Problems with cistern pump

Hello everyone!
Here’s the problem: we have a cistern with an external pump, which is located in the heating basement, and this pump is acting up. Sometimes it works, sometimes it doesn’t.
When it runs, it runs perfectly fine, enough pressure, everything’s great. When you try to start it the next day, it does NOT make any sound. Turning the power off and on doesn’t help, and the cistern is definitely not dry.

I am somewhat handy, as they say, but so far I have had no experience with pump technology or anything similar, as we only recently purchased the house. Therefore, if possible via “remote diagnosis,” I would appreciate any tips and ideas that could help me with troubleshooting and save me from having to replace the pump.

It is a:

Grundfos Hydrojet 4651 BAAE with a
Condor MDR21-EA automatic switch
Year of manufacture 1995

If you need any more information, I will try to respond as quickly as possible :-)
Thanks in advance!
Regards, Sebastian

Hi,

I am also just an amateur in this. To me, it sounds like a loose connection or possibly a defective spot in the cable. Although the two ends still have contact, they can separate due to "movements" or similar (e.g., by "shaking" when the pump is turned off). Then the contact is lost and the pump does not run.

This is all expressed very amateurishly and hopefully understandable. On the other hand, this is probably the first thing you checked.

A dry cistern should have no influence on the pump running. In that case, the pump would simply "run dry," but in my opinion, the pump motor still operates.

That is also my suspicion, it is basically always the first thought when something works sometimes and sometimes not. It could have been that pumps like these have a "classic" problem with exactly this error pattern. Kind of a tip that the problem is usually found at point XY, e.g. pressure sensor in the switch or similar.

A dry cistern should have no influence on the pump running.

He does have an automatic switch, but one can surely understand anything under that. For me, that would be a dry run protection. Therefore, it could also be that this automatic "is acting up".

Will probably open the switch first and see if it’s clogged or something. I’ll report back...

He does have an automatic switch, but underneath one can surely understand anything else. .

I'll put it this way, there is certainly a more appropriate term for it that I'm not familiar with. The switch is connected upstream of the pump and is located together with the manometer on the pressure vessel. Since the switch has two positions (Auto + Off), I assume there is a pressure sensor or a membrane that controls the power supply to the actual pump.