Unless you have a quite late model pump, last couple of years, it has a polarity sensitive contact arc suppression diode under the plastic end cap. It is a small black plastic cylinder with red & black wires on one end. If the pump is connected reverse polarity, this diode will be ruined. The only way you would know is that the pump's electrical contact life would be shortened a lot.
Replacement diodes are available from Moss for either positive ground, #377-135, or negative ground #377-145. You may need to get a new negative ground diode.
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