Mark - Don't go buying parts
yet. Before any more logical troubleshooting can be done you really need a circuit diagram.
The black item is simply a relay - pins 1-2 when charged activate the coil - then 3-4 are connected - I suspect based on the wire thickness in and out of the black box that this is house bus power (red lines) - I suspect one comes from alternator
output and the other goes to the 1-2-both switch or the main bus and then from main bus a line to the 1-2-both switch - this is likely how the alternator charge also gets to teh batteries through the 1-2-both switch.
That cylinder is clearly a solenoid and I guess the double wire going in is solenoid power (blue) and alternator sense brown - but that's a guess (you can check the back of the alternator and see if a brown wire is coming out of the plastic plug
that goes in the back of the alt. When you hit the master, the cylinder is charged, the big black wires make contact and then the black box is powered by the blue ware coming off the cylinder to the black relay - That lights up the DC bus. Then watever battery
bank (1-2-both) is selected on the manual switch is now supplying one side of the cylinder and out to the starter on the other. The brown wire is now sensing bus voltage, because it is now exposed to the bus due to all teh contacts being closed.
It is here you would check "sense" voltage. You will likely get a good voltage read to ground from brown. This is what the alternator is seeing. If this voltage is low the alternate regulates up. If this voltage is high the alternator regulates down.
You can also check bus voltage on each side of the cylinder - If the voltages are different going in than going out, that solenoid (cylinder) has high resistance. I am betting it is OK because you report the fault only on one bank.
It makes sense that the sense line goes to a unit that is common to both battery
banks and "central" to the electrical system
- this solenoid appears to be it. The blue line in the double likely goes to the master switch on teh DC bus panel. The blue single
goes to the bus realy (black box) andis powered by the master relay (cylinder).
Note there is one big cable going in and one big cable going out of the cylinder. I suspect one of the big cables
goes to the 1-2-both switch. The other end likely goes to the starter relay.
So - This cylinder is a single
point piece of hardware
- i.e. it is operating whether you are on 1-2 or both. Since your fault lies with only one condition (2) then a common piece of hardware
to both settings cannot be the fault.
I hope someone with a 393 wiring
diagram shows up soon.
BTW - Solenoids get hot. There is a current racing
through a coil to keep the solenoid contacts closed and there is current
running across the secondary contacts especially right after teh starter has been activated. How hot is too hot takes an experienced hand to figure out. I would say that if you can put your hand on it for up to 5 seconds it is not too hot. If you can fry eggs on it, you have an issue. If it makes smoke it's too late...
BTW - Have Nicolle operate your bus master switch while you listen to this cyclinder - you will hear a loud click when she powers on.
Remember - Your fault is on one bank only. Parts
that are common to both banks are not going to be the problem.
At this point I still vote 1-2-both switch or it could be the cable going from house bank to 1-2-both switch. Can you tell the condition of that?