How to Troubleshoot a Sump Pump Diaphragm Switch

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Sump pump diaphragm switches typically use a piggyback plug that connects to the sump pump and then to an outlet. This type of switch is triggered by pressure caused by water in the sump pit and is usually attached low on the pump body or on the pump’s side. A bad diaphragm switch may cause your pump to cycle too frequently or not run at all, but these issues can be caused by other factors as well. Troubleshooting can help identify whether you have a bad switch or if the problem is related to the pump itself.

Things You'll Need

  • Extension cord
  • Voltmeter
  • Bath towel
  • Garden hose or bucket

Pump Not Running or Cycling Too Slowly

Make sure that the piggyback plug is fully inserted at the switch outlet and the power source.

Check the circuit breaker or fuse panel for the sump pump to make sure there is power going to the circuit. Reset the breaker or replace the fuse if it has tripped or blown.

Unplug the sump pump and test the outlet with a voltmeter or by plugging another appliance, such as a lamp, into the outlet. If there is no power, contact an electrician to inspect and repair the outlet wiring.

Remove the cover to the sump pump pit and plug the inlets to the pit with a large towel or a test plug.

Fill the pit with a garden hose or bucket and wait for the pump to cycle on. When the water rises in the sump pit, it increases the pressure on the switch. Once the pressure is high enough, it compresses a bladder within the switch that turns the pump on. As water is pumped out, the pressure decreases, and the pump shuts off. If there is not enough water in the pit to apply pressure, the pump may not cycle on.

Remove the screen from the bottom of the pump to check the impeller. If this is blocked by debris, it may not move freely, and this can affect cycling.

Remove the towel from the pit inlets and replace the cover if the impeller is not blocked, the sump pit is full and the pump is still not cycling.

Unplug the pump cord from the piggyback plug on the float switch and plug the pump directly into the outlet. Use an extension cord if the pump cord will not reach the outlet. If the pump runs when plugged in directly, the switch is defective.

Disconnect the pump plug and replace the diaphragm switch. Switches that are wired in cannot be replaced. In this case the pump should be returned to the manufacturer for repair.

Pump Runs Constantly or Cycles On and Off Too Frequently

Make sure the pump cord is plugged into the piggyback connector on the back or the side of the switch plug.

Plug the cord from the switch, with the pump cord attached, into the outlet. If the pump and switch are plugged into separate outlets, this can cause the pump to run constantly and will eventually burn out the pump.

Check that there is water being ejected from the pit through the discharge outlet. If it is discharging but the water level doesn’t decrease, you may have more water coming into the pit than the pump can handle.

Contact a plumber to enlarge the sump pit to prevent pump failure due to overuse.

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