Position and Pressure Switches

Postition Pressure Switches Installation, Care, and Diagnostics By Mike Weaver March 2001 Electromechanical switches — from the common toggle and push button switches to the more advanced position (limit) and pressure switches...


Field repairs
Another option in the case of a switch failure or for preventative maintenance is to work on a switch in the field (if possible). These types of repairs are usually limited to repairing the external actuation/de-actuation mechanisms, enclosures/housings, wire leads, and other electrical connections. In general, the internal environmentally- or hermetically- sealed switch element is not usually repairable in the field but there are exceptions.
Before starting any work, there are a few things to keep in mind. First, and as mentioned before, follow the instructions provided in the CMM — it is the best insurance policy. CMMs are written by experts to ensure the proper operation of their product. So use the recommended equipment and follow the procedures described.
As for more general tips, first do no harm to the switch. Avoid damaging any connector threads during disassembly. When cleaning parts, make sure not to get solvent on parts such as O-rings that become damaged. Be sure to avoid kinking or pinching any connecting wires inside the cover. In the case of a pressure switch, don’t try to repair the internal threads or sealing surfaces on the pressure port. That is a sure way to get chips inside the switch and really cause problems. In repairing a switch, cleanliness is your top priority. Keep the switch free of contamination, such as solder flux and particulate matter, which can cause resistance problems or even cut off current flow. Be especially careful to avoid contamination with any silicone-based compounds. If silicone is present, the electrical energy used in the switch will change it to silica over time and cause a failure. Since silicone vapor can cause such contamination, no silicone-based materials should be present in the same room with an open switch. Also, external actuation mechanisms can be altered by contamination. For instance, the operating points of a "plunger" type position switch can be changed by foreign matter working its way into the cavity between the plunger and the switch shell/housing.

Problem? Call a Specialist!
In the event a system problem is confirmed to be caused by a switch, the best procedure would be to send the switch back to the manufacturer for analysis (if the switch is in warranty, displays a unique failure mode, or failed at very low cycles) and/or overhaul. Switch manufacturers have the knowledge, equipment, and personnel resources to evaluate and service a switch. Due to the myriad of switch designs in the field, it is almost impossible for every field maintenance repair facility to carry all the necessary equipment and possess all the know-how to service all the switches flying today. Since a manufacturer designed and built the switch, they can more easily pinpoint and deal with a problem. Another option might be to send the switch to a manufacturer’s authorized repair center. Also, keep in mind not all switches are reparable, either for economic reasons or due to their design. In this case, a failed switch should be properly disposed of and replaced with a new switch.

Detailed information
In the event a switch needs to be returned to the manufacturer, the more information provided to the manufacturer on the nature of the failure and the operating characteristics of the switch before failure, the better. Key points to include are the reason for removal, specific problems observed, hours in operation, and any operating conditions worth noting (high temp, over voltage, etc.). This information will cut down on the detective work, but more importantly, will give the manufacturer insight into performance trends and possible improvements.
Today’s aircraft use a wide array of switches in various applications. Although they have a variety of functions and constructions, they all share a common thread: with proper installation, regular inspection, and preventive maintenance, they will provide many hours of reliable service.

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