dc42:
From the photo, it looks as if those solenoids are held together by 4 screws with hex or Torx heads. It might be worth disassembling one, to see whether there is room to fit a diode inside (although that will probably invalidate the warranty). If space is limited, then depending on the current taken by the solenoid, you may be able to use a smaller diode. You can also get 1N400x diodes in SMD packages.
@DC42, I opened up the solenoid, and inside is one complete unit, so there is no way to place the diode inside. What I did was to place the diode as shown in my previous photo (where the scissors symbol is pointing at) to see if I can completely suppress the EMF. To my surprise, this new setup actually made it worse. Now I see garbage on the LCD all the time. Attached is a drawing of my old and new setup. The diode on the left-hand side is the old setup where I place the diode about half way between the solenoid and the 12V power supply. This setup works sometimes. The diode on the right-hand side is the new setup, which always fails, i.e. garbage on LCD. This is completely opposite to what I expect to happen. Any suggestion? Thanks.
Protel Schematic.pdf (9.41 KB)