MST Damper Motor Problem - HVAC - DIY Chatroom Home Improvement Forum

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Old 06-06-2011, 09:22 AM   #1
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MST Damper Motor Problem

I have a dual zone damper-controlled system with a damper downstairs and one in the attic for the upstairs system. The system has been fine for 5 years I've lived here but within the last few weeks it was not getting any air to upstairs system. In checking out the upstairs damper (damper is free and easy to move), when the thermostat calls for cooling and sends air upstairs, the damper motor doesnt move. When I short terminals 4 and 5 (or move it slightly with my hand) it will open but rather than stop, the motor rotates 360 degrees back to closed position. Any ideas? One other curiosity, terminal #2 (24V Hot) is jumped to terminal #5 (switch common), seems odd to me but it has been ok for years. I replaced the MST motor and board with new unit over the weekend, nothing changed. Thanks in advance for any assistance.


Last edited by Mountie0329; 06-06-2011 at 09:25 AM. Reason: Forgot important detail
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