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I am trying my hand at fixing a La Cimbali M28 Select, which is a professional espresso machine.

The remaining issue is that, when I press the 'hot water dispenser button' it does not dispense hot water but it pulls an espresso shot from the left group (meaning, the left coffee dispenser unit). When I push the button, the solenoid of the left group is activated now instead of the hot water solenoid.

This seems to me like an electrical issue. Why?

The boiler is directly connected to the hot water dispenser via a solenoid that is activated by that button. When I put the machine in 'maintenance mode' it dispenses hot water just fine, meaning the solenoid is working fine and the pipes aren't clogged. I should also mention it was dispensing hot water in 'regular mode' just fine a day ago without changes that could possibly affect it (unseizing the pump, fixing the gauge, ..).

In the schematics, pushing the button (PA) should activate the hot water solenoid (Eac), and should trigger the water tank fill solenoid (Evc) and its indication light (L2). None of which is happening. Instead it triggers the left coffee group solenoid (G1).

My DMM is broken and I cannot get a quick replacement due to COVID. I used a buzzer as a continuity tester. I used my working steam dispenser, with a near identical set-up, as a reference.

Having connected the buzzer to the two opposite pins - to be more precise, to the incoming wires that are connected to these pins - of the 3-pin solenoid (leaving out GND), I hear a very loud beep when pressing the button. When doing the same with the hot water dispenser, I hear soft crackling when the button isn't pressed and soft crackling when the button is pressed. Surely that is not right? The only difference is that the steam button is activated upon pressing, but the hot water button you need to press and release to activate and press and release to disactivate.

The only thing I can think of, is that I needed to disassemble the machine to reach parts. Some of the machine housing has a GND wire connected to it. I left this untouched, but removed the panels from the machine. Putting everything back together is a lot of work, so I touched the GND wires to the inner machine casing but that didn't make a difference.

EDIT: Completely rebuild the machine and it didn't solve the issue. I did find a GND plug-in on one of the metal sheets that doesn't have a corresponding green-yellow wire. Can this kind of behaviour ever originate from not all GND terminals being connected?

Any insights are more than welcome!

enter image description here

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  • \$\begingroup\$ It may be a malfunction of the microprocessor board controlling the solenoids. The buttons are not connected directly to the solenoids, the buttons are inputs for the board. \$\endgroup\$ – Uwe Mar 23 '20 at 9:33
  • \$\begingroup\$ Correct. That has me puzzled, since nothing happened to the board or was modified. \$\endgroup\$ – Nils Deschrijver Mar 23 '20 at 12:38
  • \$\begingroup\$ A microprocessor board may fail without modification or "happenning". I would try to power down the espresso machine by pulling the plug for at least 10 minutes. The probability is low, but it might help. \$\endgroup\$ – Uwe Mar 23 '20 at 15:46
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Apparently, the little water that leaked contained bits of copper from the piping. The dried bits caused a short. Cleaning the board resolves the issue.

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