When I bought my 1979 911 SC, the heating system was not functioning, partly because the hose from the engine fan's shroud to the heater fan was missing. However, even if the missing hose were to be replaced, the heating system would not have worked optimally.
Installed in the engine compartment, over the left-hand side
of the engine, there is a heater fan that uses a "squirrel cage" to help force
hot air into the passenger compartment (the engine fan's air pressure supposedly
only really comes into play after about 3000 rpm). As the heater fan's
motor aged, it started wearing out the spherical bearing at the top of the
motor (by the brushes). Eventually, there was enough play in the bearing
that one of the brushes popped out of its carrier, at which point the motor
ceased working. The bearing's side was almost completely worn away.
It is notable that the bearing at the bottom of the motor shows no wear
whatsoever.
As you can see in the picture below, the bearing was worn
enough that the motor's shaft was spinning against the motor's top plate and
starting to wear the plate away (wear can be seen between 12:00 and 1:00 o'clock
around the new bearing.
The repair involved getting a new bearing built out of brass
as I had a very hard time locating a replacement bearing. The original
bearing was made out of some lightweight material, but it contained some
iron as the motor's magnet attracted it. Both of the motor's brushes
were also replaced for good measure, along with a weak brush spring (the
unequal spring tension possibly compounded on the uneven bearing wear).
Finally, one of the motor's terminals was resoldered as the connection seemed
weak. You can see a close-up of the new bearing with the new brushes
in the picture below:
