Grundfos MQ pump starts every 15 seconds

Pressure is bleeding back through the pump or system to the tank or well or where ever the water is coming from. Should be some sort of one way valve to keep water from going back to the source, mounted somewhere so that once pressure is pumped up, it will hold that pressure in the part of the system that has the pressure switch that controls the pump.

That or you have a leak where you can't see it. (Actually you do, it is just internal and not leaking outside the pipes!)

Pressure switch failure or waterline leak

Mine did that. Little known that the Grundfos MQ pump has a small pressure tank on the end. If the pressure is low and the air volume is therefore low, then any tiny loss of water results in pressure drop and pump cycles on. Check the pressure in the tank. This is not well described in the Grundfos service manual.

To pump up the pressure bladder:
1. remove the rear cylindrical cover (swift bash with a fist downwards)
2. unscrew plastic cap on the end (about 2 inches diameter with tapered sides
3. attach pump to schrader valve and pump up (22-25 psi according to service manual)

Reassemble.

Suggest teflon tape to schrader valve stem to try to create a secondary airtight seal.

I have the same problem with my Grundfos pump JP5PM RAIN. Follow the manual instruction I increased the pressure reducing valve and the problem been solved. Hope you so!