Hello,
My AC is no longer blowing air through the middle or top vents. I searched the C3 forum looking for a 1979 heater/AC vacuum diagram but all the diagrams I found are for early models with the wipper door. Could one of the members provide me either the correct diagram for a 79 or point me in the right direction to get one? I also looked in YouTube with the same results. Found one in Google for 77-79, but it also shows connections to a wipper door, there's no wipper door on a 79. Last question: If I decide to replace all the vacuum hoses for the heater/AC system, do I have to remove the entire instrument panels, or just removing the center console will be sufficient
Thanks All.

 

Here is the diagram I got from Doc. A little worse for wear since it is old. But it shows just what you need to know. And you don't need to remove center gauge cluster. 2 screws under it and then take out the center console.

 

If your HVAC system loses vacuum the default is for the air to blow on the floor.
So, the most common cause of a sudden loss of all vacuum in the HVAC system is that the plastic rivet on the control switch breaks from old age and no longer holds the vacuum switch together. This is under your console. Taking both sides off the console will give you access. The plastic rivet can be replaced with a small machine screw and nyloc nut. With a couple washers. I have a pic somewhere but I'm at work right now.
Or you can purchase a new switch assembly which may or may not work.

 

If it were my car, And yes I have had this happen. More than once.
The first thing to check is the line under the hood. Open the hood. From the vacuum check valve, follow the small vacuum line that connects into a even smaller vacuum tube that goes behind the distributor and disappears into the firewall. That's the vacuum source for the HVAC. Make certain it's all connected, not broken or inadvertently knocked off.
If that looks good.
Then pull both sides of the console so you can access the heater/air-con control unit. There are 3 screws coming up from the bottom that retain the switch assembly to the bottom of the console. The unit has a 2 electric switches and a cable and of course the vacuum switch.
I couldn't find a old photo of mine repaired, sorry.
But it's straight forward. A plastic rivet, or dart goes through the center of the vacuum switch and holds it together. It's like a sandwich with a rubber disc that has a lot of different channels cut into it as the filling.
here's what happens to the plastic rivet.


They get old, brittle and just let go. You loose all your vacuum in the HVAC system when this happens and your air only blows on the floor.
You replace it with a little machine screw and matching nyloc nut. You need to use a nylock nut because you can't over tighten this assembly. And it can't be allowed to lossen up. With a little silicone rubber grease on the disc, you replace the plastic fastener with the screw, a couple washers and the nyloc nut. Just so it's just snug so it seals, but you can still turn it. Reassemble the switch to the control unit and the unit to the console.

 

I found these pic's on a old thread on here. Now of course this may not be your issue. But it's highly likely.
So, this is the vacuum switch once removed from the unit.

You can see the ends of the plastic dart in the center.
Now here's some pics of someone elses repair.


This guy used a heap of loctite instead of a nylock nut.
But clearly you can see the repair, the placement of washers and which side to put the nut on.
This is a really common failure. The electric switch that controls power to the air-con compressor and such is on the flip side of the unit. Also has a plastic rivet. I had that rivet snap as well. That one is a little tougher to repair as there is no clearance for the nut. But that's a topic for another day.

 

Hello Kanvasman,
I really appreciate the vacuum diagram and the information! I been looking for this for some time now without any luck. If I understand correctly the diagram shows the vacuum connections inside the car, not the ones in the engine compartment, correct? Also the line that shows to source, where this one connects to?
Thank you again!

 

Hello from the USA 4-vettes! The information you provided me is awesome, it will really help me in troubleshooting and fixing the issue. A picture is worth a thousand words, I appreciate them, it will make my life easier if I have to do the repair, hopefully it will be something easy like a disconnected vacuum hose. I am out on business travel, will let you know how it goes once I get back.
Thanks!

 

Correct, the source is where you are getting your vacuum in the engine compartment. Look for the GROMMET in the diagram. That is the firewall. The green line goes to the outside air valve, which is located inside the right fender. It is a flapper thing that lets outside air into the car. The WHITE line goes to the heater control valve. SO there should only be 3 lines inside the engine compartment. I am also sending a pic of the valve that 4 Vettes posted showing what each port is for. As said, it is easy to take apart. I did mine and just put some dialectric grease on the rubber orings. WIth this info you could just take a hand vacuum pump to each line coming off the valve and verify if everything else works. Good luck.

 

Hello,
I hope I am doing this right. i was looking at my 79 L48 vacuum connections inside the engine bay, I am not an expert and this is my first Corvette but looking at the diagram that kanvasman sent me, I don't think that things are looking right. On picture 1 I can see the vacuum hose filter connection to the vacuum check valve. The two connections coming out of the vacuum check valve one goes to the grommet (X marks the spot) and the other goes to the vacuum hose fitting with the screw (what is the deal with this screw? Is this supposed to be a T type fitting?

