on the car board pin B is labeled LDZ on a 4 key board on a 3 key board there is no pin labeled B that I can see. it looks as though LDZ was added to the 4 key circuit board to disable the ULD circuit. the 3 key board does not have a ULD output circuit but does have the same ULD input circuit tied to a terminal that 4 key does. I cant find where the ULD terminal is on the print though, still looking.
One thing to look at is why the U terminal is being grounded to cause the U relay to energize and move the car up until level down input comes on. I would go to the power board and check the signal to start the car up. the signal is U, or US and is on pin T. If that pin is going hi, then the amplifier circuit will drive U terminal low and pick up the U relay. If you see that pin T is staying low and does not change when the U relay picks up, you want to check out the rest of the amplifier circuitry, In order for the circuit to work the IN inputs must be in the proper state for the amplifier to work correctly. first the SP signal must be connected to pin X of the power board through a normally open in2 relay contact it should be 5 volts.
the other part is the common or PCP of the amplifier circuit. It need to be connected to PC through another normally open in2 contact. If PCP signal is not connected to the PC terminal through the in2 relay contact the amplifier may trigger U to go low without Pin T the up start signal initiating the sequence. If you have a power board logic print look at the amplifier circuit at the bottom of the page and you will see the PCP terminal there, it doesnt show you where it originates though, you have to go to where the in2 relay is on the print to see what I mean, it is next to the U and D relays on the print i have. I suspect you dont have a connection to pcp.. hope this might have helped still looking for that damn uld terminal on my prints though,
