To be able to vary the volume of the air coming from the dashboard
vents, the eatc controller has to be able to control the speed of
the blower motor. This is accomplished by a relay, a darlington transistor,
and some support components. The controller in question is mounted
to the underhood evaporator housing which keeps the components cool
since air going to the dashboard vents is constantly forced past it when
the blower motor is turning.
Below are a couple pictures of the blower speed controller
with it's casing removed. The black component towards the lower left
is the high speed relay and the component in the top center is the power
darlington transistor. The power transistor does get hot as previously
mentioned, but it is mounted on a large heatsink and a large amount of
air flowing past it to help keep it cool.
Note that if your blower motor fails, the eatc speed controller will
likely fail shortly after from overheating. Wornout motors typically draw
considerably more power than they should, they also don't move the volume
of air that they should either.
Below are some pictures of the case of the eatc speed controller
with the circuit board removed.