One of these hoses either the hose marked with the blue X or the one with the blue dot on picture 2 goes to the rear fitting of the vacuum hose filter, but I am not sure

Please look at picture 3 connections marked in purple, is this even right? and what is that thing with the ? mark?

Finally guys it appears that there was a vacuum connection at the top of the water shutoff valve (marked with the green square), but it was cutoff. The diagram show that it goes from the water shutoff valve through the grommet and from there to a water shutoff switch? Not really sure.

Any help I can get will be greatly appreciated! By the way even though with the vacuum issues the head lights work great!
Whishing you all a good night from Texas.
Jose

 

You have yourself a vacuum hoses mess there, sorry...
Purchasing a Assembly Manual for your year car is a plus...
Lets start with the HVAC problem
That screw is inserted into what was once was a "T"
The "T" is a Auveco #12924 and is 3/16"x3/16"x1/8"
That 1/8" line is your vacuum source line to the HVAC switch (Black) in the drawing above

 

#4 goes from the hose that was cut off back thru the firewall to a switch that is located at the top of the heater box inside the car. Had a little nub on it that the lever from the control switch pushes against. Then the temp control lever in the car. And Mr BMO is correct. Almost every hose that you are showing is cracked and or falling apart. And there are many hoses. It is pretty obvious that the previous owner didn't know what he was doing. Get yourself a diagram ( I actually had a small booklet at one time) of the hose diagrams. It wasn't even an assembly manual but rather a drawing of the hoses. And you need to understand WHERE the initial vacuum comes from and how it gets to each system. THen get a hand held vacuum pump so you can sit there, without the engine running ( that is where the vacuum comes from) and diagnose each system.It isn't that complicated once you understand it and can separate each system ( like the heater controls vs the headlight system). This project is actually a great one for someone not very experienced in these old cars. You really can't make it any worse than it is now and the only mistake you can make is cutting a hose too short! But you need a reference book of some type. Jump in, the water is fine.....

 

Hello bmotojoe,
Appreciate your valuable inputs. I know is a vacuum nightmare and I am working on getting it fix. Based on your advice I just ordered the 1979 Chevrolet Corvette repair manual, I am hoping it will have all the vacuum diagrams I need to fix this vacuum mess. I also ordered 3 of the Auveco "T" fittings (just in case) and some other components for the HVAC system. Hopefully I can get the issue corrected.
Best Regards,
Jose

 

Hi kanvasman,
Appreciate all your help! I agree with you about the poor state of the vacuum hoses. I just ordered the vacuum hose kit from Top Flight so I can replace them all with new ones. I also ordered the 1979 Chevrolet Corvette repair manual, hopefully it will have the vacuum diagrams needed to fix this nightmare. I also ordered Auveco "T" fittings, a vacuum check valve, the vacuum filter and a heater/AC control vacuum switch. As you said, I need to understand the system and honestly I do not, hopefully the book will teach me what I need to know. For the cut hose In photo #4, you won't happen to have the part number for the heater box inside the car? Looking at the diagram it is the white color hose that goes from the HVAC heater control valve through the firewall and into a water shutoff switch, correct? Is the water shutoff switch the same as the heater box inside the car? Doing a Google search for "79 corvette heater box" comes up with nothing. Could you provide me with the parts numbers for both? Something to notice also is that in all diagrams I have seen the grommet is for 3 hoses, but the one in my Vette is for 2 hoses only, I don't even know what to thing about that.
It is what it is my friend,
Best Regards,
Jose

 

Yes that’s my blue loctite..need to keep thin
here is all switch thread in detail
https://www.corvetteforum.com/forums...-switch-2.html

and vacuum

 

Hello,
I purchased the vacuum hoses kits for both the emissions and AC systems. Before I start getting into this vacuum mess as correctly stated by bmotojoe I want to make sure that I have the right vacuum diagram to do the job. I also purchased the 1979 Corvette Repair Manual, I found 2 vacuum diagrams in the book, the first one titled "C60 Vacuum Line Routing" (what is a C60?) which has 3 vacuum lines going into the grommet (see picture below). All the diagrams I have received from the members, THANK YOU!, have 3 lines going into the grommet but my car only have 2 lines going into the grommet, so what am I missing, or not understanding?


As indicated above my Vette has only two vacuum lines going into the grommet, much like the photo below. I was feeling with my finger under the two lines, but I can not feel an opening for a third line.

I am trying to understand the system before I get to work on it. The car is equipped with AC, Any Help I continue to get from the members is greatly appreciated.
Thanks
Jose

 

C60 is the options code for A/C.
Are you sure you're looking in thr correct location for the 3 hose grommet?
Look behind the distributor shielding on the firewall, it should be there...
The lower diagram is for a NON A/C car....