Group Solenoid Misbehaving

Equipment doesn't work? Troubleshooting? If you're handy, members can help.
sar5w
Posts: 88
Joined: 15 years ago

#1: Post by sar5w »

I thought I would share as this one was new to me in all the machines I have worked on so far.

After rebuilding yet another CMA machine, I went to pull my first shots so as to taste my results. Because the machine has volumetric dosing, I started the extraction by pushing the button on the touchpad. I watched the glorious beginnings of the extraction, rich, sweet, flowing like honey and then it stopped. No the dose was not complete as the light on the touchpad was still on and the pump was running. The group solenoid had closed for no apparent reason! No issue using the manual brew switch, hmm . . .

I went through panic first thinking maybe I had finally come across a machine with a bad, expensive to replace board. Then I got to troubleshooting. First I checked for loose connections at the board, nothing. Since it is a two group, it made the process a bit easier. The left group functioned normally so I started swapping wires and connectors, eliminating the touchpad. It ended up being a bad coil on the solenoid. Comparing resistance across the coil to the problematic one to the other (and then one new in box), I found the value was significantly greater and fluctuating. I replaced the coil and seems as if the machine is good to go.

So here's my question. Why would the coil function properly with the manual brew switch, but have issues with autodosing and running through the board?

JRising
Team HB
Posts: 3665
Joined: 5 years ago

#2: Post by JRising »

sar5w wrote: So here's my question. Why would the coil function properly with the manual brew switch, but have issues with autodosing and running through the board?
Either it's a coincidence that it functioned with the manual switch, or the manual switch has slightly less resistance than the Triac that controls it when dosing and that is just enough difference to not be able to open the valve with that cracked/damp coil.
(The fluctuating resistance almost certainly indicated that the coil is cracked somewhere and has gotten damp, like trying to measure resistance in a cracked element).

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BaristaBoy E61
Posts: 3512
Joined: 9 years ago

#3: Post by BaristaBoy E61 »

Have you measured and compared the coil resistance of the 'bad' solenoid coil to the 'good' or new solenoid coils?

Can you post the numbers or pics from your DDM (meter) display?
"You didn't buy an Espresso Machine - You bought a Chemistry Set!"

sar5w (original poster)
Posts: 88
Joined: 15 years ago

#4: Post by sar5w (original poster) »

I believe the new coil (220v) had a resistance of 860 ohm and the troublesome one was reading 1800 ohm or so.

The machine was a $350 Facebook Marketplace buy which I didn't really bother testing prior to stripping the frame, powder coating, and rebuilding. There were some signs that it had leaks that were not fixed by the previous owner. I will not be surprised to find a few more issues as I test it out and put it through the paces.



Shot timers and barista lights are installed and I'm getting lovely shots from both groups.

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BaristaBoy E61
Posts: 3512
Joined: 9 years ago

#5: Post by BaristaBoy E61 »

sar5w wrote:I believe the new coil (220v) had a resistance of 860 ohm and the troublesome one was reading 1800 ohm or so.
Then just change out that solenoid and be done with it.

Machine looks great btw.
"You didn't buy an Espresso Machine - You bought a Chemistry Set!"