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How do we know if a fan is going to work? We are having relay problems.

You would normally test a fan by applying +12 volts to its terminal. The negative voltage goes to its chassis ground, or to its negative terminal if there is one. If the motor is good, the fan will turn.

Note: if you mix up + and - voltages on an isolated fan, the fan will still probably work but it will turn backwards. Not a problem.