Structural Maintenance 12-19
Symptom – Damper motor does not turn when defrost is initiated (see “Defrost Cycle Checkout Procedure” in Electrical
Maintenance chapter for additional information):
• Make sure that the evaporator coil temperature is below 3 C (TG-IV), 6 C (TG-V) or 7 C (TG-VI, µP-IV, µP-V, µP-VI or
SR-2).
• Check the defrost fuse.
• Check the 29 wire at the damper motor connector and make sure that voltage (12 volts dc) is present when defrost is initiated.
If the above items are satisfactory, check the following items:
1. Remove the damper motor assembly and check the wiring harness for damage.
2. Examine the outer micro switch.
a. Make sure that the lever (follower) is not damaged.
b.Make sure that the lever actuates the switch. You should hear a click when the lever is pressed, and another click when the
lever is released.
c. Check the outer cam. Make sure it is tight on the output shaft. If the set screw is not tight enough the cam may remain
stationary while the output shaft turns. Therefore, the cam will not actuate the switch.
NOTE: Do not overtighten the set screw or the threads in the cam may be stripped.
If the above items are satisfactory, disassemble the damper motor and check the following items:
1. Check the brushes to see if they are damaged or worn out.
2. Check the armature to see if it is damaged or if the windings are burned out.
SYMPTOM – When the unit goes into defrost, the damper door closes and opens a few times before closing properly:
1. Check the outer micro switch. It may be staying in the active position.
a. If the outer cam is loose, it may fail to rotate with the output shaft and hold the micro switch in the active position.
b.Something may be stuck on the switch that causes the switch to stay in the active position.
2. Check the inner micro switch. It may be staying in the normal position.
a. If the inner cam is loose, it may fail to rotate with the output shaft and fail to actuate the micro switch.
b.Something may be stuck on the switch that causes the switch to stay in the normal position.
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