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.