La San Marco Flow Meter Voltage

Equipment doesn't work? Troubleshooting? If you're handy, members can help.

#1: Post by 3spd »

I have been cleaning up a single group 1991 La San Marco 85-16m. I was pleasantly surprised after tearing it down and descaling, replacing the element, a solenoid coil, the sight glass, and a few seals everything worked. Except the flow meter was leaking.

Replaced the o-ring, fixed the leak, but now its throwing a flow meter code (each button runs the pump for the same time then starts flashing the manual button).

That's when I found out that the two red wires that go to flow meter and have the same connector are directional - and I installed them backwards. And the board let a little magic smoke out. As far as I can tell the smoke was from a resistor getting hot. It melted the solder but still reads the correct resistance, so I just re-soldered it. There was also a small smoke smudge on one of the capacitors but it measures fine and the diode (a ZPY 12) next to it looked a little cooked, so I replaced the diode. Still not working.

I tried re-programming the shot volumes, wont take.

I read a "good" flow meter should have 2.2k of resistance, mine did not. Luckily I have another 3 group machine that I can steal flow meters off of (its a Nuova Simonelli) - and it's flow meters measure good. Swap over a flow meter lid and it still wont work.

I wasn't able to find a wiring diagram for the La San Marco but I did find a diagram for a Simonelli: ... trical.pdf

There are two red wires going to the flow meter one labeled (1) and the other (5) - these two labels were conveniently located underneath the plastic insulators where I couldn't see them until removing them for better probing access. With the machine on between ground and (1) I get 5V and between ground and (5) I get 12V. This doesn't sit right with me. Per the Simonelli diagram I like the 12V going to the + terminal on flow meter. But why would it be inputting 5V to the signal out terminal?

I do have an oscilloscope and failed to see a square wave coming out of the flow meter - so maybe I fired this flow meter too.

I am no electrical engineer and its been awhile since I last took a circuits course. Can anyone provide insight? I am going to take a deeper dive on the board and see if I can suss out any other bad components.

Any help would be greatly apricated. Thanks!

3spd (original poster)

#2: Post by 3spd (original poster) »

After much monkeying around I found the problem was my fault.

I thought the center pin on the sensor I stole off the Simonelli (on the left in the below image) was ground and the right pin was signal out. As that is how the La San Marco is arranged and how its shown in the diagram I found. Should have opened my eyes and seen, the now embarrassingly obvious, "OUT" cast into the sensor clearly showing the center pin is signal out and the pin labeled "-" is the ground pin.

My embarrassment is only outweighed by the relief that I didn't fry control board.

So it would seem that 5v at the (1) wire and 12v at the (5) wire is correct.

Hope this saga helps someone in the future.