Since I use the same brand heater, are you talking about the little oragne light on the side? That light is an indicator to let you know when the heater is heating. When there is a light the heater is increasing the tempurature of the water. If the light is off the heater shut off so it doesn't over heat your tank. You want the heater to turn off occasionally so it doesn't boil your tank. :)
Also Hydor heaters heat the water very slowly. You tank isn't going to increase the temp from 70 to 83 in 5 minutes. It took my hydor heater about 3 hours to heat my room tempurature water to 80 degrees. Slow heating is another safety feature to help prevent overheating.
Based on what you are describing you may have a heater with a fualty plug. Does the plug feel really lose and is it easy to wiggle? If so you may be able to return it to the manufacture for a refund or exchange. Since you already purchased the new heater, you may be better off with the refund at this point. ^_^;