Pump Won’t Shut Off? Welded Switch Contacts Guide

The primary cause of welded contacts in a Pumptrol FSG2 switch is electrical arcing, a phenomenon rooted in the physics of inductive loads. A pump motor is a large inductor; when the pressure switch contacts open to shut it off, the motor’s collapsing magnetic field induces a high-voltage back-EMF (electromotive force). This voltage is high enough to jump the air gap between the separating contacts, creating a miniature lightning bolt—a plasma arc with temperatures exceeding 6,000°F. This intense heat vaporizes and melts the silver-cadmium oxide contact surfaces. When this happens infrequently in a healthy system, the damage is minimal. However, when a system is short-cycling, this violent event occurs repeatedly, often dozens of times per hour. The cumulative heat damage leads to significant pitting and material transfer, until one day the molten contacts fuse together as they cool, creating a permanent, welded-shut connection that prevents the pump from ever turning off.

This destructive short-cycling is almost always a direct result of a failed pressure tank. A properly functioning tank contains a bladder with a specific air pre-charge, creating a cushion that allows several gallons of water to be drawn before the pump needs to run. When this bladder fails or loses its charge, the tank becomes ‘waterlogged.’ With no air cushion, the system pressure drops instantaneously the moment a faucet is opened, causing the pump to start. It runs for only a few seconds to satisfy the immediate demand and high-pressure shutoff setting, then stops. This violent start-stop sequence, happening over and over, is the mechanical equivalent of torture for the pump. The immense start-up torque places extreme stress on the motor shaft, splines, and thrust bearings, leading to accelerated wear, vibration, and eventual mechanical failure.

Electrically and internally, the consequences of short-cycling are just as severe. Each startup draws a massive inrush current, typically 5-7 times the motor’s normal running amperage. This current surge generates significant heat in the motor windings. Over thousands of rapid cycles, this heat degrades the enamel insulation coating the copper windings, leading to turn-to-turn shorts and, ultimately, a complete motor burnout. This heat also conducts through the motor shaft to the mechanical pump seals. The elevated temperature causes the seal elastomers and O-rings to become brittle and crack, allowing water to penetrate the motor housing. This intrusion compromises the dielectric oil, shorts the windings, and causes a catastrophic failure that requires a complete pump replacement.

• Emergency Power Cut-Off: Your first and most critical step is to locate the dedicated two-pole circuit breaker for the well pump in your main electrical panel and shut it off. A pump that cannot turn off can over-pressurize your plumbing system, causing leaks or bursting pipes. Verify power is off at the switch with a multimeter before proceeding.
• Visual Contact Inspection: Carefully remove the gray plastic cover from the Square D pressure switch. Visually inspect the set of four electrical contacts. Look for signs of severe black carbon scoring, deep pitting on the contact faces, and melted or distorted metal. If the contacts are physically touching and fused together, you have confirmed a welded failure.
• Pressure Tank Pre-Charge Test: With the pump breaker off and the entire plumbing system depressurized (open a downstairs faucet until water stops), locate the air valve (Schrader valve) on your pressure tank, usually on top. Use a reliable tire pressure gauge to check the air pressure. The reading should be exactly 2 PSI below your pump’s cut-in pressure (e.g., 38 PSI for a 40/60 PSI switch). A reading of 0 PSI, or if water spits out of the valve, confirms a failed tank bladder, which is the root cause of the short-cycling that destroyed your switch.
• Monitor Post-Repair Cycle Time: After replacing the switch and addressing the tank issue, you must time the pump’s performance. Turn on a faucet and use a stopwatch to measure the pump’s run time from when it kicks on to when it shuts off. A healthy residential system should have a minimum run time (drawdown cycle) of 60 seconds. Anything less, especially under 30 seconds, indicates a persistent problem that will quickly destroy the new switch.
• Check Amperage (Advanced): If you are comfortable and trained in using a clamp-on ammeter, clamp it around one of the two hot wires (L1 or L2) leading to the pump. Compare the running amp draw to the Full Load Amps (FLA) rating printed on the pump motor’s nameplate. An amperage reading that significantly exceeds the FLA suggests a failing motor, binding pump, or low voltage condition, all of which contribute to excessive heat and arcing at the switch contacts.

The cost of repairing a welded pressure switch is highly dependent on the root cause. For a straightforward service call where the pump does not need to be pulled from the well, a homeowner can expect to pay between $400 and $750. This price typically includes the initial service fee, 1-2 hours of a licensed technician’s labor, the cost of a new Square D pressure switch and pressure gauge, and the labor to properly replace, adjust, and test a new pressure tank if it has failed. The bulk of this cost is skilled labor and diagnostic time.

If the diagnostics determine the submersible pump motor has failed and must be replaced, the cost increases dramatically, typically ranging from $2,800 to $6,000+. This significant jump in price reflects the intensive labor and heavy equipment required. The cost includes 4-8 hours of labor for a two-person crew, the use of a specialized pump hoist rig, the price of the new submersible pump and motor assembly (which can be $800-$2,500 alone depending on horsepower and brand), and all necessary materials like new drop pipe fittings, a torque arrestor, pitless adapter components, and a code-compliant heat-shrink splice kit. The depth of the well and the pump’s horsepower are the primary factors influencing this final cost.