Each flow meter has a positive (+), negative (-) and signal (0) terminal. The type of connector and pinout used by the coffee machine / flow meters varies by manufacturer and model.
See https://flowcoffee-co-nz.atlassian.net/wiki/spaces/FGCI/pages/461242401/Espresso+machine+knowledge+base#Flow-meter-connection-variations for an overview of the different types of flow meter connections found in common makes/models of espresso machine.
GDS 207 connector compatible
The majority of espresso machine manufacturers use a GDS 207 connector compatible flow meter.
Parts required
Two grouphead Flow meter cable, per two groupheads
Installation
GDS 207 connector orientation
Nb. If space constraints inside the coffee machine prevent the connector on the Flow meter cable from being able to be connected to the coffee machine’s flow meter, the orientation of the cable entry sleeve on the connector can be rotated. To do this:
Unscrew the cable entry grommet, so that the cable can move freely.
Pop the pinout insert out of the bottom of the connector by removing the centre screw and pressing down on the flange which surrounds the screw.
Rotate the pinout insert to the desired orientation and reinsert it into the connector housing, taking care not to crush any of the wires on the centre screw flange.
Tighten the cable entry grommet and reinsert the centre screw.
GDS 207 connector compatible with individual quick connect (blade) connectors
Some coffee machines have GDS 207 connector compatible flow meters but instead of a GDS 207 connector they have individual quick connect connectors on the cable loom.
Parts required
Two grouphead Flow meter cable, per two groupheads
GDS 207 connector compatible - Alternate pinout
Some espresso machines use a different pinout on the flow meters, even though they use the same type of plug. This applies to machines which use a GDS 207 connector as well as ones which use individual quick connect (blade) connectors.
This type of flow meter is present in most La Marzocco and Unic espresso machines.
Parts required
Two grouphead Flow meter cable, per two groupheads
Alternate pinout adapter, per two groupheads
Two quick connect (blade) connectors plus ground lug
This type of flow meter is present in most Rancilio, and some La Marzocco espresso machines.
In the case of a La Marzocco machine with this type of flow meter, the Alternate pinout adapter (above) is not required.
Parts required
Two grouphead Flow meter cable, per two groupheads
Two terminal plus ground lug flow meter adapter, per grouphead
1x insulated quick connect female, per grouphead
Installation
Four pin header
This type of flow meter is present in Cimbali and Faema espresso machines.
Parts required
Two grouphead Flow meter cable, per two groupheads
Four pin header flow meter adapter, per grouphead
GDS 207 connector, per grouphead