Chris, I recently found that my control cable from the control panel to the
water control valve ( water **** ) had broken at the bellcrank under the
dash, and not actuating the water vlv. when selecting heat or defrost. You've
got A/C so there is a vacuum signal to a vacuum controlled vlv. upstream of
the water control vlv.. You might be loosing vac signal or a siezed vlv. at
either the vacuum **** or water **** that controls coolant flow into the
heater core.

 

First step is water circulation.. Like Mr heater valve. Second step is blower fan action. Third step is the opening of under dash cowls or DUCTS,(One on left of center, one on right). These are vacuum operated but there are things one can do to "Block open" these little buggers! I guarantee if you do these things you WILL HAVE HEAT!!! Norm K. P.S.; I got the vacuum routing diagram from Z Clock and it worked 3 months! I'm not giving up easy though I do have heat and A/C-the Super Pain! You can do it! Tap into the greatness of the hot Z Engine water! It's so close you can touch it! Let's go!

 

Yeah... check that cable it is known to fall off first. Also check for binding of that cable. Since you are changing from one season to the next.You probably just had it at a set place during the summer months and now you want it to slide all the way to the right. It controls the heater **** lever. Also down there is clip sometimes it fall off. If you can't find a clip to replace the one that's not there. You can always use your hand or a flat tip screw driver to push the heater **** lever to the the desired position. At least, that is a temporary fix.Then of course you could use nrkrell's method.

Enjoy the ride!

 

I agree with heater valve as a starting position, also make sure that you have good flow through the heater core. They can plug up. Also check the mode door ( vacuum operated) for operation, this door allows the air to circulate around the heater core. Default position on my 78 was a/c. I opened the door by hand and now have heat. Do you have control of the vents during ac operation? If so, this will inform on the condition of the other vacuum servo's. Could be just a bad diaphragm on the servo.

 

Hey,
If you turn the switch to heat and air still comes out the vents, then the mode door isnt moving. If i were you, id check the vacuum lines. Mine have split where they go into t joints several times already.