1972 Cutlass with A/C. When I move the temperature selector from cold to heat I can hear a vacuum leak under the dash somewhere. Service manual says this selector is a cable. Any ideas where the vacuum leak could be coming from?
Thanks

 

The little vacuum switch that is depressed when the slider is at full-cold, which closes the hot water valve.

- Eric

 

Thanks. I'll check that out.

 

Hope Eric is right and you find it quickly. The heater box & flappers are controlled a combination of cables, linkages and vacuum hoses mostly behind the passenger side dash. One or more of those hoses could have cracked or come disconnected over the years. Prepare to spend some time poking (and listening) under the dash to find the culprit, and maybe pop out the glove box to get better access.

 

Ok. Spent the last couple of hours under the dash. Taking apart duct to get to vacuum lines. Looking and sucking on lines. Cant find the leak. I can hear it though. Bad pics but is this the vacuum switch you were talking about mdchanic? Two vacuum lines going to it and it's round?

 

Ummmm... I think so.

There were a few different arrangements in different years and models.
¼
I would recommend taking the controller down out of the dash (3 ¼"-head screws, if I'm not mistaken, and perhaps an Allen screw in each control lever end) and examining it in better light at a better angle, then you can also clean and lubricate it, which will make it feel much nicer, and if you can't fix the leak that way, you'll be all set to install a replacement.

- Eric

 

Ok. I was afraid to attempt to take it out but if it's only 3 screws and the ***** I'll give it a shot. If the switch is bad do they reproduce them or do i need to get a used one?

 

Does it hiss all the time or just when changing positions?

 

It does not hiss when changing from def to vent or heat. It's only when moving from cold to warm or hot. I started to remove the controller and it stressed me out. Too many wires in the way. Or too many hours under the dash already. So i took the two hoses off the heater switch and connected them together, bypassing the switch. No hissing. So I guess it's the switch that's bad. Now this will cause the hot water to constantly run through the heater core, correct? Any other negative consequences to doing this temporarily?

 

not sure but what about that ball in the hood compartment check that to and if u have it apart replace all those hoses with new ones while u have it apart

 

I checked the lines going to the engine compartment. Seems to be the switch on the back on the temp control.

 

You can get replacement electrical switches but if you're careful, you can bend apart the little fingers and disassemble your original switch and clean it up.
Just be careful because the internal parts can be easily reversed, so put all the pieces down in a row and don't rotate any of them while cleaning, or mark them with a file on one edge so you know which end is which.

Also, leaving the two hoses connected will either close or open the water valve (it varies by years), but, of course leaving them disconnected, with one plugged, will do the opposite, so just set them up the way you want.

- Eric

 

I have the same problem n my 70 442. When I move the lever to heat it starts hissing under the dash. What do you have to do to get to the vacuum hoses. I can hardly get my hand behind the dash. I have ac also.

 

I had to take out all the ductwork, bottom A/C plastic piece and then it's still hard to get to. Lots of wires in the way.

 

Does the switch have 2 vacuum hoses or 3 and how did it work to hook them together?

 

If you take out the front seat/seats it will be much easier on your back to access under the dash.

 

Mine had 2 vacuum hoses. One of them goes to a T. I hooked them together and it stopped the water valve from opening and does not allow heat. I need to plug them still so I'll get heat.

 

I worked on mine today. It has 2 hoses coming through the firewall. One went to the round vacuum pod on the firewall. The other went to what I would call the hot water switch. I connected the 2 but I still have no heat, so I plugged both off and still no heat. The hot water seems to be circulating through the hoses but no warm air inside. The leaking noise went away when I unplugged the hose from the vacuum pod.

 

Is the blend door moving when you move the selector?

 

It feels like a cable is moving but how can I tell if is opening a door or not?

 

You may have to partially take it apart. Might be able to see the axis pin turn somewhere.