semipro
New Member
Our MQ3-35 that has operated reliably for several years recently started short cycling, resulting in a very distinctive pressure fluctuation at the use points. It was most notable in the shower. I installed a pressure gauge on the outlet side of the MQ and noted that the unit shut off every time the pressure reached the boosted pressure. I read in the Groundfos literature that the pump should continue to operate even after that pressure is reached as long as their is sufficient flow. This made me suspect that the either the flow sensor or control board was bad.
I removed the flow meter (a 1o minute job) to make sure it wasn't physically obstructed or bound up somehow. It wasn't. I looked for diagnostic data on the sensor but could find none. I'm pretty sure its uses a hall-effect sensor so I checked with a multi-meter to see that it was receiving about 5 volts to the sensor when the pump was energized - it was. I don't have an oscilloscope or I would have tried to measure the sensor output while the pump was operating (and the flow sensor impeller turning).
Since the control board is pretty expensive I took a shot and ordered a new flow meter for $80. I installed it and the pump seems to be working properly as before.
I thought I'd share as I could not find any related information for this particular problem on the web.
I removed the flow meter (a 1o minute job) to make sure it wasn't physically obstructed or bound up somehow. It wasn't. I looked for diagnostic data on the sensor but could find none. I'm pretty sure its uses a hall-effect sensor so I checked with a multi-meter to see that it was receiving about 5 volts to the sensor when the pump was energized - it was. I don't have an oscilloscope or I would have tried to measure the sensor output while the pump was operating (and the flow sensor impeller turning).
Since the control board is pretty expensive I took a shot and ordered a new flow meter for $80. I installed it and the pump seems to be working properly as before.
I thought I'd share as I could not find any related information for this particular problem on the web.