Did you Know that some manufactures use extra terminals on there compressor contactors?

These are used as a junction point for other wires to terminate without coming in contact with other line or control voltage connections on the contactor. This is common on Carrier units and is labeled as “DU” (Dummy Terminals) on the wiring schematic. They are commonly used for the Line voltage side of Transformers, Condenser Fan Motors and other single phasing line voltage devices. Searching the drawing to locate these terminals is sometimes difficult and not always shown next to the contacts or the coils on the drawing. Many technicians have mistakenly connected these wires to the wrong terminal on the contactor when replacing it with another brand. This causes blown fuses and or burnt transformers and thermostats, not to mention unit operational problems which can be difficult to diagnose.

These terminals are mistaken for additional connections to the contactor coil, which some manufactures do provide on there contactors. Other Technicians have connected the extra wires that were placed on the “DU” terminals to the extra terminals that are located and connected to the main Line Voltage terminals of the contactor. Care must be taken and visually inspecting the old and new contactors before replacement of the contactor takes place. If you come across what you believe are “DU” terminals on the contactor, research the wiring schematic. Check the old contactor by using your Ohm Meter by checking these connections and measuring the continuity from that terminal to the others on the contactor. You will get an infinity reading, the terminal is not connected to the others on this contactor. Last, if you are unsure, call and ask questions from other technicians. This will help with the change out and prevent damage to the equipment and yourself.