You need to leave the Step.Result.Numeric property in the step, because the status expression expects it to be there. Go ahead and leave your Coil_Voltage property as the data source, just add a "Numeric" property.
Or, you could change the status expression to "CheckLimits(Coil_Voltage, step.limits.high, step.limits.low, step.comp)". If you can't edit the status expression, you need to go to the Type Palette (MyTypes.ini) and edit the properties of your custom step type to get to the status expression.