Simple system.. Not many things can go wrong. Hot water flows thru the
heater
core. Heater fan provides air flow... Air doors direct it to the right
place. On a '77, the air doors are likely vacuum controlled, rather than
cable. If you find a softball sized vacuum cannister underhood, I'd
check
for leaks.
Mark Mullins
The CSM has a troubleshooting chart that's pretty extensive. It says to
rapidly move the temp lever back & forth and you should be able to hear the
door hit the stops at both extremes. Hot inlet and warm outlet hoses means
clogged heater core. Both hoses warm, check the vacuum at the "water valve"
on the manifold (AC cars).
Air flow to the vents is controlled by vacuum actuated diaphram thingies.
The little black vacuum tank that Mark mentioned was a problem on my 78. The
hose that feeds it is only like 1/8" and they get brittle and break. With no
vacuum the vents won't switch. Not sure if the 77 is the same or not.
drc
