Structural Maintenance 12-8
DEFROST DAMPER
Check the damper during scheduled maintenance inspections for shaft wear, end play and the ability to stop the airflow. The
damper should close and seal so the airflow is stopped on the top and bottom edges with the solenoid plunger bottomed out. If
the damper does not seal properly, adjust the damper assembly.
SL Defrost Damper
The SL (up to August 2000) use a damper motor assembly to open and close the defrost damper. Later model SL units (from
August 2000) and SLe units have been fitted with a different model damper gear motor. The details of this are covered in the next
section.
The damper motor does not reverse. The output shaft on the damper motor assembly rotates in the same direction (counter
clockwise viewed from the gear case end) to both open and close the damper. When defrost is initiated, the output shaft rotates
180 degrees to close the damper. When defrost is terminated, the output shaft rotates 180 degrees to open the damper.
The main components of the damper motor assembly are the electric motor, gear case, cams, and micro switches. Two micro
switches are activated by cams on the motor end of the output shaft. The outer cam leads the inner cam by 20 degrees.
Electrically the micro switches are placed in the circuits between the defrost relay and damper motor. The micro switches stop
the damper door at the open and closed positions. The micro switches also cause the damper door to fully open or close (it may
close and then open) when the unit is turned on. This keeps the damper from staying in a partially open (or closed) position if the
unit was turned off while the damper was opening or closing.
Theory of Operation
Micro Switch Logic
Each micro switch is a double-pole, single-throw switch that is activated through a lever. The lever is also called a follower. One
set of contacts in the switch are normally closed and one set are normally open.
When a switch is in the normal position, the yellow wire (pole 1) is connected to the red wire (throw) and the blue wire (pole 2)
is open.
Defrost Damper Open – SL
1. Damper Door Bracket*
2. Damper Door
3. Non-freeze Strip
4. Damper Link
5. Ball Joint Link
6. Damper Motor Bracket**
*Adjusts Arc of Damper Operation
**Adjusts Damper Vertically
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