I'm sorry no one has chimed in on this one. My company has also had this problem on a few mod jobs, and I was interested to find the solution.

The jobs we have had this problem with have had discrete hall calls, though, not serial link. Also, I don't think the A car being on inspection affected the hall calls, it would only happen if the A car was the slave and the B car was the dispatcher. We only noticed it after emergency power testing when building normal power came back on and the dispatching was swapped.

I know that it is important that the 2 busses are in phase between the two cars- with the meter on AC Volts 2 to ST2 should have no potential between them. But you mentioned that this is the case already, and in the cases I have seen the 2 busses are in phase.

Everyone is so busy here that no one has had the time or patience to call MCE tech support and wait the two to four hours on hold with a meter and print at the ready in the machine room! But that is probably what needs to be done, and then hope you get someone who knows what they are doing.

Good luck, if you figure it out report back here please, if I do I will as well.