slicks
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What a great thread!! This is why this forum and the people who participate are the best. Having a very similar problem with my 69 - heat coming up from the defroster vent. I now know the short term solution (the external shut off valve) and what to do to correct the problem in the long run. Can finally take my wife out to a cruise without her worrying about her makeup and hair!
Burning Brakes
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I don't remember which way the turnbuckle must be turned. Yes, you need to remove the glove box (4 phillips screws and the light assembly) so you can see what's happening with the plunger in the vacuum valve. As I said before, the plunger must be fully depressed in order to supply vacuum to the underhood valve. That's how the underhood valve is held closed so coolant does not flow through the heater core. The temperature control lever must be fully to the left (cold) as you adjust the turnbuckle.
Here's a link to a replacement vacuum valve so you can see what it looks like:
http://willcoxcorvette.com/product_i...ducts_id=26130
Here's a link to a replacement vacuum valve so you can see what it looks like:
http://willcoxcorvette.com/product_i...ducts_id=26130