The system voltage is measured on the main bus. Each shelf of 4 rectifiers has it’s own bus. The shelf plugs into the main bus via three Crown Clips on each side of the shelf.
The original crown clips sometimes did not plug into the shelf all the way. The gap between the end of the crown clips and the valley between the three fingers should be less than 1/8 inch.
After a time (possibly years) the crown clips will overheat, burn up and stop making contact. To prove this is the problem, measure the voltage on the buses that go horizontally along the back of the shelf. The voltage will probably be above 27 volts (the controller is turning up the rectifiers to compensate for the low system voltage). Sometimes the shelf can be removed and jiggled to make better contact as a temporary fix. Also look at the max batt recharge rate and set it to as low as possible to ease the strain on the connectors. Recommended max batt recharge time is total ampere hours divided by 20 hours.
As a permanent fix new buses that are slightly longer are available. The part number is 0M-9142, and two per shelf are required.