Why won't my pressure switch turn on?

If your pressure switch will not turn your pump on, it could be one of two causes.

#1 - That the tube to the pressure switch (jet pump) or the nipple between the tank tee and the pressure switch has become blocked with sediment, iron, or rust. Then, the pressure switch will not sense when the system pressure is low to activate (start) or high to deactivate (stop) the pump. Try tapping on the pressure switch, if it starts it could be that a small piece of scale if floating around and from time to time it blocks the pressure from getting to the pressure switch. This would obstruct the switch operation intermittently. 

You can remove the pressure switch to inspect the 1/4" nipple or tube that supplies pressure to the pressure switch diaphragm to make sure it is not clogged with debris. You may be able to unclog it using a piece of wire, however best practice is to replace the nipple or tube.

#2 - If you inspect the system for debris and find no evidence of dirt or a blockage and the switch starts when you tap on it, it could be that the switches linkage mechanism has worn from years of use and is starting to stick periodically. If it is a new switch, it will not be the result of wear but could indicate that either a small piece of debris has got into the switch linkage mechanism or that the switch is defective and is not activating on pressure drop. A defective switch will typically fail from new and normally is not an intermittent problem, however it is possible. In either case this will require replacement of the pressure switch.

Do not confuse the above condition with the symptoms below!

Pressure and water flow drop off before the switch turns the pump on.

If you are noticing that your water pressure drops to zero and your flow momentarily stops before the pump starts, it is almost always caused by an improper relationship between your tank’s pre-charge pressure and the cut in pressure of your pressure switch. This condition results when your pressure switch’s cut-in setting is too close to the tank's pre-charge pressure. The tank's pre-charge pressure should be about 15% lower than the pressure switch cut in setting. See the article at the link below.

https://support.boshart.com/trouble-shooting-my-water-flow-stops-before-my-pump-starts

https://support.boshart.com/why-is-my-low-pressure-cut-out-switch-nuisance-tripping