Hoonbash

Hi
I know this has been covered many times but I cant find the answer.

My 78 will not switch vents for AC or heat, it blows through the floor vents.I assumed it was a faulty controller but a new one did not solve the problem.
My next step was to vacuum test the lines at the controller & I found the main feed line (#1 port-black line) will not hold vacuum.I removed the glove box & can see that the black line is still connected in with a "T'' to the water temp valve.

I also separated the main feed line in the engine bay to test that section into the interior


I'm at a loss what to do or look for next?

sgt kohler

I need to know the same thing. I charged the ac up on my 77 and it blows cold but only on the floor. im getting very very little out of the rest of the vents.

minitech

Check the small white vac tube going into the engine compartment to the heater hose shut iff valve. It is white and the white on on your excellent diagram.

Plug that if it is cut and just hanging. I would pull it off the valve and plug it to test if it is still connected.

Problem is if that leaks it is a vac leak for the system and there is not enough vac to close the actuator near the driver's toe that closes the door from bottom to top. Default is bottom and your air is getting out there.

After that look at the vac valve attached to the A/C switch getting to that by taking off one of the trans tunnel side panels. Valve is the one with the four lines all going to it blue, orange, green, black

After that (Going in difficulty) try to get to that actuator that is not closing your heat/a/c door and see if it works. The actuator that is the problem here is the one with the blue and black line going to it.

sgt kohler

im going to work on that today. I will keep you posted. are there supposed to be seals that go between the ac / heat ducts? I was replacing my temp gauge and noticed that there some gaps between the ducts.

biackbenz

I just finished fixing the same problem. Minitech is exactly right. A PO removed the heater core water valve so I had constant hot water fkowing through the heater core and into the car. The vacuum hose was left hanging open. I installed a new control valve and now I have temperature control, and all the vents work.

Hoonbash

I'm going to work on this today as well,Thanks for the input guys

Hoonbash

Disconnected the white from the hot-water shut off valve & that line wont hold vacuum either.I'm at a stand-still until I can find some replacement hoses,impossible where I live.

On a plus ,applied vacuum to the blue & had air from the defrost vent & I could hear the spring from the actuator when vacuum released.

Dave J

You may be looking at the same problem the 69-72 car guys have with the wiper door safety valve. Yours is called a bypass valve vacuum switch or water temp valve. When the valve gets old, it leaks massive amounts of vacuum. If that is your case, none of the other vacuum stuff can operate.

A simple check with a hand held vacuum pump is as follows:

1 - Ignore the open vent closest to the plunger.
2 - With the plunger fully extended, connect to the middle port and check for vacuum. It should hold.
3 - Depress the plunger down halfway and check the middle port with the lower port blocked. It should hold.

Where do you live?

Hoonbash

Thanks dave I'll give that a shot as well,I live in small town called Redcliff,Alberta where it takes 2 weeks to get anything

I read on the forum here that the by-pass valve vaccum switch will also supply vacuum to the hot water shut off valve so maybe the white vac hose is ok & it could be the by-pass valve leaking

Dave J

You can easily check the viability of hoses by simply blocking one end and applying the vacuum pump to the other end. I'd put money on your vacuum hose to the shutoff valve being good.

You can also eliminate the switch by connecting the two hoses going to it, therefore bypassing the switch for testing purposes.

You may not need a/c where you live, but you certainly would need your cabin HVAC controls to work.

How's the fishin?

Hoonbash

Believe it or not I've never caught a fish in my life but my buddies do a lot of fishing around here,big-azz sturgeon in the south Saskatchewan river system,I try to spend my time drag racing when I get a chance on days off.I need that A/C it gets plenty hot here,was hovering around 98 F for about 2 weeks.

I think you nailed my problem Dave I can hear air entering at the plunger.I hope they have the spring problem corrected on the new part I'm ordering,any suggested vendors?
The nasty to get at Culprit



Interested to see SGT Kohler outcome on his problem?

minitech

[QUOTE=Hoonbash;1587654765]Disconnected the white from the hot-water shut off valve & that line wont hold vacuum either.I'm at a stand-still until I can find some replacement hoses,impossible where I live.

If you are saying the hot water shut off valve won't hold vacuum you might have found the problem. The valve letting vac leak off.

If you connected your gauge to the white hose to the car and it won't hold vac it may not do that because it is not opening a valve in that direction.

Try pulling the white hose off the heater valve and plugging it. Turn on the car and see if that solves it. Simple to try.

Dave J

You have a reproduction switch, also known as Chinese. You can bypass the switch and send vacuum to your shutoff valve. Your shutoff valve will not be able to turn off now, but who cares? When you get the new switch, you can simply test and then replace it if you get a good one.

PM sent on the switch.....

sgt kohler

I couldn't find that valve anywhere . where was yours located?

Hoonbash

Its on top of the heater,you will need to remove the glove box

sgt kohler

ok ,I will look there thanks

Hoonbash

Update:
Replaced the by-pass valve(water-temp valve) & still have the same problem.It wont switch the AC off the floor vents.
When on heat it will now switch from floor to defrost.

What else can I check?

g5067

I don't mean to hijack this thread, but I have been following this discussion with great interest as I am trying to work out some similar problems on my 79 corvette. I wanted to know if anything connects to the top (next to the spring) port connection of the plunger? There is nothing connected to mine and it was leaking vacuum, so I plugged it off and everything seems to work a little better. Is this the way it is supposed to be or am I missing a vacuum hose somewhere? Thanks for any response.

Hoonbash

No your not hi-jacking at all the more info we can get flowing the better as this seems to be a wide spread problem.

I don't think that port is open,on mine the rubber grommet that has the white & black lines also has an opening that slides around this port

Dave J

The port closest to the plunger is supposed to be OPEN. This valve is a carryover from the ''69-'72 years as a safety switch for the wiper door system. There was a rubber boot over the spring opening to try to keep out rain & crud that might contaminate the valve. There was a hose connected to it, but was simply routed to the engine compartment and left open. The reason your top port is leaking vacuum here is atmosphere pressure is getting past the piston seals inside the valve. Your middle port should hold vacuum when the plunger is extended all the way out, and pass vacuum through to the lower port when depressed. Here's some pics of the valve with/without the boot.