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Do exactly what 70gmcer said, then check the ground. With the fan on, touch the volt meter leads to the negative battery post and to the ground on the blower motor. By doing this you are checking voltage drop. It should be minimal. If it reads anything over about one half of a volt, you have a ground problem. You can do this on any circuit. The volt meter measures the difference between the two leads. The voltage at one end of the wire should be the same at the other, that is why the reading should be as close to zero as possible. Hope that helps !