I just bought a 1978 vette. The heater blower works fine. The air will only come out on the floor. Moving the heater control does not change anything. How do I check out the heater control? I believe it is vacuum operated. Where does the vacuum come from to operate the heater control unit? I also have a post about the blower motor. Thanks

 

Try this link, I think your find everything you need to know...
http://home.comcast.net/~chadwick.ro...entilation.htm


Bob

 

thanks, i'll look at that

 

Is there a way to know if the heater control itself is working properly?

 

If I understand you correctly...
Just run the controls through the motons, defrost, heat, vent ect:

Sounds like yours is not working correctly
Its not a big job to replace the control....... at least not on the 73-76

 

On the back of the heater control switch is a vacuum switch. This part of the switch controls the vacuum to the damper motors. Without a vacuum supply (or bad vacuum switch) the default would be lower vents only. To access the switch you will need to remove the carpeted side panels on the center console (it’s no fun). First place I would check is on the engine and look for a broken vacuum hose. Then check the vacuum switch.

 

There are a lot of vacuum hoses in the engine compartment. Is there a way to know which one or what area to look? Where does the line i am looking for go through the firewall?

 

Look for vacuum lines that enters into the fire wall and work backwords

 

It should be a thin black plastic line...not a typical rubber hose.

Hate to say this but sometimes the problem is with the actuators that are tucked up under the ductwork. The actuators work like those for your headlights and actually operate the vent doors that open and close depending on the control switch position. If you've got vacuum to the switch...next place to look is the actuators.

I check all mine out when I replaced the heater core in my 78...so I'm quite familar with them.

 

My guess is that one of the vacuum actuators is frozen in the closed position. That's how mine was.

 

I posted a pic of the controls in your other post. Something to look for, the vac switch itself in the console sometimes leaks. You may need to drill out the rivit that holds it together, clean/lubt the seals and put it back together with a screw and locknut to get it functioning.

 

Yep, Thats the problem about 90% of the time.

 

Thanks everyone. I'll try some of those things. Great to have friends on a site like this!!

 

not sure of the rest but the actuator that controls the temp control
is on top of the heater box just behind the glove box.
pulled the glove box and found that mine was way out of adjustment
so i was always getting hot water through the system.

once i fixed this it cut back about 80% of the heat i was getting in the foot well area

 

I had a problem with my 81 - same issue - fan worked fine on the floor but i could not switch it. I traced it to the Vacumn Switch on the Heater control. Put in a new Vacumn Switch and i was back in business. BUt be careful bolting the heater control back in - i have the CB radio and the small vacumn lines are right up against the CB - the lines can get kinked.

 

I had the same problem on my 78...On the passenger side of the firewall, near the hood light switch there is a very thin (like a coffee "straw") vacuum line (mine was white with a blue stripe, not the usual black, a replacement of some sort) that runs down to a small (3 inch diameter or so) actuator to the right of the engine. In mine, the hose was degraded where it connects to the fitting. I snipped off the bad part, used a screw to pull the leftovers in the connector out, and reconnected. Made a big difference. I've got a leaky heater core though, so I'll be going through those lines "while-I'm-at-it" and replacing...

 

I have 4 vacuum hoses attached to the vacuum switch behind the heater control and 5 connections. I do not see any unhooked hoses and in the AIM I only see 4 hoses. Does the 5th connection for a hose need to be blocked or is it just not used and no vacuum to it?

 

I have several problems with my 1978 Corvette that I recently purchased. Heater duct operation and wiper operation. I have my AIM but am having problems learning how to use it. I also have a wiring diagram for my car.

I installed a different vacuum switch on the heater control. Nothing different. I found the green stripe hose in the wiper well area that goes to the flapper in the upper area of the right fender. If I trace that hose back to the firewall there is also a small black hose that goes into the cockpit. If I follow the small black hose thru the engine compartment from there it goes to a tee. I assume that this is the supply vacuum for the heater control system. If I suck on the small black hose I can get no vacuum.
SO!!!!!!!!!!
How do I trace the hoses from here? Where does the vacuum hose for the heater hose shutoff valve run? The line off the valve has been cut and plugged but I cannot find where the hose came from. There is no other hose in the area that has been cut and plugged. I think that is a good place to start looking for my leak.
1. Where can I find the actual original routing for the heater vacuum hoses?
2. Where can I find instructions for taking apart the dash to get to them?

 

Here is the basic vac control stuff

Here is some ac control stuff

 

Thanks, but I have that stuff in my AIM too. I am looking for actual practical experience as to where to physically find the hoses, and step by step how to remove the dash. Thanks anyway, that would be a great help if I did not have it.