I have a catchment water system using a jps2-a grundfos pump. Not pumping. Where do I begin to troubleshoot?

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Definately sounds like a leak in the pipes from the pressure side of the pump onwards, even the smallest leak will cause it to turn on to regain adequate pressure.

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Are you able to check for power to the pump with a mult-meter. If you have power you the know the pump has siezed (flat spot on the motor) if there is no power then you know there are issues with the controls.
I hope this may have helped,
Tom

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check air press in built in tank.

SOURCE: New grundig pump is now making a kind of clicking

I was ready to pass your question and go to the next person because you said it was making a clicking, and I could not relate or diagnose that type of sound for that kind of pump. Thank God I decided to read to the end where you said rattling sound. that is what you should have said in the beginning.
1st, did you replace the pump cartridge or the whole pump unit? 2nd, is the pump in a different oriented position than the original pump position? The inner cartridge floats inside the pump as it rotates and if it is in an oriented position that allows it to bounce (to & frow) it will make this rattle sound; not serious but can be annoying. It is was a replacement cartridge, perhaps the new gasket and the old gasket are in place, thus allowing too much space for the cartridge and it therfore moves too much and rattles. Or, if it was replaced as a whole new unit. there is a problem with it that you should not be responsible for, except for the positioning orientation. Finally, if there is still too much air in the system, the air bubbles will jossle the cartridge untill you get rid (bleed ou ) the remaining air. Goodluck<Macgivor

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Grundfos is a p. o. s. I have a new system cu 301 and I have had to get it repaired twice by Fogle Pump and Supply. These guys are real rip of artists in Washington State and should be reported to the BBB. Dont Use Fogle if you need a well