Grundfos MQ3-35  3/4 HP Pressure Booster Pump  Logo
Chip Fieberg Posted on Nov 02, 2013
Answered by a Fixya Expert

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Grundfos MQ3-35 hums loudly and does not pump. Alarm light does not go on. Pump seems "happy" but when a tap opens and the pump turns on, there is no boost in pressure and the pump just hums loudly... definitely a different noise from normal operation. Seized? Vapor lock? Any help is greatly appreciated.

  • 2 more comments 
  • Chip Fieberg
    Chip Fieberg Nov 02, 2013

    Thanks so much Bart. The pump is still installed in line. When I open the port to pour water into the pump for priming.... Water just gushes out. Pump operates for as long as there is a tap opened and then shuts off when tap is closed. When the pump is off and unplugged, I still hear a soft werrying.

  • Chip Fieberg
    Chip Fieberg Nov 02, 2013

    Clarification: when a tap in the plumbing is opened, pump hums without pumping... When tap is closed humming stops

  • Bart Fonteyne Nov 03, 2013

    When you open a tap is there water comming out?

  • Chip Fieberg
    Chip Fieberg Nov 04, 2013

    yes, there is water coming out. The Grudfos is in line as a booster, not primary pump. Grundfos in installed "downstream" of pressure tank and filtration system. Water will flow with out the pump inline.. just at a low pressure, hence the need for the pump

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Raul Sierra

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  • Posted on May 25, 2017
Raul Sierra
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Hi, seems like the capacitor gone bad or a seized motor. I just fixed mine cranking the motor manually with the use of a flat head screwdriver. In the back of the motor there is a rubber plug about the size of a dime. Take it off an insert a long flat-head screwdriver and crank the motor clockwise a few turns. Remove the screwdriver and try to operate the pump as usual. Good luck ...

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  • Posted on Nov 02, 2013
Bart Fonteyne
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Pump probably runs dry, try to fill up the pump with water a few times and see what it gives. If the pump heats up then the rotor is turning but no water moves, pressure is not building up. Pressure switch is maybe faulty. If you have the possibility to connect a pressure gauge after the dicharge you can see if the pressure is building up or not.

  • Bart Fonteyne Nov 03, 2013

    If you open a tap is there water comming out?

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5 Related Answers

Anonymous

  • 31 Answers
  • Posted on Nov 10, 2009

SOURCE: Voltage at pump is 120 & air pressure in tank

it sounds like the pressure sensor or switch is bad.

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Anonymous

  • 990 Answers
  • Posted on Mar 09, 2010

SOURCE: With the water running the MQ pump runs intermittently

This is a problem with your bladder in the pressure tank or the pressure switch. Ether one will produce this cycling on and off. Good luck.

Anonymous

  • 43501 Answers
  • Posted on May 26, 2010

SOURCE: pump cut out. primed pump and restarted but keeps

Hi,
Here is a tip that will help you..

Jet Well Pump Well Jet Water Pump
http://www.fixya.com/support/r3695577-jet_pump_jet_water_pump

Anonymous

  • 760 Answers
  • Posted on Aug 01, 2010

SOURCE: Grundfos pump leaking?

If it is starting and stopping, it most likely does have some type of leak. The check vavlve is normally installed on the suction side of the pump, and sometimes instead of a check vavlve or in addition to, there is a foot valve. A foot valve is just a check valve (usually with a screen) at the end of the line in the bottom of the cistern. If you have much suction lift, and the check valve leaks (or any leak in the suction piping) then the water will drop back into the cistern. This will also cause the discharge side of the pump system to drop in pressure, causing the pump to restart. You mentioned removeing the pressure tank, that tank keeps the pump from short cycling when you have low demand on the system. If the pump is higher than the cistern, and you don't see a check valve between cistern and pump then you most likely have a foot vavle in the cistern. Hopefully it's accessable by removing the pipe.

Azrael SRL

  • 11800 Answers
  • Posted on Oct 05, 2010

SOURCE: I have a Grundfos MQ3

The only thing that would cause a problem like that is when the pump is no longer primed - the water has drained out of it and it runs on empty = overspeeds.
You need to replace the valves, they are leaking. Also make sure that the rain tank is clean, dirt and debris might block the valves.

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