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My A/C system uses an electric furnace blower motor for heating and cooling. The motor jerks violently when cycling off when the thermostat is set on auto for cooling. The motor was recently replaced but the problem remains. There are two separate thermostats for heat and cool. The problem does not happen when using the heat thermostat nor when I manually turn on or off the fan (there's a 'auto' and 'on' switch located above the ECM). It does happen when I turn the fan on using the thermostat. Have no clue what's causing the jerk when it cycles off.

Any help will be greatly appreciated.

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    A bit more information is needed some HVAC systems powered by single phase blowers actually use 3 phase motors and the make & model will give us info on the starting stopping curves. – Ed Beal Jun 23 '17 at 19:06
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I would look for a wiring problem between the heating and cooling fan relays and I would combine both the heating and cooling thermostats into 1 combination thermostat. 2 separate thermostats is a disaster "waiting to happen". Have a service tech redo the wiring to make sure it is correct.

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