Try swaping relays around, possible the tips of K3 are oxidized causing the low B+ voltage. But the problem is that you are not getting B+ at the trac and drive amps when K3 is energized/on. Power goes through Fuse 5 on the relay card, to K3 when it is on/energied and to the trac and lift amps terminal 1 B+, which wakes the amps up. Veryify you have 36volts/battery voltage across fuse 5 on the relay card, with everything connected and the unit powered up, but you should since fuse 5 also powers the key switch.
If their was a short in one of the amps in the B + cicruit it should be blowing fuse 5, they show a thermistor in the wiring diagram on a lot of cicruits on the relay card, but none on the B+ circuit to the trac and lift amps, otherwise i would say that is why you are not getting full power at the amps. Their is still a issue with the relay card, you could try making a small jumper with a 10-15 amp fuse direct from the battery to the B+ terminal at the lift and trac amps, with the unit key'd up the leds, should light up on the amps. Look very closely at the K3 relay/terminals on the relay card/inspect the b + wires between the relay card and the amps.