mint79

Hi guys, as i continue on my journey of repairing, replacing, or doing all the little things that i have wanted to do,while i am replacing the speedo cable on my 1979 l-48,and i am under the dash,for the last several yrs. i have had no defrost, i have heat at the floor, A/C is cold, what can i look for to fix this problem, never been a issue not having no defrost as the car is only driven in nice weather, but what the heck, lets see if we can get it to work-Thanks

wfo76

This thread might help....

http://forums.corvetteforum.com/c3-t...nside-fan.html

mint79

checked all the vacuum lines[i think!] found that the black line with white stripe from the heater valve to the firewall was connected with rubber hose and cable ties, it was under
convoluted tubing, did notice a small leak when i gave it the 'huff test', what does this valve do and if it was not functioning do to a vacuum leak would i lose my defrost and middle vents, again i have heat at the feet, a/c at the feet, nothing above-Thanks

7T1vette

Most likely a bad seal on the heater/A/C control head vacuum distribution manifold. It's a little round 'thing' with 8 or 9 vacuum hoses going to it. If that seal is bad, the system leaks and vacuum gets distributed nowhere! On the other hand, check to see if vacuum is getting to that manifold. If no vacuum is present, you'll get the same symptoms.

mint79

Where is that little vacuum distribution tee?

mint79

and is it correct that the defrost -heat- are controlled by vacuum and not a lever, wire. ect.?

7T1vette

The ducting paths are determined by vacuum signals to the various diaphragm servos which operate duct 'doors'. Those doors open or close different duct paths according to the vacuum signal which is sent by the heating/A/C control head in your floor console. You can see if the vacuum supply is being sent to the console by tapping into that vacuum supply hose [engine compartment at firewall] where it passes into the cabin. If you have vacuum at that point, the problem lies in the lower console somewhere. {The other remote possibility is that the vacuum supply line is broken under the dash before it gets to the lower console....but then you would be hearing a big vacuum leak/hiss in your dash.}