This part is connected to a relay, that is connected to your Cristec
battery isolator, model RCE/100-2E-3.
Both engines have the part.
The
battery isolator receives the output from the 2 engines and directs it to the least charged of the three
batteries.
The part excites your alternator on command from the battery isolator - because the isolator / alternator
installation does not have ignition alternator excitation.
I suspect that if you remove the part, the isolator wont charge your
batteries.
Note. The Cristec isolator is rated to 100 amps. Standard
Yanmar alternators output 60 amps each. If you replace your engines with the latest models, these have 120 amp alternators. So now you will need to upgrade your battery isolator to cope. The
engine upgrade is a nice upgrade because the new engines use a serpentine belt to drive the
water pump and alternator - say goodbye to all that vee belt dust, grief, and on-going
replacements. You can get
parts to upgrade an old engine to use the serpentine belt.