La Marzocco GS3 AV line pressure pre infusion

[Nawaf]

Long time reader first time poster,
I have 2017 gs3 av plumbed in,
As i read here in the forum the pre infusion uses line pressure which is 3.5 bar,
But when i brew the gauge rises to 9 bars in the 2 stages pre infusion and the brew,
Is there any way to make pre infuse use line pressure,
Or it's not as i read,
Thanks in advance,

[Jake_G]

Hey Nawaf,

Welcome to HB!

Line pressure PI was only available on the early MP models of the GS/3, prior to the conical valve upgrade.

The AV uses a pump on/pump off preinfusion that has user-set times for customization. I don't like it, so I modified my machine to be a flow profiler, but it works the same as the current connected Linea Mini machines, so I guess it is better than nothing.

Cheers!

- Jake

[Nawaf (original poster)]

Thank you for your reply,
Then the pre infusion is useless in this case,
I saw some linea mini users modified their machine to pre infuse with 3 bars
Is there is a way to modify the gs3 av the same way
Thanks

[Peppersass]

There's a way to make the GS/3 do line pressure pre-infusion: install a motor cutoff switch.

When the cutoff switch is in the On position, hitting the brew switch opens the 3-way valve to let water into the portafilter and turns on the pump motor. That's normal operation. But in the Off position, the 3-way valve opens but the motor doesn't start. Water flows at line pressure into the basket. When the boiler gauge rises to max line pressure (perhaps waiting until you see the first drops at the bottom of the basket), you flip the switch to turn on the pump.

You do need to make sure that the switch is always in the On position when not brewing. Otherwise, the steam boiler won't fill when needed.

Wiring a cutoff switch isn't terribly hard, but it requires some knowledge of basic AC circuits and the skill and willingness to make a modification inside the machine.

The best way to do it, though not the simplest or easiest, is to disconnect the motor hot lead from the terminal block screw on the logic board inside the brain box and connect one wire from an external AC toggle switch to the terminal block screw and the other lead from the switch to the motor hot lead (an AC wire nut is the best way to make that connection, so it's quick and easy to remove the mod and go back to stock configuration if desired.) The two switch leads can be run alongside the other wires exiting the brain box and run under the machine to the toggle switch, which should be mounted in an insulated enclosure like a sturdy plastic box.

Removing/reinstalling the brain box, which you must do to access the logic board, is a little tricky and requires some care. Remove the cup tray, the left and right side panels, and the top panel. Remove the two screws at the bottom of the rear panel and pull up to remove it. That lets you slide the brain box out from under the machine. Do that slowly and don't pull too hard because that could disconnect one or more wires running between the brain box and the rest of the machine. Remove the screws in the cover of the brain box and gently lift the cover. You'll probably have to remove the leads that connect to the main power switch in order to remove the cover, so note the connector positions before removal (take a photo.) Once the cover is off, the terminal block is the large green one all the way to the left of the logic board. After making the connections, check that all the connectors on the logic board are fully inserted (especially the ribbon cable connectors), reconnect the main power switch, replace the cover, screw it down (don't overtighten the screws or the plastic pillars will break), and gently insert the brain box back into the machine, taking care not to pull on any wires. If you find after firing up the machine that there are any problems with the display or front panel buttons, it's likely that one or both ribbon cables pulled off the logic board in the brain box or at the front panel.

A much simpler and easier way to do the mod is to cut the hot lead where it runs along the chassis floor to the left of the steam boiler. The leads can be run through the hole in the chassis floor alongside the pump supply hose. It's not too big a deal to reverse the mod -- just reconnect the two ends of the hot lead with a wire nut and tape it out of the way. But the machine will no longer be cable of returning to stock condition if you do it that way.

Forget about trying to disconnect the leads at the motor end. The connection is inside the motor and that part of the motor is very difficult to access.

Hope this helps.

[Nawaf (original poster)]

Thank you so much for taking time to explain everything,
I have rebuilt some old machines, so I think ill take the first way to do it, it's sound easy to do,
Regards,

[Jake_G]

You do need to make sure that the switch is always in the On position when not brewing. Otherwise, the steam boiler won't fill when needed.

Why not?

When your plumbed in at 3 bar line pressure, the boiler should fill just fine without the pump. In fact, I think there is a setting in the menu to fill the boiler sans pump (I know this exists for tea water, can't remember if it does boiler fill, too). Anyway, I ran my Rancilio commercial machine with the pump kill switch off for silent boiler fills.

Cheers!

- Jake

[Peppersass]

You're right. Sorry. The boiler will fill, just not as fast.