Heater and A/C vacume lines

[Motor City GMC]

Can anyone tell me where these originate from and enters the cab and then how they run after that?
I have heat and the cold to hot control works but i cannot switch between floor defrost vents etc... I know i have a couple that are broken so im looking to just replace them all with new lines and want/need to know there routes and also where they come from in the engine compartment because i want to replace that lines as well and because im about to eliminate as many vacume lines in the engine as possible but dont want to lose this one.

[Titomars]

there is only one vacuum line from the engine. It comes off the intake and heads up to the firewall next to the auto trans dipstick. It goes across the firewall to a round vacuum tank above the blower motor area from there it goes inside the cab and goes to the 9 port valve on the control from there every blend door in the system is controlled via the 9 port valve.
If some doors are working you can either have a 9 port failing or one or more vacuum motors failing. If nothing works check the line from the engine to vac tank. then check if you are getting vacuum to the 9 port valve. I highly doubt any of the vacuum lines under the dash are bad. Generally the only way those can fail is from damage cause by animals or kids.

A good hint: When the 9 port fails, they generally start leaking vacuum. So you can try listening for vacuum leaks under the dash

[Motor City GMC]

Thanks a bunch, thats what i was thinking as far as the one from the engine compartment, but wasnt sure....and your exactly right....looks like a mouse got to it and chewed it.
Im gonna replace this one and then see where im at. I might just play it safe since ill be down there anyway and replace them all at once, at least that way i k ow there all good.
Does the 9 port or any of the things the lines hook into (i dont know what they are called, circular ufo shape with a nipple on it) ever fail and go bad or is that very rare?

[Motor City GMC]

Thanks a bunch, thats what i was thinking as far as the one from the engine compartment, but wasnt sure....and your exactly right....looks like a mouse got to it and chewed it.
Im gonna replace this one and then see where im at. I might just play it safe since ill be down there anyway and replace them all at once, at least that way i k ow there all good.
Does the 9 port or any of the things the lines hook into (i dont know what they are called, circular ufo shape with a nipple on it) ever fail and go bad or is that very rare?

[Motor City GMC]

Oops...damn cell phones..lol

[Titomars]

GM used a preformed vacuum harness for inside the cab and the plug that hooked all the hoses to the 9 port was formed one piece connector like the emission thermo switches that had the molded plug. also the hosing under the dash is not the same size or make up and standard under the hood vacuum lines.
You might be time and effort ahead hitting the wreckers for a vacuum hose harness. if you are going that far into it, if it were mine i would just replace the 9 port valve too they do wear out and tend to leak when old.

[Motor City GMC]

Im gonna start with that main line from the engine compartment and see what happens.
His may be a dumb question, but how do you check those lines under the dash for vacume?
If i replace that main line and nothing happens what would be your advise on what to do next?
Of course it would be nice if all vents worked, but really all i care about is the floor and defrost, im taki g the a/c out anyway.

[Titomars]

The easiest way is to pull off the main and check for vacuum while the engine is running with your finger. First I would check at the tank that will tell you if the check valve and line from the engine is good. Oh popping the line off the tank should also give you a hiss for a few seconds as the tank empties (technically fills with air). If it doesn't hiss then you are in the right area for finding the problem. If all is good under the hood the next check would be the 9 port valve connection. removing the dash bezel for that is needed. pull the connector on the 9 port and check for vacuum there.
Vacuum motors (UFO thing with the nipple) seldom fail. Inside the truck out of all the vacuum components the most likely part that would fail is the 9 port valve.

[gramps77]

Thanks for posting this, I have similar issues with my 77. Sometimes it will switch to defrost or vent other times it never does for the whole drive. Its very sporadic. Any pics of the 9 port valve? Or better yet, link to a part number? Its been the only reason Ive been thinking of swapping all the HVAC over to a newer Vintage Air kit. I still have all the original stuff on it, and was hoping to keep it. This info may help

[Titomars]

Originally Posted by gramps77

Thanks for posting this, I have similar issues with my 77. Sometimes it will switch to defrost or vent other times it never does for the whole drive. Its very sporadic. Any pics of the 9 port valve? Or better yet, link to a part number? Its been the only reason Ive been thinking of swapping all the HVAC over to a newer Vintage Air kit. I still have all the original stuff on it, and was hoping to keep it. This info may help

For a 77 the GM part number is #340428.
typically GM 9 port valves look like the attached photo but I am pretty sure C/K trucks from 73 to 78 used a more box like valve. instead of a rotary select the mode selector arm actually slid a slider inside the valve.
EDIT: Yes I was right the actual valve is a box type here is a link to one on ebay:
http://www.ebay.com/itm/1974-Chevell...-/191357432100

[Motor City GMC]

So know that i know what is causing me to not be able to switch between different vents in the truck, i am wondering if anyone knows if vacume lines have anything to do with the fan speed? My guess is no, but figured i would ask....it stays at the same speed no matter what position it is in from low to high, if not then could it be that my whole control unit is just bad and needs to be replaced or is this rare to have to replace?

[Titomars]

Originally Posted by Motor City GMC

So know that i know what is causing me to not be able to switch between different vents in the truck, i am wondering if anyone knows if vacume lines have anything to do with the fan speed? My guess is no, but figured i would ask....it stays at the same speed no matter what position it is in from low to high, if not then could it be that my whole control unit is just bad and needs to be replaced or is this rare to have to replace?

No the vacuum system does not control blower motor operation directly.
if the blower motor only runs on high your blower resistor may be bad.
if the blower motor only runs on low or medium most likely the blower switch is bad

[Motor City GMC]

Blower switch bieng the lo-hi lever/switch in the cab at the controls, or something internal in the blower motor?
It feels like its blowing about medium to me...so would it be easiest/cheapest/best to just replace the entire control unit?

[Titomars]

the blower switch is the speed select switch on the dash. they go bad and "high" is where they generally fail at. It is due to the high amps going through them. If you pull the connector at the back of the switch you can usually tell. A couple of the pins will be discolored and/or the connector could be slightly melted. the switch is replaceable and is only held in by 2 screws.

[Motor City GMC]

First i have to say thanks a bunch!! This has helped me out tremendously!! And secondly, this should be my last question...Do you know the size of the line that is the main line coming in the cab from the "tank"? I know that it is gonna be smaller that the one feeding the tank because the nipple is smaller, but is it the same size and type as all the rest under the dash and inside the cab?
Im thinking mine has been jerry rigged at some point because coming off the tank is a clear line that is larger than the ones inside, but it comes off the tank and threw the firewall and then is reduced (by electrical taping together) down to the same size line as the rest inside the cab before heading to the control unit.